EP2669028A1

EP2669028A1 - Crystal grain refining agent for casting and method for producing the same

Info

Publication number: EP2669028A1
Application number: EP12738824.7A
Authority: EP
Inventors: Yoshimi Watanabe; Hisashi Sato; Takahiro Matsuoka
Original assignee: Nagoya Institute of Technology NUC
Current assignee: Nagoya Institute of Technology NUC
Priority date: 2011-01-25
Filing date: 2012-01-19
Publication date: 2013-12-04
Anticipated expiration: 2032-01-19
Also published as: EP2669028B1; WO2012102162A1; EP2669028A4; JP5850372B2; JPWO2012102162A1

Abstract

[Object] To provide a grain refiner for casting; and a method for producing the same. The grain refiner contains heterogeneous nucleator particles having a smaller degree of mismatching δ with respect to pure aluminum or an aluminum alloy than those of customary grain refiners.

[Solution] The grain refiner is produced by rapid and low-temperature sintering through spark plasma sintering. The grain refiner contains heterogeneous nucleator particles that have a small degree of mismatching and originally fail to exist in equilibrium with aluminum. Typically spark plasma sintering gives a grain refiner for casting which includes an aluminum matrix; and particles of an L1₂ structure Al₅CuTi₂ intermetallic compound dispersed in the matrix. The grain refiner, when added to a molten metal, gives a pure-aluminum or aluminum-alloy cast material having a refined and uniformized microstructure.

Description

Technical Field

The present invention relates to a grain refiner for casting and a production method thereof.

Background Art

Melt processing is one of representative processes for metals and alloys. The melt processing advantageously gives complicated, smooth-shaped products and is applicable to any hard-to-work metals and alloys. Solidification is a most important phase change in the melt processing. The solidification is associated with nucleation which includes homogeneous nucleation and heterogeneous nucleation. Heterogeneous nuclei, when added, refine the solidified microstructure, i.e., cast microstructure. The material structure refinement is one of material strengthening techniques and is known as a technique for strengthening a material without impairing its ductility/toughness. There is a Hall-Petch relation between a yield stress σ_y and a grain size d of a polycrystal, where the relation is specified by Mathematical Expression (1) as follows:
[Math. 1] $σ_{y} = σ_{0} + k d^{- 1 / 2}$

where σ₀ is a friction stress for dislocation movement; and k is a so-called Hall-Petch coefficient; and both σ₀ and k are material specific. To act as an effective heterogeneous nucleator in solidification, the heterogeneous nucleator should have a small interfacial energy with respect to the cast material. Non Patent Literature (NPL) 1 mentions that how a heterogeneous nucleator is effective can be discussed or determined by a degree of mismatching δ between the heterogeneous nucleator and a cast material. The degree of mismatching δ is defined in crystal lattice in a low-index plane of an atomic arrangement toward one direction. The degree of mismatching δ is specified by Mathematical Expression (2) expressed as follows.
[Math. 2] $δ = (|a - a_{0}| / a_{0}) \times 100 (%)$

where a is the lattice constant of a low-index plane of the heterogeneous nucleator; and a₀ is the lattice constant of a low-index plane of the cast material. With a decreasing δ, the atomic arrangement more satisfactorily matches and the interfacial energy decreases. A heterogeneous nucleator having a degree of mismatching δ of 10% or less effectively acts as heterogeneous nuclei.
Al₃Ti, TiB₂, and Al₃Zr are known as substances to possibly serve as heterogeneous nucleators in aluminum grains. Al-Ti alloys and Al-Ti-X (X=B, C) alloys are employed as grain refiners in practical production of cast aluminum (e.g., Patent Literature (PTL) 1 and 2). The intermetallic compounds Al₃Ti and Al₃Zr have D0₂₂ and D0₂₃ structures, respectively.
Of these substances, Al₃Ti having the D0₂₂ structure is highly valued as a grain refiner. With reference to FIGS. 1A and 1B, pure aluminum and Al₃Ti having the D0₂₂ structure have lattice constants of 0.40496 nm and 0.384 nm, respectively, with a degree of mismatching δ between them of about 5%.
The grain refining performance of a grain refiner depends on the number of heterogeneous nuclei in the grain refiner. PTL 1 indicates that the number of heterogeneous nuclei can be controlled by applying severe plastic deformation to a grain refiner to allow the grain refiner to exhibit better grain refining performance.
D0₂₂-structure intermetallic compounds have poor crystal symmetry, as illustrated in FIG. 1B. For better crystal symmetry, investigations have been intensively made to allow them to have a satisfactorily symmetric L1₂ structure (FIG. 1C) by adding an additional element thereto (e.g., NPL 2, 3 and 4). Ll₂-structure intermetallic compounds having various lattice constants have been found based on the investigations. Unfortunately these Ll₂-structureintermetallic compounds have not yet been in practical use. This is because they suffer from pores formed upon solidification and exhibit insufficient tensile ductility. The Ll₂-structure intermetallic compounds, if to be used in bulk, fail to exhibit strengths at expected level.

Citation List

Patent Literature

PTL 1: Japanese Patent Application Publication No. JP-A-2005-329459
PTL 2: Japanese Patent Application Publication No. JP-A-H10-317083

Non Patent Literature

NPL 1: Turnbull and Vonnegut, Ind. Eng. Chem., 1952.
NPL 2: Metal. Trans. A, Vol. 23A, 1992, 2963.
NPL 3: Mater. Sci. Eng. A, Vol. A192/193, 1995, 92.
NPL 4: M. Yamaguchi, Kinzoku (in Japanese; "Metals"): Materials science & technology, Vol. 62, No. 10, 1992, 2.

Summary of Invention

Technical Problem

The D0₂₂-structure Al₃Ti has a degree of mismatching δ of about 5% as described above. A compound having a degree of mismatching δ of equal to or less than this level can serve as a refiner (refining additive) having higher performance than that of Al₃Ti.
Aluminum alloys have lattice constants different from that of pure aluminum, and there should be optimal refining additives for respective aluminum alloys.
The D0₂₂-structure Al₃Ti has lattice constants differing from a crystal face to another, i.e., has lattice constants a of 0.384 nm and c of 0.8596 nm as illustrated in FIG. 1B. This means that the D0₂₂-structure Al₃Ti has degrees of mismatching δ differing from a crystal face to another and has functions as a heterogeneous nucleus differing from a crystal face to another.
Exemplary process variables that affect the grain refinement of cast materials include the molten metal temperature, amount of the grain refiner, as well as holding time (retention time) after the addition of the grain refiner to a molten metal. An Al-5 percent by mass Ti-X (X=B, C) alloy is currently employed as a grain refiner. This alloy is added to the molten metal in an amount of 1 percent by mass or less to give a molten metal containing titanium in a content of 0.05 percent by mass or less. At this content, Al₃Ti heterogeneous nuclei fail to exist in equilibrium in the molten metal (see FIG. 2). Heterogeneous nuclei in nonequilibrium, however, are utilized in the practical production of cast aluminum by regulating the holding time. After further consideration, this indicates that a grain refiner itself does not have to be a system in equilibrium. Namely, even an intermetallic compound that originally fails to exist stably in equilibrium should be usable as heterogeneous nuclei.
Under these circumstances, an object of the present invention is to provide a refining additive (refiner) including heterogeneous nuclei which have a lower degree of mismatching δ than those of heterogeneous nuclei of existing grain refiners and which have a minimized degree of mismatching δ with respect to pure aluminum or aluminum alloys. Another object of the present invention is to provide a method for producing the refining additive.

Solution to Problem

The present invention employs an Ll₂-structure intermetallic compound from a viewpoint entirely different from that of the investigations of Ll₂-structure intermetallic compounds.
Specifically, a 1st aspect of the present invention provides a grain refiner for casting which is a bulk solid and includes a matrix mainly comprising aluminum; and particles of an intermetallic compound dispersed in the matrix, the intermetallic compound having an L1₂ structure and represented by Formula (1) expressed as follows: ${(Al, Y)}_{3} Z$

where Y is one element selected from the group consisting of Cu, Fe, Ni, Zn, Pd, Cr, Mn, Co, Ag, Rh, Pt, Au, and Hf; and Z is one element selected from the group consisting of Ti, Zr, and Zn (claim 1).
In the grain refiner for casting, Z in Formula (1) may be Ti (claim 2). The intermetallic compound may be Al₅CuTi₂, Al₂₂Fe₃Ti₈, or Al₆₇Ni₈Ti₂₅ (claim 3, 4, or 5).
An intermetallic compound can have a varying lattice constant by replacing part of aluminum in the D0₂₂-structure Al₃Ti or replacing Ti in the D0₂₂-structure Al₃Ti each with another element, as indicated in the L1₂ structure of Formula (1). This allows the intermetallic compound to have a lattice constant nearer to that of pure aluminum than the D0₂₂-structure Al₃Ti does. Appropriate selection of the other element to be replaced allows the intermetallic compound to have a lattice constant near to that of a specific aluminum alloy.
This can provide a refining additive including heterogeneous nuclei which have a smaller degree of mismatching δ than those of heterogeneous nuclei in the existing grain refiners and have a minimized degree of mismatching δ with respect to pure aluminum or an aluminum alloy.
Typically, D0₂₂-structure Al₃Ti, when combined with a third element Y, gives L1₂-structure(Al,Y)₃Ti intermetallic compounds as illustrated in FIG. 1(c). The intermetallic compounds have lattice constants a and degrees of mismatching δ with respect to pure aluminum as follows. The intermetallic compounds have lattice constants nearer to that of pure aluminum than the D0₂₂-structure Al₃Ti does. This demonstrates that the intermetallic compounds serve as heterogeneous nuclei having degrees of mismatching δ smaller than that of the D0₂₂-structure Al₃Ti.

Y=Cu: Al₅CuTi₂, a=0.3927 nm, δ=3.0
Y=Fe: Al₂₂Fe₃Ti₈, a=0.393 nm, δ=3.0
Y=Ni: Al₆₇Ni₈Ti₂₅, a=0.394 nm, δ=2.7
Y=Zn: Al₆₆Zn₉Ti₂₅, a=0.396 nm, δ=2.2

₂

FIG. 1(c)

The grain refiner for casting can be produced through spark plasma sintering. The grain refiner for casting may be not a dense sintered compact but a semi-sintered compact.

Brief Description of Drawings

[FIG. 1A]
FIG. 1A illustrates the crystal structure and lattice constant of fcc-structure Al.
[FIG. 1B]
FIG. 1B illustrates the crystal structure and lattice constant of D0₂₂-structure Al₃Ti.
[FIG. 1 C]
FIG. 1C illustrates the crystal structure and lattice constant of an Ll₂-structure (Al,Y)₃Ti.
[FIG. 2]
FIG. 2 is an Al-Ti binary equilibrium diagram, illustrating an area where Ti is present in a content of 0 to 30 percent by weight.
[FIG. 3]
FIG. 3 is a chart illustrating X-ray diffractometric data of an Al₅CuTi₂ sample obtained in Example 1, which sample had been subjected to homogenization.
[FIG. 4]
FIG. 4 is a chart illustrating X-ray diffractometric data of a bulk grain refiner obtained in Example 1.
[FIG. 5A]
FIG. 5A depicts a scanning electron photomicrograph of the bulk grain refiner obtained in Example 1.
[FIG. 5B]
FIG. 5B is a schematic view of FIG. 5A.
[FIG. 6A]
FIG. 6A depicts a scanning electron photomicrograph of a cross section of an aluminum cast material obtained in Comparative Example 1.
[FIG. 6B]
FIG. 6B is a schematic view of Area A1 in FIG. 6A.
[FIG. 7A]
FIG. 7A depicts a scanning electron photomicrograph of a cross section of an aluminum cast material obtained in Example 1.
[FIG. 7B]
FIG. 7B is a schematic view of Area A2 in FIG. 7A.
[FIG. 8]
FIG. 8 is a graph illustrating average grain sizes of aluminum cast materials obtained in Examples 1 to 12.
[FIG. 9A]
FIG. 9A depicts a scanning electron photomicrograph of a cross section of the aluminum cast material obtained in Example 9.
[FIG. 9B]
FIG. 9B is a schematic view of Area A3 in FIG. 9A.

Description of Embodiments

The present invention provides, in an embodiment, a grain refiner including a heterogeneous nucleator with a small degree of mismatching (i.e. disregistry). The heterogeneous nucleator may have a degree of mismatching of 5 or less and preferably 4 or less.
This grain refiner is a bulk solid (solid mass) and includes a matrix and particles of an intermetallic compound dispersed in the matrix, in which the matrix mainly includes Al, and the intermetallic compound serves as a heterogeneous nucleator, has an L1₂-structure, and is expressed by Formula (1).
The intermetallic compound originally fails to exist stably in equilibrium with Al. The intermetallic compound is exemplified by L1₂ structure (Al,Y)₃Ti and L1₂ structure (Al,Y)₃Z, which are in turn typified by Al₅CuZr₂: a=0.404 nm, Al₂HfZn: a=0.4033 nm, Al₅NiZr₂: a=0.406 nm, as well as intermetallic compounds given in Table 1. These intermetallic compounds have lattice constants near to that (0.40496 nm) of pure aluminum.

[Table 1]

Intermetallic compound	Lattice constant a	Degree of mismatching δ
Al₅CuTi₂	0.3927	3.0
Al₂₂Fe₃Ti₈	0.393	3.0
Al₆₇Fe₈Ti₂₅	0.394	2.7
Al₆₇Ni₈Ti₂₅	0.394	2.7
Al₆₇Zn₈Ti₂₅	0.392	3.2
Al₆₆Zn₉Ti₂₅	0.396	2.2
Al₆₇Pd₈Ti₂₅	0.393 or 0.3950	3.0 or 2.5
Al₆₇Cr₈Ti₂₅	0.396	2.2
Al₆₆Mn₉Ti₂₅	0.3955	2.3
Al₆₇Co₈Ti₂₅	0.395	2.5
Al₆₇Ag₈Ti₂₅	0.399	1.5
Al₆₇Rh₆Ti₂₇	0.3965	2.1
Al₆₇Pt₆Ti₂₇	0.3965	2.1
Al₆₆Au₉Ti₂₅	0.3975	1.8
Al₅CuZr₂	0.404	0.2

The intermetallic compound for use herein is preferably one that originally fails to exist stably in equilibrium with Al, but may be one that can stably exist in equilibrium with Al, as long as possibly becoming heterogeneous nuclei with a smaller degree of mismatching δ than that of D0₂₂-structure Al₃Ti.

As used herein the term "matrix mainly including Al" refers to a matrix including Al in a largest amount as a component and containing pure aluminum or an aluminum alloy. To suppress compositional variation of the cast material, the matrix preferably has the same composition with that of the cast material. Specifically, to produce a pure aluminum cast material, pure aluminum is preferably used as the matrix. To produce an aluminum-alloy cast material, an aluminum alloy having the same composition with that of the target cast material is preferably used as the matrix.
A powdery L1₂-structure intermetallic compound represented by Formula (1), if directly added as particles to the molten metal, is not mixed with the molten metal and floats thereon because of its poor wettability. To prevent this, the present invention employs a grain refiner which has a solid structure (bulk or mass) and includes a matrix; and particles of an L1₂-structure intermetallic compound represented by Formula (1) dispersed in the matrix. This allows the Ll₂-structure intermetallic compound represented by Formula (1) in the grain refiner to disperse in the molten metal and to effectively serve as heterogeneous nuclei.
The particles of the intermetallic compound, if contained in an excessively large volume fraction based on the total volume of the grain refiner, may fail to satisfactorily disperse in the molten metal; whereas if contained in an excessively small volume fraction, may require a large amount of the grain refiner to be added, thus being industrially undesirable. To prevent these, the particles of the intermetallic compound may be present in a volume fraction of 5% to 40% of the entire grain refiner.
An intermetallic compound represented by Formula (1), such as L1₂ structure (Al,Y)₃Ti, cannot exist in equilibrium with Al. To produce a bulk including aluminum and particles of this intermetallic compound dispersed in the matrix aluminum, sintering should be performed at a low temperature in a short time such that the intermetallic compound does not decompose.
In view of this requirement, spark plasma sintering (SPS) enables rapid and low-temperature sintering and can give a bulk even in a nonequilibrium system. A grain refiner for casting can be produced by mixing a powder of the intermetallic compound with a powder of the matrix to give a powder mixture; compacting the powder mixture to give a powder compact; and sintering the powder compact through SPS.
A material powder compact, when fired at a low temperature in a short time through another sintering technique than SPS, generally gives a sintered compact having low mechanical strengths. Such semi-sintered compact, however, can serve as a grain refiner, because the mechanical strengths of the (semi-)sintered compact do not affect properties of the cast material. The grain refiner for casting may also be produced through not SPS but another sintering technique such as hot pressing, or atmospheric sintering after hot or cold isostatic compaction. As used herein the term "semi-sintered compact" refers to a compact having a packing density of 70% to 90%. The "packing density" is determined by taking an image of the microstructure under an optical microscope, analyzing the image to measure an area fraction of pores; and subtracting the area fraction from 100%. The term "hot isostatic compaction" refers to a technique in which a work is isostatically compressed using a high-temperature high-pressure gas as a medium to densify the work. The term "cold isostatic compaction" refers to a technique in which a powder is filled in a rubber mold, and hydrostatic pressure is applied thereto to compact the powder.
The grain refiner for casting is added to a molten pure aluminum or aluminum alloy, the resulting molten metal is poured into a mold and yields a pure-aluminum or aluminum-alloy cast material. This allows the pure-aluminum or aluminum-alloy cast material to have a refined and uniformized microstructure. The grain refinement of the cast material can be optimized by regulating the holding time in this process, as demonstrated by Examples 1 to 10 mentioned below.

[Examples]

(Example 1)

Example 1 employed Al₅CuTi₂ as a sample to be prepared. Al₅CuTi₂ has a relatively wide compositional range in the equilibrium diagram among Ll₂-structure intermetallic compounds. This compound is illustrated merely by way of example and is not intended to limit heterogeneous nucleators to be used. A bulk Al-40 percent by mass Cu alloy, powdery pure aluminum, and powdery pure titanium were used as materials to prepare the sample, but they are not intended to limit starting materials to be used. These materials were subjected to arc melting in an argon atmosphere and yielded a bulk sample. To give a uniform sample, melting in the arc melting was performed at least seven times after the respective materials were melted and mixed with each other.
The as-melted sample was cut into a rectangular solid. The cut sample was placed on an aluminum plate, arranged at the center of a soaking zone in an infrared gold image furnace, and homogenized at 1100°C in a vacuum for one hour.
The crystal structure and microstructure of the prepared Al₅CuTi₂ were evaluated. The observation revealed that the as-melted sample had a second phase, but the sample after homogenization had no second phase. This indicates that the homogenization allows atoms to diffuse sufficiently. Part of the sample was cut out, pulverized with a hammer, and the crystal structure thereof was evaluated through X-ray diffractometry. FIG. 3 illustrates X-ray diffractometric data of the homogenized sample. The sample had an Al₅CuTi₂ peak pattern, as illustrated in FIG. 3. The sample was found to have a lattice constant a of 0.3917 nm as calculated from the result, which approximates the literature value.
To allow the prepared L1₂-structure intermetallic compound Al₅CuTi₂ to act as heterogeneous nuclei in an aluminum molten metal, the compound should be pulverized into a powder so as to have smaller particle sizes. However, the powdery Al₅CuTi₂, if directly added to the aluminum molten metal, may highly possibly fail to disperse therein because the powder is not mixed with the molten metal but floats thereon due to its poor wettability. To prevent this, Al₅CuTi₂ particles were dispersed in an aluminum matrix through spark plasma sintering to give a grain refiner in the following manner.
Initially, the prepared bulk Al₅CuTi₂ was pulverized into particles, sieved using sieves of 150 µm and 75 µm openings, and yielded a powder having particle sizes of 75 µm to 150 µm. The prepared Al₅CuTi₂ powder was mixed with a powdery pure aluminum to give a powder mixture having a volume fraction of the Al₅CuTi₂ powder of 10%, and the powder mixture was compacted to give a powder compact. The powder compact was sintered using a compact spark plasma sintering system (DR. SINTER Series, SPS-515S, Sumitomo Coal Mining Co., Ltd.) and yielded a bulk refiner. This process was performed at a compaction pressure of 45 MPa, a rate of temperature rise of 100°C per minute, a sintering temperature of 500°C, for a holding time of 5 minutes.
The prepared bulk grain refiner was subjected to X-ray diffractometry, the data of which are indicated in FIG. 4. The sample had an intense Al peak pattern with a volume fraction of 90% and also had a clear peak pattern of Al₅CuTi₂. The data demonstrate that Al₅CuTi₂ remained as unreacted in the sample.
The sample was cut, chemically polished with emery paper of #100 to #4000, and observed under a SEM. The results are indicated in FIGS. 5A and 5B. FIG. 5B is a schematic view of FIG. 5A. These figures demonstrate that the sample included powdered Al₅CuTi₂ particles as remained. The sample had clear grain boundaries, indicating that the Al₅CuTi₂ particles did not react with the aluminum matrix.
The rapid and low-temperature sintering allowed the L1₂-structure intermetallic compound Al₅CuTi₂ to remain without reaction in the sample. The remained L1₂-structureintermetallic compound will serve as heterogeneous nuclei in an aluminum cast material. This enables production of a grain refiner including heterogeneous nuclei of the L1₂-structure intermetallic compound Al₅CuTi₂.
The produced grain refiner was used in a casting experiment. Initially, 148.8 g of a pure aluminum ingot was melted at 750°C in a crucible and combined with 1.2 g (0.8 percent by mass) of the refiner. The amount of the refiner in this experiment was determined so that the resulting cast material had a sufficiently low Ti content as compared to the peritectic composition (0.12 percent by mass) in an Al-Ti binary system. The molten metal immediately after the grain refiner addition was stirred for 30 seconds without subsequent holding (held for a holding time of 0 second).
An experiment as Comparative Example 1 was performed by the above procedure to give an aluminum cast material by melting 148.8 g of a pure aluminum ingot at 750°C in a crucible, except for adding 1.2 g of pure aluminum to the molten metal. The aluminum cast material was cut at a position 5 mm high from the bottom, and a top face of which was defined as a face to be observed. The observation face was chemically polished with emery paper of #80 to #4000, buffed with 1-µm alumina, and etched with a 10% hydrofluoric acid for 90 seconds.
FIGS. 6A and 7A depict photomicrographs of sample cross sections of Comparative Example 1 using no grain refiner and of Example 1 using the grain refiner, respectively. FIGS. 6B and 7B depict schematic views of Areas A1 an A2 in FIGS. 6A and 7A, respectively. The sample using no grain refiner included a regular solidified microstructure containing equiaxial grains and columnar grains as observed. In contrast, the sample using the refiner had a microstructure that is approximately uniform and refined as a whole, while partially having columnar grains.
That sample included approximately equiaxed grains even in a region including columnar grains. The samples were subjected to average grain size measurement by the mean linear intercept technique. The sample using no grain refiner had an average grain size of 1353 µm, whereas the sample using the grain refiner had an average grain size refined to 851 µm.

(Examples 2 to 10)

In Examples 2 to 5, casting experiments were performed under the conditions as in Example 1 using the grain refiner prepared in Example 1, except for holding the mixture for holding times of 120, 210, 300, and 600 seconds, respectively, after stirring the mixture for 30 seconds immediately after the grain refiner addition.
In Examples 6 to 10, a grain refiner was prepared by the procedure of Example 1, except for using the Al₅CuTi₂ powder in a volume fraction of 20%. Using the prepared grain refiner, casting experiments were performed under the conditions as in Example 1, except for adding the refiner in an amount of 0.4 percent by mass and holding the resulting mixture for holding times of 0, 210, 300, 480, and 600 seconds, respectively, after stirring the mixture for 30 seconds immediately after the grain refiner addition.
FIG. 8 depicts average grain sizes of aluminum cast materials obtained in Examples 2 to 10, as well as those of Example 1 and after-mentioned Examples 11 and 12.
The data demonstrate that even the samples of Examples 2 to 5 had refined (smaller) grains sizes, which samples were prepared at a volume fraction of the Al₅CuTi₂ powder of 10% for holding times longer than 0 second. At a volume fraction of the Al₅CuTi₂ powder of 10%, the grain size reached a minimum of 344 µm when held for a holding time of 300 seconds.
The data demonstrate that the samples of Examples 6 to 10 having a volume fraction of the Al₅CuTi₂ powder of 20% had smaller grain sizes. At a volume fraction of the Al₅CuTi₂ powder of 20%, the grain size reached a minimum of 439 µm when held for a holding time of 480 seconds (Example 9).
FIG. 9A depicts the cross section of the sample of Example 9; and FIG. 9B depicts a schematic view of Area A3 in FIG. 9A. These demonstrate that the sample of Example 9 had a microstructure being approximately uniform and refined as a whole.

(Examples 11 and 12)

Al₂₂Fe₃Ti₈ and Al₆₇Ni₈Ti₂₅ were prepared by arc melting. They were vacuum-encapsulated and homogenized in a muffle furnace at 1200°C for 24 hours and at 1100°C for 100 hours, respectively.
The bulk Al₂₂Fe₃Ti₈ and Al₆₇Ni₈Ti₂₅ were pulverized and classified to powders of 75 to 150 µm by the procedure of Example 1. These were mixed each in a volume fraction of 10% with a powdery pure aluminum (99.9%), from which refiners were prepared through SPS.
The prepared refiners were used in casting experiments under the conditions as in Example 1.
The aluminum cast materials of Examples 11 and 12 prepared using the refiner both had microstructures being approximately uniform and refined as a whole. Their average grain sizes were measured by the mean linear intercept technique. With reference to FIG. 8, the aluminum cast material of Example 11 prepared using the refiner containing heterogeneous nuclei of Al₂₂Fe₃Ti₈ had an α-aluminum grain size of 642 µm. The aluminum cast material of Example 12 prepared using the refiner containing heterogeneous nuclei of Al₆₇Ni₈Ti₂₅ had an α-aluminum grain size of 260 µm. These data demonstrate that the refiners using Al₂₂Fe₃Ti₈ and Al₆₇Ni₈Ti₂₅, respectively, exhibited satisfactory grain refining performance.
These results demonstrate that the grain refiners prepared through a series of processes according to the examples give aluminum cast materials having refined and uniformized microstructures.
Titanium is a rare metal. The technique according to the present invention, however, may possibly allow another element to be used instead of titanium in heterogeneous nuclei. This provides grain refiners without influence by world situation on demand and supply of such rare metals. The present invention allows arbitrary use of nonequilibrium heterogeneous nuclei and is applicable to all structural metal materials based on not only aluminum, but also on iron, titanium, and other metals.
The refining performance depends on the number of heterogeneous nuclei in a grain refiner. The number of heterogeneous nuclei can be controlled by applying severe plastic deformation to the grain refiner according to the technique disclosed in PTL 1. To put grain refiners to practical use, the present invention can employ this technique for the control of the number of heterogeneous nuclei.
The present invention helps all cast materials to have higher strengths. This contributes to weight reduction of transport equipment, resulting in higher fuel efficiencies. When applied to a cast die for use typically in resin foam production, the present invention contributes to wall-thickness reduction of the die. This allows heating-energy saving and suppresses carbon dioxide evolution.

Claims

A grain refiner for casting, being a bulk solid and comprising: a matrix mainly comprising aluminum; and particles of an intermetallic compound dispersed in the matrix, the intermetallic compound having an L1₂ structure and represented by Formula (1) expressed as follows: ${(Al, Y)}_{3} Z$

where Y is one element selected from the group consisting of Cu, Fe, Ni, Zn, Pd, Cr, Mn, Co, Ag, Rh, Pt, Au, and Hf; and Z is one element selected from the group consisting of Ti, Zr, and Zn.
The grain refiner for casting of claim 1, wherein Z is Ti.
The grain refiner for casting of claim 2, wherein the intermetallic compound is Al₅CuTi₂.
The grain refiner for casting of claim 2, wherein the intermetallic compound is Al₂₂Fe₃Ti₈.
The grain refiner for casting of claim 2, wherein the intermetallic compound is Al₆₇Ni₈Ti₂₅.
The grain refiner for casting of any one of claims 1 to 5, wherein the particles are contained in a volume fraction of 5% to 40% of a total volume of the grain refiner.
A method for producing the grain refiner for casting of any one of claims 1 to 6, the method comprising the steps of:
mixing a powder of the intermetallic compound with a powder of the matrix to give a powder mixture;

compacting the powder mixture to give a powder compact; and

sintering the powder compact through spark plasma sintering.
A method for producing the grain refiner for casting of any one of claims 1 to 6, the method comprising the steps of:
mixing a powder of the intermetallic compound with a powder of the matrix to give a powder mixture;

compacting the powder mixture to give a powder compact; and

sintering the powder compact to give a semi-sintered compact.
A method for producing a pure-aluminum or aluminum-alloy cast material, the method comprising the steps of:
adding the grain refiner of any one of claims 1 to 6 to molten pure aluminum or aluminum alloy to give a molten metal; and

pouring the molten metal into a mold.