EP0570910A1 - High strength and high toughness aluminum alloy structural member, and process for producing the same - Google Patents

Abstract

A high strength and high toughness aluminum alloy structural member is produced by crystallization of an aluminum alloy blank having a metallographic structure including an amorphous phase. The structural member includes an aluminum crystal matrix and intermetallic compound crystals dispersed in the aluminum alloy matrix. If the average grain size of aluminum crystals is represented by d₁, and the grain size of the intermetallic compound crystals is represented by d₂, the average percentage P₁ of the total number of small intermetallic compound crystals having the grain size d₂ equal to or less than d₁/2 (d₂≦ d₁/2) is set at a value equal to or more than 80 % (P₁ ≧ 80 %). An increase in strength of the structural member is provided by dispersing the intermetallic compound crystals in the aluminum crystal matrix. With a reduction in size of the intermetallic compound crystals, an impact force on the structural member is absorbed into the aluminum crystals, leading to an increase in toughness of the structural member.

Description

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION
• The present invention relates to a high strength and high toughness aluminum alloy structural member, and particularly, to an aluminum alloy structural member produced by crystallization of an aluminum alloy blank having a metallographic structure including an amorphous phase, and processes for producing the same.
DESCRIPTION OF THE PRIOR ART
• There are such conventionally known aluminum alloy structural members including intermetallic compound crystals dispersed in an aluminum crystal matrix. For example, see Japanese Patent Application Laid-open No. 275732/89 and U.S. Patent No. 5,053,085, both hereby incorporated by reference.
• The prior art aluminum alloy structural member offers no problem with respect to the strength, because the metallographic structure is fine and substantially uniform. However, the prior art aluminum alloy structural member is accompanied by a problem that if the sizes of the aluminum crystal and the intermetallic compound crystal become substantially equal to each other during uniformization of the metallographic structure, an impact force on the structural member acts substantially equally on both the aluminum and intermetallic compound crystals. As a result, the crystal grain boundary of the intermetallic compound, which is brittle, is destroyed, thereby bringing about a reduction in toughness of the structural member.
• There are also conventionally known processes for producing the structural member, in which the aluminum alloy blank is subjected to a shaping and solidification, e.g., a hot extrusion with the processing temperature T set at a value in a range of $\text{Tx - 100 K ≦ T ≦ Tx + 100 K}$

  

  (see the above-described known publications).
• However, the structural member produced in the prior art process has a problem that the structural member is low in elongation, fatigue strength and toughness at a high temperature such as 423 to 473 K (150 to 200°C) due to a high internal stress of the aluminum alloy blank, a small difference in grain size between the aluminum and intermetallic compound crystals, and a high resistance to slip (deformation by slip). Therefore, such structural member is unsuitable for a structural member in high temperature applications such as an engine part.
SUMMARY OF THE INVENTION
• Accordingly, it is an object of the present invention to provide an aluminum alloy structural member of the type described above, wherein increases in strength and toughness are achieved by dispersing, in an aluminum crystal matrix, a particular amount of intermetallic compound crystals smaller than the aluminum crystals.
• To achieve the above object, according to the present invention, there is provided a high strength and high toughness aluminum alloy structural member, produced by crystallization of an aluminum alloy blank having a metallographic structure including an amorphous phase, wherein the structural member comprises an aluminum crystal matrix and intermetallic compound crystals dispersed in the aluminum crystal matrix, and if an average grain size of the aluminum crystals is represented by d₁ and the grain size of the intermetallic compound crystals is represented by d₂, an average percentage P₁ of the total number of small intermetallic compound crystals having the grain size d₂ equal to or less than d₁/2 with respect to 100 or more intermetallic compound crystals is equal to or more than 80 % in a plane of microscopic examination.
• If the small intermetallic compound crystals having the grain size d₂ are dispersed at an average percentage P₁ in the aluminum crystal matrix, an impact force, when it is applied to the structural member, is resiliently absorbed by large aluminum crystals, so that the propagation thereof to the small intermetallic compound crystals is suppressed to the utmost. This makes it possible to provide an increase in toughness of the structural member. In addition, it is possible to achieve an increase in strength of the structural member by dispersing the intermetallic compound crystals in the aluminum crystal matrix.
• It should be noted that it is impossible to produce the structural member of the invention from an aluminum alloy blank that initially includes no amorphous phase. This is because the structural member according to the invention is produced utilizes the crystallization process of the amorphous phase to control the grain size of the intermetallic compound crystals and their concentration. If the grain size d₂ of the small intermetallic compound crystals is larger than d₁/2 (i.e., d₂ > d₁/2), the degree of increase in toughness of the structural member is low, due to the relatively small difference in grain size between the aluminum and intermetallic compound crystals. The same can be said when the average percentage P₁ is less than 80 % (P₁ < 80 %).
• In addition, it is an object of the invention to provide a process for producing an aluminum alloy structural member with a high strength and a high toughness, in which the internal stress in the aluminum alloy blank can be removed, and at the same time, a large difference in grain size can be produced between the aluminum and intermetallic compound crystals by subjecting the aluminum alloy blank to a particular thermal treatment, thereby producing a structural member having increased elongation, fatigue strength and toughness at a high temperature.
• To achieve the above object, according to the invention, there is provided a process for producing a high strength and high toughness aluminum alloy structural member, comprising the steps of: subjecting an aluminum alloy blank having a metallographic structure including an amorphous phase having a crystallization temperature Tx to a primary thermal treatment under a condition of a thermal treatment temperature T₁ in a range of $\text{Tx - 100 K≦ T₁ ≦ Tx + 100 K}$

  

  , thereby crystallizing the amorphous phase and decomposing the phase, subjecting the aluminum alloy blank to a secondary thermal treatment under a condition of a thermal treatment temperature T₂ higher than Tx + 100 K, and subjecting the resulting aluminum alloy blank to a shaping and solidification under a processing temperature T₄ equal to or lower than the temperature T₂.
• The above-described metallographic structure corresponds to a metallographic structure which has a mixed-phase texture with fine aluminum crystals uniformly dispersed in an amorphous phase, or an amorphous single-phase texture, and which exhibits an exotherm of 20 J/g or more in a temperature region from the crystallization temperature Tx to the crystallization temperature Tx + 150 K in a differential scanning calorimeter (DSC) thermal analysis, when the temperature is increased at a rate of 20 K/min. If an aluminum alloy blank having such a metallographic structure is subjected to a primary thermal treatment of the type described above, the metallographic structure of a resulting aluminum alloy blank will have a fine and uniform crystalline phase. In this case, a high internal stress exists in the aluminum alloy blank as a result of the crystallization and the phase-decomposition, but if such aluminum alloy blank is subjected to a secondary thermal treatment of the type described above, the internal stress in the aluminum alloy blank is removed and the metallographic structure is stabilized. At the same time, the resistance to slip is reduced by preferentially growing the aluminum crystals to provide an increased difference in grain size between the aluminum and intermetallic compound crystals, and as a result, the aluminum alloy blank has excellent deformation capability.
• If the shaping and solidification of such an aluminum alloy blank is conducted at the above-described processing temperature T₄, a structural member can be produced without injuring the stabilized state of the metallographic structure and the large difference in grain size produced between both the crystals. Thus, the structural member exhibits excellent elongation, fatigue strength and toughness at a high temperature.
• In the producing process, the primary thermal treatment may be applied to an aluminum alloy blank in the form of a powder.
• However, if the thermal treatment temperature T₁ in the primary thermal treatment is lower than $\text{Tx - 100 K (T₁ < Tx - 100 K)}$

  

  , the crystallization of the amorphous phase and the phase-decomposition are insufficient and hence, a sudden phase change occurs during the secondary thermal treatment, resulting in a nonuniform metallographic structure of the aluminum alloy and moreover, it is difficult to remove the internal stress and to preferentially grow the aluminum crystals. If T₁ > Tx + 100 K, the resulting aluminum alloy has a fine and coalesced metallographic structure. If the thermal treatment temperature T₂ in the secondary thermal treatment is equal to or less than $\text{Tx + 100 K (T₂ ≦ Tx + 100 K)}$ , the removal of the internal stress in the aluminum alloy blank and the preferential growth of the aluminum crystals may not be sufficient in some cases. If the processing temperature T₄ in the shaping and solidification is higher than T₂ (T₄ > T₂), the stabilized state of the metallographic structure in the aluminum alloy blank after the secondary thermal treatment and the large difference in grain size between both the crystals may be injured.
• In the producing process, a step of isothermally maintaining the aluminum alloy blank after the secondary thermal treatment in a heating atmosphere at a temperature T₃ equal to or higher than $\text{Tx + 160 K (T₃ ≧ Tx + 160 K)}$

  

  may be added.
• Further, if a producing process is used in which the thermal treatment temperature T₂ in the secondary thermal treatment is changed to a range represented by $\text{T₁ ≦ T₂ ≦ Tx + 100 K}$

  

  , and a step of isothermally maintaining the aluminum alloy blank after the secondary thermal treatment in a heating atmosphere at a temperature T₃ equal to or higher than $\text{Tx + 160 K (T₃ ≧ Tx + 160 K)}$

  

  is added and then, the blank is subjected to shaping and solidification at the processing temperature of T₄ ≦ T₃, an aluminum alloy structural member having a further high toughness can be produced.
• When the aluminum alloy blank is in the form of a powder (including a compact), the primary and secondary thermal treatments can be also used as primary and secondary degassing treatments for the aluminum alloy blank.
• The above and other objects, features and advantages of the invention will become apparent from the following description of preferred embodiments, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
• Fig. 1 is a graph illustrating the relationship between the average percentage P₁ of small intermetallic compound crystals and the tensile strength;
• Fig. 2 is a graph illustrating the relationship between the average percentage P₁ of small intermetallic compound crystals and the Charpy impact value;
• Fig. 3 is a graph illustrating the relationship between the average percentage of small intermetallic compound crystals and the Charpy impact value;
• Fig. 4 is a photomicrograph mainly showing aluminum crystals;
• Fig. 5 is a diagram tracing the aluminum crystal in Fig. 4;
• Fig. 6 is a photomicrograph mainly showing intermetallic compound crystals;
• Fig. 7 is a diagram tracing the intermetallic compound crystals in Fig. 6;
• Fig. 8 is a graph illustrating the relationship between the average percentage of minute intermetallic compound crystals and the Charpy impact value;
• Fig. 9 is an X-ray diffraction pattern for an aluminum alloy blank;
• Fig. 10 is a thermocurve diagram of a differential thermal analysis for the aluminum alloy blank;
• Fig. 11 is a graph illustrating the relationship between the thermal treatment temperature T₂ in a secondary thermal treatment and the tensile strength σ _B as well as the elongation;
• Fig. 12 is a graph illustrating the relationship between the thermal treatment temperature T₂ in the secondary thermal treatment and the fatigue strength σ _f;
• Fig. 13 is a graph illustrating the relationship between the thermal treatment temperature T₂ in the secondary thermal treatment and the Charpy impact value;
• Fig. 14 is a graph illustrating the relationship between the thermal treatment temperature T₁ in a primary thermal treatment and the tensile strength σ _B as well as the elongation;
• Fig. 15 is a graph illustrating the relationship between the thermal treatment temperature T₂ in the secondary thermal treatment and the average grain sizes of aluminum crystals and intermetallic compound crystals;
• Fig. 16 is a graph illustrating the relationship between the time in a secondary degassing treatment and the tensile strength.
DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1
• A molten metal having a composition represented by Al₉₂Fe₅Y₃ (each numerical value represents an atomic %) was prepared. A powdery aluminum alloy blank was produced under a condition of a helium gas pressure of 100 kgf/cm² by use of a ultrasonic gas atomization apparatus. Then, the aluminum alloy blank was subjected to a classifying treatment to adjust the grain size of the blank to less than 26 µm.
• The aluminum alloy blank was subjected to X-ray diffraction and thermal analysis using a differential scanning calorimeter (DSC) to examine the metallographic structure thereof. The results showed that the metallographic structure of the aluminum alloy blank was a mixed-phase texture consisting of a crystalline phase and an amorphous phase, and the crystallizing temperature Tx for the amorphous phase was 653 K.
• The aluminum alloy blank was used to produce a plurality of billets by application of a cold isostatic press under a condition of 4,000 kgf/cm². The billets were then subjected sequentially to a primary and a secondary thermal treatment. Then, the billets were heated to an extrusion temperature (processing temperature) T₄= 673 K. The temperature of the container and die was set at 723 K, and the billets were subjected to a hot extrusion (a shaping and solidification) at that temperature, thereby producing various aluminum alloy structural members.
• The term "shaping and solidification" used herein means firmly binding alloy powder grains to one another in a similar condition to the sintering process, and the resulting product has enough hardness to be used as structural members.
• The primary thermal treatment was performed mainly for the purpose of crystallization and phase-decomposition, and the secondary thermal treatment was performed mainly for the purpose of adjusting the grain size of aluminum crystals and intermetallic compound crystals (which will be referred to as IMC crystals hereinafter). The primary thermal treatment was carried out under conditions of a thermal treatment temperature T₁ = 623 K and a time of 15 to 75 minutes. And the secondary thermal treatment was carried out under conditions of a thermal treatment temperature T₂ = 773 K and a time of 60 minutes.
• In this case, the thermal treatment temperature T₁ for the primary thermal treatment satisfies the relation of $\text{Tx - 100 K}$

  

  $\text{≦ T₁ ≦ Tx + 100 K}$

  

  ; the thermal treatment temperature T₂ for the secondary thermal treatment satisfies the relation of $\text{T₂ > Tx + 100 K}$

  

  , and the extrusion temperature T₄ satisfies the relation of T₄ ≦ T₂.
• The metallographic structure of each of the structural members was observed using a transmission type electron microscope and a scanning type electronic microscope, thereby finding an average grain size d₁ ( µm) of the aluminum crystals and a grain size d₂ ( µm) of the IMC crystals. The average grain size d₁ ( µm) of the aluminum crystals was determined by finding examining the size of 30 aluminum crystals on any plane of microscopic examination.
• In this case, if the aluminum crystals were not spherical, the longitudinal length thereof was defined as the grain size. This is also the case for the IMC crystals. One hundred (100) or more IMC crystals (100 in this embodiment) were selected for each of the various planes of microscopic examination, and an average percentage P of the total number n of IMC crystals having a grain size d₂ in each of various ranges in such 100 IMC crystals was determined $\text{(P = (n/100)x100 (\%))}$

  

  .
• Table 1 shows the primary thermal treatment time, the average grain size d₁ of the aluminum crystals, the grain sizes d₂ of the IMC crystals, and the average percentage P of IMC crystals having the grain sizes d₂ in each of the structural members (1) to (5). TABLE 1

  S.M.	P.T.T. (min.)	d₁ of Al Cr. ( µm)	A.P. (%) of IMC crystals (unit of d₂:µm)
  			(a)	(b)	(c)	(d)	(e)
  (1)	15	0.31	21	16	17	22	24
  (2)	30	0.30	23	26	19	22	10
  (3)	45	0.30	42	26	13	12	7
  (4)	60	0.31	51	34	12	3	-
  (5)	75	0.31	62	28	10	-	-
  S.M. = Structural member P.T.T. = Primary thermal treatment Cr. = Crystal A.P. = Average percentage (a): d₂ ≦ 0.05 (b): 0.05 < d₂ ≦ 0.1 (c): 0.1 < d₂ ≦ 0.15 (d): 0.15 < d₂ ≦ 0.2 (e): 0.2 < d₂

• If the time is varied in the primary thermal treatment, the degree of crystallization of the billet is varied. If the time is less than 60 minutes, a partial crystallization occurs, resulting in an nonuniform metallographic structure. On the other hand, if the time is set at a value equal to or more than 60 minutes, the crystallization is completed, thereby providing a uniform and fine metallographic structure of a billet.
• If the crystallization has been completed in the primary thermal treatment, a distinct difference in grain size between both the aluminum and IMC crystals appears in the secondary thermal treatment, due to a faster growth of aluminum crystals than the growth of IMC crystals. On the other hand, if the crystallization has not been completed in the primary thermal treatment, aluminum and IMC crystals are suddenly produced and coalesced in the secondary thermal treatment, resulting in an indistinct difference in grain size between both the aluminum and IMC crystals.
• The average percentage P₁ of small IMC crystals having grain sizes d₂ ≦ d₁/2 (d₂ ≦ 0.15 µm) in each of the structural members (1) to (5) as given in Table 1 was found, and the mechanical strength of each of the structural members (1) to (5) was measured, thereby providing results given in Table 2. The tensile test and the Charpy impact test were carried out at room temperature (the room temperature test condition was likewise used in subsequent Examples which will be described hereinafter). TABLE 2

  S.M.	A.P of small IMC CR. P₁ (%)	Tensile strength (MPa)	Elongation (%)	Charpy impact value (J/mm2)
  (1)	54	591	8.7	0.10
  (2)	68	598	8.5	0.12
  (3)	81	608	8.3	0.28
  (4)	97	613	8.6	0.31
  (5)	100	614	8.4	0.30
  S.M. = Structural member A.P = Average percentage Cr. = crystal

• Fig. 1 shows the relationship between the average percentage P₁ of members (1) to (5), and Fig. 2 shows the relationship between the average percentage P₁ of small IMC crystals and the Charpy impact value for each of the structural members (1) to (5). Numerals (1) to (5) in Figs. 1 and 2 correspond to the structural members (1) to (5), respectively.
• As apparent from Table 2 and Figs. 1 and 2, the structural members (3) to (5) with the average percentage Pi of small IMC crystals equal to or more than 80 % (P₁ ≧ 80 %) have a high strength and a high toughness, whereas the structural members (1) and (2) with the average percentage P₁ less than 80 % (P₁ < 80 %) have a low strength and a low toughness.
EXAMPLE 2
• Molten metals having various compositions were prepared and used to produce various powdery aluminum alloy blanks in the same manner as in Example 1. Then, the powdery aluminum alloy blanks were subjected to a classifying treatment similar to that in Example 1.
• The aluminum alloy blanks were used to produce various billets in the same manner as in Example 1. Then, the billet were subjected to primary and secondary thermal treatments, followed by a hot extrusion (T₄ = 673 K), in the same manner as in Example 1, thereby producing various aluminum alloy structural members. The primary treatment was carried out under conditions of a thermal treatment temperature T₁ of 593 to 653 K and a time of 30 to 75 minutes, and the secondary thermal treatment was carried out under conditions of a thermal treatment temperature T₂ of 773 K and a time of 60 minutes.
• Table 3 shows the composition of each of the structural members (6) to (25), the metallographic structure and the crystallization temperature Tx of the amorphous phase in each of the aluminum alloys, the conditions for the primary thermal treatment and the grain size d₁ of aluminum crystals. "Mm" in both Table 3 means a misch metal. This also applies in Examples which will be described hereinafter.

  
• In this case, in the structural members (6) to (25), the thermal treatment temperature T₁ in the primary thermal treatment satisfies the relation of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  ; the thermal treatment temperature T₂ in the secondary thermal treatment satisfies the relation of T₂ > Tx + 100 K, and the extrusion temperature T₄ satisfies the relation of T₄ ≦ T₂.
• The average percentage P₁ of the small IMC crystals in each of the structural members (6) to (25) with the grain size d₂ ≦ d₁/2 was found, and the mechanical strength of the each of the structural members (6) to (25) was measured, thereby providing results given in Table 4. TABLE 4

  S.M.	A.P of small IMC CR. P₁ (%)	Tensile strength (MPa)	Elongation (%)	Charpy impact value (J/mm2)
  (6)	68	568	6.4	0.13
  (7)	82	580	6.3	0.24
  (8)	91	589	6.6	0.30
  (9)	98	590	6.5	0.32
  (10)	58	578	7.8	0.10
  (11)	74	583	7.7	0.12
  (12)	86	591	8.0	0.26
  (13)	97	597	8.0	0.28
  (14)	42	724	6.3	0.09
  (15)	61	729	6.1	0.08
  (16)	74	736	6.0	0.12
  (17)	91	742	6.3	0.26
  (18)	64	621	8.9	0.12
  (19)	77	627	8.9	0.14
  (20)	88	634	9.1	0.28
  (21)	100	638	8.9	0.31
  (22)	63	714	6.8	0.09
  (23)	74	720	6.7	0.11
  (24)	82	721	6.9	0.25
  (25)	91	723	6.7	0.28
  S.M. = Structural member A.P = Average percentage Cr. = crystal

• Fig. 3 shows the relationship between the average percentage P₁ of the small IMC crystals and the charpy shock value of each of the structural members (6) to (25). Numerals (6) to (25) in Fig. 3 correspond to the structural members (6) to (25), respectively.
• As is apparent from Table 4 and Fig. 3, the structural members (7) to (9), (12), (13), (17), (20), (21), (24) and (25), in which the average percentage P₁ of small IMC crystals is set larger than 80% (P₁ ≧ 80%), have a high strength and a high toughness in the same composition, whereas the aluminum alloys (6), (10), (11), (14) to (16), (18), (19), (22) and (23) with P₁ < 80 % have a low strength and a low toughness in the same composition.
• Fig. 4 is a photomicrograph (130,000X magnification) showing mainly an aluminum crystal in the structural member (20) of Tables 3 and 4. Fig. 4 was taken by a transmission type electron microscope, and Fig. 5 is a diagram tracing the aluminum crystal shown in Fig. 4. One aluminum crystal exists in Fig. 4.
• Fig. 6 is a photomicrograph (130,000X magnification) showing mainly the IMC crystals in the aluminum alloy (20) in the same plane of microscopic examination as in Fig. 4, and Fig. 7 is a diagram tracing the IMC crystals shown in Fig. 6. Thirty (30) IMC crystals exist in Fig. 6, as indicated by numerals (1) to (30) in Fig. 7. Among them, the IMC crystals indicated by the numerals (1) to (27), i.e., the 27 IMC crystals correspond to the small IMC crystals with the grain size d₂ ≦ 0.15 µm.
Example 3
• Using the aluminum alloy blanks (Al₉₂Fe₅Y₃, Tx = 653 K), a plurality of billets were produced in the same manner as in Example 1. Then, the billets were subjected to primary and secondary thermal treatments as in Example 1 and further isothermally maintained for 5 minutes in a heated atmosphere having a temperature T₃ equal to 823 K. Thereafter, the billets were subjected to a hot extrusion at an extrusion temperature T₄ of 673 K as in Example 1, thereby producing various aluminum alloy structural members. The primary thermal treatment was carried out under conditions of a thermal treatment temperature T₁ equal to 623 K and a time of 60 minutes, and the secondary thermal treatment was carried out under conditions of a thermal treatment temperature T₂ equal to 623 to 823 K and a time of 60 minutes.
• Table 5 shows the secondary thermal treatment temperature, the average grain size d₁ of the aluminum crystals, the grain sizes d₂ of the IMC crystals, and the average percentage P of the IMC crystals having the grain sizes d₂ in each of the structural member (26) to (30). TABLE 5

  S.M.	S.T.T.T. (K)	d₁ of A.C. ( µm)	A.P. (%) of IMC crystals (unit of d₂:µm)
  			(a)	(b)	(c)	(d)	(e)	(f)	(g)
  (26)	623	0.45	16	10	34	28	10	2	-
  (27)	673	0.46	11	14	28	31	13	3	-
  (28)	723	0.44	8	18	27	34	11	2	-
  (29)	773	0.45	3	17	31	34	12	2	1
  (28)	823	0.46	2	7	18	48	18	4	3
  S.M. = Structural member S.T.T.T = Secondary thermal treatment temperature A.C = Aluminum crystal A.P. = Average percentage (a): d₂ ≦ 0.01 (b): 0.01 < d₂ ≦ 0.03 (c): 0.03 < d₂ ≦ 0.05 (d): 0.05 < d₂ ≦ 0.1 (d): 0.1 < d₂ ≦ 0.15 (f): 0.15 < d₂ ≦ d₁/2 (g): d₁/2 < d₂

• In this case, in the structural members (26) to (28), the thermal treatment temperature T₁ in the primary thermal treatment satisfies the relation of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  ; the thermal treatment temperature T₂ in the secondary thermal treatment satisfies the relation of $\text{T₁ ≦ T₂ ≦ Tx + 100 K}$

  

  ; the isothermally maintaining temperature T₃ satisfies the relation of $\text{T₃ ≧ Tx + 160 K}$

  

  ; and the extrusion temperature T₄ satisfies the relation of T₄ ≦ T₃.
• In the structural member (29) and (30), the thermal treatment temperature T₁ in the primary thermal treatment satisfies the relation of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  ; the thermal treatment temperature T₂ in the secondary thermal treatment satisfies the relation of T₂> Tx + 100 K; the isothermally maintaining temperature T₃ satisfies the relation of $\text{T₃ ≧ Tx + 160 K}$

  

  ; and the extrusion temperature T₄ satisfies the relation of T₄ ≦ T₂.
• As is apparent from Table 5, the isothermally maintaining treatment for 5 minutes at 825 K after the primary and secondary thermal treatments ensures that the average grain sizes d₁ of the aluminum crystals in the structural members (26) to (30) are large, as compared with those in Examples 1 and 2 and substantially equal to one another. This is because the aluminum crystals grow preferentially during the isothermally maintaining treatment. In this case, the IMC was grown simultaneously, but the degree of growth thereof was small, because the date of growth of the IMC crystals is low in comparison to the aluminum crystals, and moreover, the isothermally maintaining time was as short as 5 minutes. As a result, the grain size of the IMC crystals was maintained small, and there was a large difference produced between the grain size of the IMC crystals and the average grain size d1 of the aluminum crystals.
• The average percentage P₁ of the small IMC crystals in each of the structural members (26) to (30) with the grain size d₂ ≦ d₁/2 as given in Table 5 was found, and the mechanical strength of each of the structural members (26) to (30) was measured, thereby providing the results given in Table 6. Table 6 also shows the average percentage of minute IMC crystal having the grain diameter d₂ ≦ 0.01 µm which is shown in Table 5, i.e., the average percentage P₂ of total number (n) of minute IMC crystals with respect to 100 or more IMC crystal (100 in the present embodiment) in a plane of microscopic examination $\text{(P₂ =(n₁/100)X100\%)}$

  

  . TABLE 6

  S.M.	A.P of samll IMC Cr. P₁ (%)	A.P of very samll IMC Cr. P₂ (%)	Te.St. (MPa)	Elo. (%)	Charpy impact value (J/mm²)
  (26)	100	16	573	10.9	0.39
  (27)	100	11	576	10.5	0.38
  (28)	100	8	572	10.8	0.31
  (29)	99	3	576	10.6	0.31
  (30)	97	2	574	10.7	0.33
  S.M. = Structural member A.P = Average percentage Sr. = crystal Te.St. = Tensile strength Elo. = Elongation

• Fig. 8 shows the relationship between the average percentage P₂ of vary small IMC crystals in each of the structural members (26) to (30) and the charpy shock value of each of the structural members (26) to (30). Numerals (26) to (30) in Fig. 8 correspond to the structural members (26) to (30), respectively.
• As is apparent from Table 6 and Fig. 8, the structural members (26) and (27) with the average percentage P₁ of the small IMC crystals equal to or more than 80 % (P₁ ≧ 80 %) and with the average percentage P₂ of the minute IMC crystals equal to or more than 10 % (P₂ ≧ 10 %) each have a further increased toughness, as compared with the other structural members (28) to (30). This is attributable to the fact that the influence of the IMC crystals on the toughness of the structural member was substantially moderated, because the reduction in size of the IMC crystals in these structural members was achieved, as compared with the other structural members (28) to (30).
Example 4
• Using the aluminum alloy blanks in Example 2, a plurality of billets were produced in the same manner as in Example 1 and then subjected to primary and secondary thermal treatments as in Example 1. Further, the billets were isothermally maintained at a temperature T₃ of 823 K for 5 minutes and then subjected to a hot extrusion at an extrusion temperature T₄ equal to 673 K, again as in example 1, thereby providing various aluminum alloy structural members. The primary thermal treatment was carried out under conditions of a thermal treatment temperature T₁ equal to 593 to 653 K and a time of 60 minutes, as in Example 2, and the secondary thermal treatment was carried out under conditions of a thermal treatment temperature T₂ equal to 673 K or 773 K and a time of 60 minutes.
• Table 7 shows the composition of each of the various structural members (31) to (40), the primary thermal treatment temperature, the secondary thermal treatment temperature, and the average grain size d₁ of the aluminum crystals. TABLE 7

  S.M.	Composition (% by atom)	Pr. T.T.Tem. (K)	Se.T.T.Tem. (K)	d₁ of Al crystals (µm)
  (31)	Al₈₉Ni₈Mm₃	593	673	0.51
  (32)	Tx = 620 K		773	0.53
  (33)	Al₉₁Ni₇Zr₂	593	673	0.48
  (34)	Tx = 615 K		773	0.48
  (35)	Al₉₀Fe₇Zr₃	613	673	0.31
  (36)	Tx = 650 K		773	0.32
  (37)	Al₉₁Fe₅Mm₄	593	673	0.45
  (38)	Tx = 623 K		773	0.45
  (39)	Al90.5Fe₆Zr₂Si1.5	653	673	0.40
  (40)	Tx = 660 K		773	0.40
  S.M. = Structural member Pr. T.T.Tem. = Primary thermal treatment temperature Se.T.T.Tem. = Secondary thermal treatment temperature

• In this case, in the structural members (31), (33), (35), (37) and (39), the thermal treatment temperature T₁ in the primary thermal treatment satisfies the relation of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  ; the thermal treatment temperature T₂ in the secondary thermal treatment satisfies the relation of $\text{T₁ ≦ T₂ ≦ Tx + 100 K}$

  

  ; the isothermally maintaining temperature T₃ satisfies the relation of $\text{T₃ ≧ Tx + 160 K}$

  

  ; and the extrusion temperature T₄ satisfies the relation of T₄ ≦ T₂.
• In the structural members (32), (34), (36), (38) and (40), the thermal treatment temperature T₁ in the primary thermal treatment satisfies the relation of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  ; the thermal treatment temperature T₂ in the secondary thermal treatment satisfies the relation of $\text{T₂ > Tx + 100 K}$

  

  ; the isothermally maintaining temperature T₃ satisfies the relation of $\text{T₃ ≧ Tx + 160 K}$

  

  ; and the extrusion temperature T₄ satisfies the relation of T₄ ≦ T₂.
• Table 8 shows the average percentages P₁ and P₂ of the small IMC crystals and the minute IMC crystals, and the mechanical strength of each of the structural members (31) to (40). TABLE 8

  S.M.	A.P of small IMC CR. P₁ (%)	A.P. of very sm. IMC CR. P₂ (%)	Te.St. (MPa)	Elo. (%)	Charpy impact value (J/mm²)
  (31)	98	12	593	7.2	0.40
  (32)	96	3	596	7.4	0.33
  (33)	100	14	561	8.6	0.39
  (34)	98	5	558	8.4	0.31
  (35)	97	8	687	7.1	0.30
  (36)	96	1	691	7.0	0.28
  (37)	99	13	596	10.3	0.41
  (38)	98	6	598	10.6	0.32
  (39)	98	11	673	7.6	0.36
  (40)	96	2	671	7.7	0.29
  S.M. = Structural member A.P = Average percentage Sr. = crystal Te.St. = Tensile strength Elo. = Elongation

• As is apparent from Table 8, the structural members (31), (33), (37) and (39), with the average percentage P₁ of the small IMC crystals equal to or more than 80% (P₁ ≧ 80%) and with the average percentage P₂ of the minute IMC crystals equal to or more than 10% (P₂ ≧ 10%), each have a further increased toughness, as compared with the other structural members (32), (34) to (36), (38) and (40).
• The aluminum alloy blanks used in the above-described Examples 1 to 4 had a metallographic structure including the amorphous single-phase texture.
Example 5
• A molten metal having a composition represented by Al₉₁Fe₆Y₃ (each numerical value represents atomic %) was prepared and used to produce a powdery aluminum alloy blank by application of a high pressure nitrogen gas atomization process (under a condition of 80 kgf/cm²). Then, the aluminum alloy blank was subjected to a classifying treatment to adjust the grain size of the powdery aluminum alloy blank to at most 22 µm.
• The powdery aluminum alloy blank was subjected to X-ray diffraction and thermal analysis using differential scanning calorimeter (DSC) to provide results given in Figs. 9 and 10. It was confirmed from Figs. 9 and 10 that the powdery aluminum alloy blank had a mixed-phase texture consisting of an amorphous phase and a crystalline phase, and the crystallization temperature Tx of the amorphous phase was 658 K.
• Primary and secondary thermal treatments and a hot extrusion which will be described below, were carried out sequentially on the powdery aluminum alloy blank to produce various structural members. First, the powdery aluminum alloy blank was used to produce a plurality of billets having a diameter of about 50 mm and a length of 50 mm by application of a cold isostatic press (CIP) under a condition of 4,000 kgf/cm². Each of the billets was placed into a can of aluminum alloy (A5056), and a lid was welded to an opening in the can, thereby fabricating an extrusion blank having a diameter of about 54 mm and a length of 70 mm. Then, the atmospheric pressure in the can was maintained at most 2 x 10 ⁻³ Torrs through a connecting pipe mounted to the lid.
• Each of the extrusion blanks, i.e., each of the billets was subjected to a primary thermal treatment with a thermal treatment temperature T₁ set at 623 K (Tx - 35 K) and a thermal treatment time set at one hour.
• The billets were subjected to a secondary thermal treatment with a thermal treatment temperature T₂ set at 673 K (Tx + 15 K), 698 K (Tx + 40 K), 723 K (Tx + 65 K), 743 K (Tx + 85 K), 753 K (Tx + 95 K), 763 K (Tx + 105 K) and 773 K (Tx + 115 K), respectively and with a thermal treatment time set at one hour.
• Then, each of the resulting billets was placed into a container having a temperature of 673 K and subjected to a hot extrusion under conditions of die diameter of 15 mm and an extrusion temperature T₄ equal to 673 K (T₄ ≦ T₂), thereby producing various structural members.
• Each of the structural members was subjected to a tensile test under a condition of a test temperature of 423 K to examine a tensile strength and an elongation at breakage, thereby providing results shown in Fig. 11. As is apparent from Fig. 11, the structural member produced with the thermal treatment temperature T₂ set larger than > Tx + 100 K in the secondary thermal treatment has an elongation of at least 5 %. Whether or not the structural member is suitable for an engine part is based on whether or not the structural member exhibits an elongation on the order of 5 % at such test temperature and therefore, according to the invention, it is possible to produce a structural member suitable for an engine part or the like.
• Then, the structural members were subjected to a tension and compression fatigue test at a test temperature of 423 K to measure fatigue strength. The tension and compression was repeated 10⁷ times, thereby providing results given in Fig. 12. In each of the structural members, the tensile strength σ _B was gently and gradually reduced with increasing thermal treatment temperature T₂, as shown in Fig. 11, but the fatigue strength σ _f was varied little at the thermal treatment temperature T₂ ≦ Tx + 100 K, on the one hand, and increased considerably at the thermal treatment temperature $\text{T₂ > Tx + 100 K}$

  

  , on the other hand, as can be seen from Fig. 12. The structural member produced with the thermal treatment temperature T₂ set at a value equal to 773 K in the secondary thermal treatment showed a high value of σ _f/σ_B equal to 0.51. For a structural member such as an engine part, the fatigue strength is generally regarded as more important than the tensile strength at temperature of 423 to 473 K, and according to the invention, it is possible to produce a structural member which can sufficiently accommodate such a demand.
• Further, the structural members were subjected to a Charpy impact test under a condition of a test temperature of 423 K, thereby providing results given in Fig. 13. As is apparent from Fig. 13, the Charpy impact values showed a tendency similar to that of the fatigue strength, and the structural member produced with the thermal treatment temperature T₂ set at a value more than Tx + 100 K has a high toughness.
• For comparison, various structural members were produced in the following manner:
     Using the above-described powdery aluminum alloy blanks, a plurality of extrusion blanks were fabricated through steps similar to those described above. The atmospheric pressure in the can in each of the extrusion blanks was likewise maintained at most 2 x 10 ⁻³ Torrs. The extrusion blanks, i.e., the billets, were subjected to a primary thermal treatment with varied thermal treatment temperatures and with a thermal treatment time set at one hour. Then, the billets were subjected to a secondary thermal treatment with a thermal treatment temperature set at 773 (Tx + 115K) and with a thermal treatment time set at one hour. Thereafter, the resulting billets were subjected to a hot extrusion under the same conditions (the extrusion temperature $\text{T₄ = 673 K, T₄ ≦ T₂}$

  

  ) as those described above, thereby producing various structural members.
• These structural members were subjected to a tensile test at a test temperature of 423 K to measure a tensile strength and an elongation at breakage, thereby providing results given in Fig. 14.
• As apparent from Fig. 14, if the thermal treatment temperature T₁ in the primary thermal treatment is set at a value in a range of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  , it is possible to increase the tensile strength and the elongation of each of the structural members. However, if the thermal treatment temperature T₁ is less than Tx - 100 K, a sudden change in phase occurs in the secondary thermal treatment, resulting in an nonuniform metallographic structure of the aluminum alloy blank and eventually, in a largely reduced elongation of the structural member. On the other hand, if $\text{T₁ > Tx + 100 K}$

  

  , the primary thermal treatment results not only in a nonuniform metallographic structure of the aluminum alloy blank, but also in a coalesced crystal texture in the aluminum alloy blank, and eventually in largely reduced tensile strength and elongation of the structural member.
• The metallographic structure of each of the structural members was observed using a transmission type electron microscope to examine the relationship between the thermal treatment temperature T₂ in the secondary treatment temperature and the average grain size of aluminum crystals and IMC crystals, thereby providing results given in Fig. 15.
• As is apparent from Fig. 15, the average grain size of the aluminum crystals can be suddenly increased, as compared with the average grain size of the IMC crystals, by setting the thermal treatment temperature T₂ in the secondary treatment temperature at a value more than $\text{Tx + 100 K (T₂ > Tx + 100 K)}$

  

  .
• If an increased difference between the average grain size of the aluminum and IMC crystals is produced by setting the thermal treatment temperature T₂ at a value more than Tx + 100 K (T₂ > Tx + 100 K) in this manner, it is possible to reduce the resistance to slip and thereby permit the dislocation of IMC crystals at the aluminum crystal interface, thus providing an increased deformation capability of the structural member. Therefore, it is believed that the above-described properties could be obtained. IMC is Al₈YFe₄, Al₃Y, etc., and the shapes and average grain size thereof were substantially identical.
• Although the thermal treatment time in the primary and secondary thermal treatments are dependent upon the thermal treatment temperature, the primary thermal treatment time t₁ is suitable to be 0.01 hour≦ t₁ ≦ 3 hr, and the secondary thermal treatment time t₂ is suitable if it is within the range 0.01 hour ≦ t₂ ≦ 2 hours. These time restraints were applied in the Examples which will be described hereinafter. Alternatively, the primary thermal treatment may be carried out with the temperature continuously increasing at a rate of 10 K/min or less. It is not necessarily required to perform the primary and secondary thermal treatments continuously, but it is desirable to perform the secondary thermal treatment and the shaping and solidification continuously. This is because if a time interval is provided between the secondary thermal treatment and the shaping and solidification, the control of the metallographic structure is complicated due to a heat hysteresis applied to the billet during the lowering, and a rising of the temperature.
Example 6
• The powdery aluminum alloy blank (Al₉₁Fe₆Y₃) in Example 5 was subjected to a primary thermal treatment in a nitrogen gas atmosphere under the same conditions as in Example 5, i.e., with the thermal treatment temperature T₁ set at 623 K (Tx - 35 K) and with the thermal treatment time set at one hour.
• The hardness of a powdery aluminum alloy blank resulting from the primary thermal treatment was measured using a micro Vickers hardness meter. The result showed that the hardness was 240 DPN which was a reduced value, as compared with the hardness of 300 DPN determined before the primary thermal treatment. Thereupon, using a mono-axial die press in place of the cold isostatic press (CIP), a billet having a diameter of 50 mm and a length of 50 mm was produced under a condition of 30 kgf/mm² from the powdery aluminum alloy blank resulting from the primary thermal treatment. The billet was placed into a can of an aluminum alloy as in Example 5 to fabricate an extrusion blank. The atmospheric pressure in the can of the billet was maintained at 2 x 10 ⁻³ Torrs or less.
• Then, the billet was subjected to a secondary thermal treatment with a thermal treatment temperature T₂ set at 763 K (Tx + 105 K) and with a thermal treatment time set at one hour, and after a lapse of such thermal treatment time, the billet was immediately placed into a container having a temperature of 673 K and subjected to a hot extrusion under the same conditions as in Example 5 to produce a structural member.
• This structural member was subjected to various tests at a temperature of 423 K. From the results, it was confirmed that the structural member exhibited a tensile strength σ _B of 470 MPa, an elongation of 6.5 %, and a fatigue strength σ _f of 220 MPa upon tensions and compressions repeated 10⁷ times and hence, had mechanical properties equivalent to or superior to those of the structural member produced under the same conditions in Example 5.
• If the aluminum alloy in the form of a powder is subjected to the primary thermal treatment in the above manner, the production of a billet can be easily carried out by the mono-axial press without use of the cold isostatic press, and therefore, it is possible to permit an in line production of the structural member to provide an improved mass production thereof.
Example 7
• Molten metals having various compositions were prepared and then used to produce powdery aluminum alloy blanks in the same manner as in Example 5. Thereafter, the powdery aluminum alloy blanks were subjected to a classification to control the grain size to equal to or less than 22 µm. The powdery aluminum alloy blanks were examined for the metallographic structure in the same manner as in Example 5, and as a result, it was ascertained that each of the powdery aluminum alloy blanks had a mixed-phase texture, as did those described above.
• Table 9 shows the composition and the crystallization temperature Tx of each of the powdery aluminum alloy blanks (41) to (60). TABLE 9

  Al alloy blank	Composition (by atomic %)	Tx (K) of amorphous phase
  (41) to (44)	Al₈₉Ni₈Mm₃	620
  (45) to (48)	Al₈₉Ni₈Zr₃	590
  (49) to (52)	Al₈₉Fe₇Mm₄	622
  (53) to (56)	Al₉₁Fe₇Zr₂	650
  (57) to (60)	Al90.5Fe₆Zr₂Si1.5	660

• The powdery aluminum alloy blanks were used to produce a plurality of extrusion blanks through steps similar to those in Example 5, and the atmospheric pressure in the can in each of the extrusion blanks was maintained at 2 x 10 ⁻³ Torrs or less. The extrusion blanks, i.e., the billets, were subjected sequentially to primary and secondary thermal treatments and a hot extrusion under the same condition as in Example 5 (the extrusion temperature T₄ = 673 K) to produce structural members (41) to (60). The structural members (41) to (60) correspond to the aluminum alloy blanks (41) to (60), respectively.
• Table 10 shows the conditions of production of each of the structural members (41) to (60), the average grain sizes of aluminum and IMC crystals and the mechanical properties of each of these structural members at 423 K. It should be noted that the thermal treatment time in the primary and secondary thermal treatments was set to one hour. In the "estimation" column of Table 10, a mark " ⃝" means that the mechanical strength is excellent and a mark "X" means that the mechanical strength is poor.

  

  
• As is apparent from Table 10, by setting the thermal treatment temperatures T₁, and T₂ in the primary and secondary thermal treatments and the extrusion temperature T₄ in the above-described range, the internal stress of the aluminum alloy blanks can be removed, and a large difference in average grain size can be allowed to appear between the aluminum and IMC crystals, thereby enabling the production of the structural members (43), (44); (47), (48); (51), (52); (55), (56); (59) and (60) each having an excellent mechanical strength.
• It should be noted that a powder forging process, a hot press, a hot isostatic press (HIP) or the like may be applied as the shaping and solidification in Examples 1 to 7.
• The powdery aluminum alloy blank having the amorphous phase has been used in each of Examples described above, and such a powdery aluminum alloy blank contains hydrogen gas therein due to the producing process. When the structural member is produced from the powdery aluminum alloy blank through a compaction and a hot extrusion, this hydrogen gas is left in the structural member to cause a reduction in strength. For this reason, the hydrogen gas should be removed. Such removal of the hydrogen gas is generally performed by heating the powdery aluminum alloy blank. However, if the degassing temperature is too high, the amount of hydrogen gas left in the aluminum alloy blank is reduced, on the one hand, and the fine structure is destructed due to the quenching and solidification of the aluminum alloy blank, resulting in a failure to produce a structural member having a high strength by effective utilization of such fine structure.
• According to the invention, the primary and secondary thermal treatments are performed when a structural member is to be produced, and therefore, these two-stage treatments can be also used as primary and secondary degassing treatments.
• More specifically, if the temperature T₁ is set at a relatively low level such as a value in a range of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  in the primary degassing treatment, the gas removal is slowly conducted, but it is possible to provide a fine and stable crystal texture from the fine structure produced by the quenching and solidification. The temperature T₂ is set at a relatively high level such as a value in a range of T₂ > Tx + 100 K in the secondary degassing treatment and therefore, the gas removal is conducted with a good efficiency. In this case, the crystal texture of the aluminum alloy blank is stable and hence, the diffusion of atoms at a high temperature is suppressed to prevent the coalescence of the texture.
• An embodiment of the gas removal will be described below.
Example 8
• A molten metal having a composition represented by Al₉₀Fe₇Y₃ (each numerical value represents an atomic %) was prepared and then used to produce a powdery aluminum alloy blank under a condition of a helium gas pressure of 100 kgf/cm² by use of a ultrasonic gas atomization apparatus. Then, the powdery aluminum alloy blank was subjected to a classifying treatment to control the grain size to at most 22 µm.
• The aluminum alloy blank was subjected to X-ray diffraction and thermal analysis using a DSC to examine metallographic structure thereof. The result showed that the metallographic structure was a mixed-phase texture consisting of a crystalline phase and an amorphous phase, and the crystallization temperature Tx of the amorphous phase was 653 K.
• Then, using the aluminum alloy blank, various structural members were produced through steps which will be described below. These steps included a step of subjecting the aluminum alloy blank to a cold isostatic press (CIP) under a condition of 4,000 kgf/cm² to produce a green compact, a step of subjecting the green compact to a degassing process, and a step of subjecting the green compact to a hot extrusion under a condition of an extrusion temperature T₄ equal to 673 K. In this case, the green compact is an aggregate of grains of the powdery aluminum alloy blank and hence, the degassing process was applied to the aluminum alloy blank in the form of a powder.
• The degassing process was carried out at two stages, i.e., a primary degassing treatment (a primary thermal treatment) and a subsequent secondary degassing treatment (a secondary thermal treatment) in vacuum of 10 ⁻³ Torrs or less.
• Table 11 shows the conditions for the degassing process and the characteristics of each of the structural members. TABLE 11

  S.M.	Gas-removing process				Structural member
  	Primary		Secondary		Tensile strength (MPa)	Elong. (%)	Amount of hydrogen left (ppm)
  	Tem. T₁ (K)	Time (hr)	Tem. T₂ (K)	Time (hr)
  (61)	823	1	-	-	571	8.6	2
  (62)	723	1	823	1	648	8.1	2
  (63)	673	1	823	1	650	8.0	2
  (64)	748	1	873	1	597	9.2	1
  S.M. = Structural member Tem. = Temperature Elong. = Elongation

• In Table 11, the structural members (62) to (64) each have a high strength, because the temperature T₁ in the primary degassing treatment satisfies the condition of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  , and the temperature T₂ in the secondary degassing treatment satisfies the condition of $\text{T₂ > Tx + 100 K}$

  

  . The structural member (61) has a low strength, because the temperature T₁ in the degassing treatment is too high, causing the fine structure to be destroyed due to the quenching and solidification.
• Then, using the aluminum alloy blanks, various structural members were produced with the degassing treatment time being varied, with a difference Δ T between the primary and secondary degassing treatment temperatures T₁ and T₂ (i.e., T₂ - T₁) being set at 50 K and 150 K and with other conditions being set at the same as those described above.
• Table 12 shows the degassing treatment conditions and the characteristics of each of the structural members produced with the temperature difference Δ T set at 50 K, and Table 13 shows the degassing treatment conditions and the characteristics of each of the structural members produced with the temperature difference Δ T set at 150 K. TABLE 12

  S.M.	Gas-removing process				Characteristics of S.M.
  	Primary		Secondary		Tensile strength (MPa)	Elong. (%)	Amount of hydrogen left (ppm)
  	Tem. T₁ (K)	Time (hr)	Tem. T₂ (K)	Time (hr)
  (65)	723	0.1	773	1	661	8.4	4
  (66)	723	0.2	773	1	658	8.1	4
  (67)	723	0.3	773	1	721	8.6	4
  (68)	723	0.5	773	1	720	8.3	4
  (69)	723	0.7	773	1	722	8.2	4
  (70)	723	1	773	0.5	724	8.3	4
  (71)	723	1	773	2	718	8.1	4
  (72)	723	1	773	3	716	8.0	4
  (73)	723	1	773	4	695	8.5	4
  (74)	723	1	773	5	660	8.3	4

  TABLE 13

  S.M.	Gas-removing process				Characteristics of S.M.
  	Primary		Secondary		Tensile strength (MPa)	Elong. (%)	Amount of hydrogen left (ppm)
  	Tem. T₁ (K)	Time (hr)	Tem. T₂ (K)	Time (hr)
  (75)	623	0.1	773	1	656	8.4	4
  (76)	623	0.2	773	1	658	8.1	4
  (77)	623	0.3	773	1	716	8.4	4
  (78)	623	0.5	773	1	721	8.6	4
  (79)	623	0.7	773	1	723	8.0	4
  (80)	623	1	773	0.5	720	8.1	4
  (81)	623	1	773	1.5	721	8.5	4
  (82)	623	1	773	2.5	713	8.3	4
  (83)	623	1	773	3.5	692	8.4	4
  (84)	623	1	773	4.5	662	8.2	4
  S.M. = Structural member Tem. = Temperature Elong. = Elongation

• In Tables 12 and 13, the structural members (67) to (73) and (77) to (83) each have a high strength. From this, it can be seen that the time t₁ in the primary degassing treatment may be set at a value more than 0.2 hours (t₁ > 0.2 hr).
• If the time t₁ in the primary degassing treatment is equal to or less than 0.2 hours (t₁ ≦ 0.2 hr), the metallographic structure of the resulting structural member is not stabilized, and the left solid solution atoms that are left are diffused in the secondary degassing treatment, resulting in a structural member having a reduced strength, as the structural members (65), (66), (75) and (76).
• On the other hand, if the time t₂ in the secondary degassing treatment is more than 4 hours (t₂ > 4 hr), the diffusion of atoms which has been suppressed heretofore occurs, resulting in a structural member having a reduced strength, as the structural members (74) and (84).
• Fig. 16 shows the relationship between the time in the secondary degassing treatment and the strength of each of the structural members. Characters (70) to (74) and (80) to (84) in Fig. 16 correspond to the shaped products (70) to (74) and (80) to (84).
• As is apparent from Tables 12 and 13 as well as Fig. 16, it is preferable that the time t₂ in the secondary degassing treatment is set at a value equal to or less than 4 hours (t₂ ≦ 4 hr). In addition, it is preferable that the lower limit value of the time t₂ is set at 0.5 hr (t₂ = 0.5 hr).
• It is noted here that the invention is not applied for a degassing process for an aluminum alloy blank such as those which exhibit an exothermic of 10 J/g in the differential thermal analysis after the primary degassing treatment.
Example 9
• A molten metal having a composition represented by Al₈₉Ni₈Mm₃ (each numerical value represents an atomic %) was prepared and then used to produce a powdery aluminum alloy blank in the same manner as in Example 8. Then, the powdery aluminum alloy blank was subjected to a classifying treatment similar to that in Example 8.
• The aluminum alloy blank was subjected to X-ray diffraction and a thermal analysis using a differential scanning calorimeter (DSC) to examine the metallographic structure. The result showed that the metallographic structure was a mixed-phase texture consisting a crystalline phase and an amorphous phase, and the crystallization temperature Tx of the amorphous phase was 620 K.
• Then, using the aluminum alloy blank, various structural members were produced in the same manner as in Example 8.
• Table 14 shows the conditions for the degassing treatment and the characteristics of each of the structural members. TABLE 14

  S.M.	Gas-removing treatment				Characteristics of S.M.
  	Primary		Secondary		Tensile strength (MPa)	Elong. (%)	Amount of hydrogen left (ppm)
  	Tem. T₁ (K)	Time (hr)	Tem. T₂ (K)	Time (hr)
  (85)	723	1	-	-	676	7.2	7
  (86)	698	1	733	1	672	7.6	7
  (87)	673	1	733	1	740	7.5	7
  (88)	673	0.2	733	1	665	8.0	7
  (89)	673	0.3	733	1	741	7.6	7
  (90)	623	1	733	1	742	7.0	7
  (91)	598	1	733	1	747	7.3	7
  (92)	673	1	733	3	740	7.4	7
  (93)	673	1	733	4	735	7.7	7
  (94)	673	1	733	5	650	7.9	7
  (95)	598	1	733	4	741	7.6	7
  (96)	598	1	733	5	660	7.9	7
  (97)	598	0.2	733	2	666	8.3	7
  (98)	598	0.3	733	2	744	7.6	7
  S.M. = Structural member Tem. = Temperature Elong. = Elongation

• In Table 14, the structural members (87), (89) to (93), (95) and (98) each have a high strength, because the temperature T₁ in the primary degassing treatment satisfies the condition of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

  

  and the time t1 satisfies the condition of t₁ > 0.2 hr, as well as the temperature T₂ in the secondary degassing treatment satisfies the condition of T₂ > Tx + 100 K and the time t₂ satisfies the condition of 0.5 hr ≦ t₂ ≦ 4 hr.
• The shaped product (85) has a low strength, as the structural member (61) in Example 8. In the structural member (86), the difference between the temperature T₁ in the primary degassing treatment and the temperature T₂ in the secondary degassing treatment is too small, and in the structural members (88) and (97), the time t₁ in the primary degassing treatment is too short. In the structural members (88) and (97), the time t₂ in the secondary degassing treatment is too long.

Claims (8)

1. A high strength and high toughness aluminum alloy structural member, produced by crystallization of an aluminum alloy blank having a metallographic structure including an amorphous phase, wherein said structural member comprises an aluminum crystal matrix and intermetallic compound crystals dispersed in the aluminum crystal matrix, and if an average grain size of aluminum crystals is represented by d₁ and a grain size of the intermetallic compound crystals is represented by d₂ an average percentage P₁ of the total number of small intermetallic compound crystals having the grain size d₂ equal to or less than d₁/2 with respect to 100 or more intermetallic compound crystals is equal to or greater than 80 % in a plane of microscopic examination.
2. A high strength and high toughness aluminum alloy structural member according to claim 1, wherein said small intermetallic compound crystals include minute intermetallic compound crystals having the grain size d₂ equal to or less than 0.01 µm, an average percentage P₂ of the total number of said minute intermetallic compound crystals with respect to 100 or more intermetallic compound crystals is equal to or greater than 10 % in the plane of microscopic examination.
3. A process for producing a high strength and high toughness aluminum alloy structural member, comprising the steps of:
      subjecting an aluminum alloy blank having a metallographic structure including an amorphous phase of with crystallization temperature Tx to a primary thermal treatment under a condition of a thermal treatment temperature T₁ in a range of $\text{Tx - 100 K ≦ T₁ ≦ Tx + 100 K}$

   

   , thereby crystallizing the amorphous phase and decomposing the phase,
      subjecting the aluminum alloy blank to a secondary thermal treatment under a condition of a thermal treatment temperature T₂ higher than Tx + 100 K, and
      subjecting the resulting aluminum alloy blank to a shaping and solidification under a processing temperature T₄ equal to or lower than the temperature T₂.
4. A process for producing a high strength and high toughness aluminum alloy structural member according to claim 3, wherein said primary thermal treatment is applied to the aluminum alloy blank in the form of powder.
5. A process for producing a high strength and high toughness aluminum alloy structural member, comprising the steps of:
      subjecting an aluminum alloy blank having a metallographic structure including an amorphous phase with a crystallization temperature Tx to a primary thermal treatment under a condition of a thermal treatment temperature T₁ in a range of $\text{Tx - 100 K≦ T₁ ≦ Tx + 100 K}$

   

   , thereby crystallizing said amorphous phase and decomposing the phase;
      subjecting said aluminum alloy blank to a secondary thermal treatment under a condition of a thermal treatment temperature T₂ in a range of $\text{T₁ ≦ T₂ ≦ Tx + 100 K}$

   

   ;
      isothermally maintaining said aluminum alloy blank in a heating atmosphere at a temperature T₃ equal to or higher than Tx + 160 K; and
      subjecting said aluminum alloy blank to a shaping and solidification under a condition of a processing temperature T₄ equal to or lower than the temperature T₃.
6. A process for producing a high strength and high toughness aluminum alloy structural member, comprising the steps of:
      subjecting an aluminum alloy blank having a metallographic structure including an amorphous phase with a crystallization temperature Tx to a primary thermal treatment under a condition of a thermal treatment temperature T₁ in a range of $\text{Tx - 100 K≦ T₁ ≦ Tx + 100 K}$

   

   , thereby crystallizing said amorphous phase and decomposing the phase;
      subjecting said aluminum alloy blank to a secondary thermal treatment under a condition of a thermal treatment temperature T₂ higher than Tx + 100 K;
      isothermally maintaining said aluminum alloy blank in a heating atmosphere at a temperature T₃ equal to or higher than Tx + 160 K; and
      subjecting said aluminum alloy blank to a shaping and solidification under a condition of a processing temperature T₄ equal to or lower than T₂.
7. A process for producing a high strength and high toughness aluminum alloy structural member according to claim 3, 4, 5 or 6, wherein said aluminum alloy blank is in the form of powder, and said primary and secondary thermal treatments also serve as primary and secondary degassing treatments for said aluminum alloy blank, respectively.
8. A process for producing a high strength and high toughness aluminum alloy structural member according to claim 7, wherein a difference ΔT in temperature between said primary and secondary degassing treatments is set in a range of 50 K≦ ΔT ≦ 150 K; time t₁ of the primary degassing treatment is set at a value more than 0.2 hours; and time t₂ of the secondary degassing treatment at a value equal to or more than 0.5 hours.

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