US3039868A - Magnesium base alloys - Google Patents

Description

United States Patent M 3,039,868 MAGNESIUM BASE ALLOYS Ronald James M. Payne, Brecon, South Wales, and Norman Bailey, Charlton, London, England, assignors to Magnesium Elektron Limited, Manchester, England No Drawing. Filed May 11, 1959, Ser. No. 812,117 Claims priority, application Great Britain May 16, 1958 9 Claims. (Cl. 75-168) The object of the invention is to provide magnesiumbase alloys having improved propertiesin particular an improved proof stress. In both the cast and wrought forms the proof stresses of the magnesium alloys used up to the present time have not been as high as is desirable in relation to the ultimate stress of the material and accordingly it is proof stresses which have set the limit to operating stresses in engineering design.

Particular aims of the present invention are to provide cast alloys in which a high proof stress and other good mechanical properties are combined with good foundry qualities, and to provide wrought alloys of inherently high proof stress which are not, as hitherto, dependent on cold working for their properties. Further aims are to provide cast and wrought products in which properties are uniform to a greater degree than obtain at the present time.

One possible step towards the development of strong alloys would be to make use of two-stage (solution and precipitation) heat-treatment processes such as are widely used with aluminum-base, copper-base and other alloys. With magnesium as base metal, however, it has proved difficult to establish alloys deriving their properties in this way, as with many alloy systems the final properties show little or no advance over those obtainable in other and simpler ways. It has, for example, long been known that cast alloys of the magnesium-aluminum type can be subjected to two-stage heat treatments, but such heat treated alloys have never found wide use as proof stresses rise to only 8 tons per square inch and the material in its final state is somewhat brittle. Magnesium-zinc- Zirconium alloys of high zinc content exhibit a more marked response to such a two-stage heat-treatment and show good mechanical properties in test-bars: alloys of this type are, however, somewhat diflicult to cast being susceptible to hot cracking in the mould and to microporosity. Furthermore, alloys of this class are not weldable and large and expensive castings may be rejected through the inability to rectify minor faults by welding. As a consequence, very little use has been made up to now of cast magnesium alloys developing their properties by two-stage heat-treatment and the usual practice is to rely on alloys subjected to precipitation treatment only. A shortcoming of alloys of this type is variability in properties, particularly between thick and thin sections.

The present invention provides alloys which respond to heat-treatment processes to a degree permitting the development of proof stresses exceeding those of alloys in general use; alloys in which, in the cast condition, a greater uniformity of properties as between thick and thin sections is made possible by the use of'two-stage heat-treatment processes, and alloys which behave well in the foundry. In particular, the alloys concerned are flee from hot cracking tendencies and are fully weldable.

Similarly, little use has been made of wrought magnesium base alloys subjected to solution plus precipitation heat-treatment processes. The reason for this is that in the alloy systems explored so far it is generally difiicult to improve upon the mechanical properties of hot or cold worked material by heat-treatment. In many cases the hardening achieved in a precipitation treatment does no more than, or little more than, offset the work hardening removed in the solution process; indeed in several in- 3,039,868 Patented June 19, 1962 stances the properties of fully heat-treated alloys, although of the precipitation hardening type, may be inferior to the material as worked.

Some very limited use has been made of magnesiumrare earth and magnesium-thorium alloys in the fully heat treated condition (generally in the cast form) and where the principal interest is in resistance to creep: these however are low strength alloys and it is to remedy shortcomings in this respect that the alloys with which we are here concerned have been devised.

For age-hardening to occur it is required that there shall be a marked difference in the solubility of the solute element in the base metal at high and low temperature. Among magnesium-base alloys having such a solubility/ temperature relationship is the magnesium-R.E. system, where the symbol R.E. stands for one or more of the elements of the rare earth metals group. Simple magnesium-R.E. alloys, for example those made from mischmetall, although possessing useful powers of creep resistance, are not, however, distinguished by particularly high tensile properties either in the cast or wrought form, and with or without zirconium present as a grain refiner; furthermore, and in spite of the favourable solubility/temperature relationship, the response to heat-treatment is poor. Nor have investigations of the properties of ternary or more complex alloys based on the magnesium-R.E. system disclosed any alloys responding to heat-treatment to a worthwhile degree and having good mechanical properties. As a consequence, magnesium-rare earth alloys have not been used where high proof stress and strength was a particular consideration, and it has not been recognized that in selected compositions there could exist a useful series of strong age-hardening alloys.

We have, however, found that the element silver when added in controlled proportions to magnesium-R.E. alloys exercises a particular and specific effect, improving the response to two-stage heat treatment to the point where real benefit results from the application of the treatment. The effects have been secured only with silver in combination with R.E. and have not been obtained with any thorium) 0.25 to 10 percent but generally within the range 0.25 to 5 percent. A range of 0.5 to 3.5 percent gives particularly good results. Thorium (used preferably in conjunction with mischmetall) not more than 5 percent and generally not more than 2.5 percent. Silver 1.5 to 3.5 percent. The quantity of R.E. elements and silver together must be at least 2 percent and preferably at least 3 percent; or, Where thorium is used in addition, the quantity of R.E. elements, silver and thorium together must be at least 2.5 percent.

If desired zirconium may be present in amount up to 1 percent for grain refining purposes. For castings it is particularly desirable to include at least 0.4 percent zirconium.

Other elements soluble in magnesium may be present provided that they do. not, by forming compounds with the silver or the R.E. elements, or otherwise, interfere with the hardening process, nor depress the melting point of the alloy to such a degree as to make heat-treatment for the dissolution of the Mg-R.E. metal compound in- 3 elfective. Elements meeting these requirements are as soluble or nearly insoluble elements, such as copper, follows: which would tend to embrittle the alloy should preferably be absent, but can be tolerated in amounts not seriously Percent tsu t 5 afiecung ductility, e.g. 0.25 percent. 5 f P 5 The efiects of silver when used as an addltion to cast a i i m amoutn S o magnesium R.E. alloys are illustrated in Table I WhICh amounts up shows the properties of magnesium-mischmetall-zlrconi- ,amoun s 1 um, magnesiurn-mischmetall-thorium-zirconium and magi m amofn S O nesium-neodymium-zirconium alloy with and without I z fl amom: S 0 10 silver additions. The mischmetall usedwas of the ordif Si 1I1.amun Z '6 nary grade containing about 50 percent cerium. The L i m amotun s 0 neodymium was derived from a technical oxide mixture i s containing a nominal content of 75 percent neodymium m amoun S up 0 oxide. The term neodymium is used here to distinguish The total of all alloying ingredients other than mag- 15 the material used from didymium mixtures which may nesium will not exceed 12 percent. It is to be noted contain lower proportions of neodymium. that zinc, hitherto regarded as a most useful component The mechanical property values quoted'relate' to tenof magnesium alloys containing zirconium, is found to site specimens taken from standard sand cast test-bars 1" be undesirable in alloys according to the invention: caddiameter x 6" long machined to 0.564" diameter x 2" mium too, hitherto thought to be a useful addition (algauge length for testing. Proof stresses were determined though less widely used), is not found to be an advanby the otfset method.

TABLE I Composition 0.1% Ultimate glzroof 'gensile Elonga- 1 ress, ress, lon, MM, Th, Nd, Zr, Ag, Heat'TTeatment Tons/ Tons/ Percent Percent Percent Percent Percent Percent square square inch. inch.

2 hrs. at 560-570 0. Water 1311111133111: 313? 3:13:31: 1:33;: 3:2 a-115i ggegr egedzthours ii iii; 20

a 2 hrs. at 570 0. Water iii; 1:33:13: 313 "'"m' g gs g 24 hours at i 3:? iii? i5 2 hrs. erase-570 0. Water E 1. 40 1. 20 0. e e. 4 15. 0 10 quenched. Aged 16 hours F 1.44 1.65 0.6 2.04 WWW 11.0v 16.8 6.5

8 hrs. at 540-560 0. Water G 3. 08 0.6 quenched. Aged 8-16 8.3 15.4 3 H 3.15 0.6 1.96 hours at 200 C. to give' 12.0 17.0 4'

Max. proof stress.

MM= Mis chrnetall.

1 Values in this column represent the total content of RE. metal of which neodymium comprises approximately 75 percent.

2 Nominal contents.

tageous addition. For castings it is preferred to omit at It will be observed thatin each case the silveraddition least manganese and galhum. has brought about a marked improvement m properties;

When zirconium is present in the alloy, elements which particularly with regard to proof stress. Of the four form high melting point compounds with zirconium, silver-bearing compositions referred to in Table I, alloys'F thereby inhibiting grain refinement, should be absent. and H, both of which show proof stresses exceeding 10 Thus, for example, the alloy should not contain tin, and tons per square inch, are regarded as particularly suitable if manganese be present its content must be adjusted to for casting purposes. The properties ofalloy H are parthe desired zirconium content in accordance with the ticularly striking and exceed those ofany known magteaching of British Patent No. 759,411. nesium alloy having'acceptable casting properties.

Iron may be regarded as an impurity for the purpose The alloys may be preparedin the wrought form by the of the present invention and may be present in the usual hot and cold working techniques. An indication of amounts tolerated in ordinary magnesium alloys (up to the properties obtainable in wrought alloys'is given'in the about 0.05 percent but considerably less than the latter tensile test results quoted in Table II which relate to pieces quantity when the alloy contains zirconium) other inforged under the hammer from 2 /2" cast bar.

TABLE II Composition Mechanical Properties 0.1% Ultimate Alloy 7 ST 7 Heat Treatment. slzroof giansile Elongalver, .irconiuin ress tess, t R'E Metals percent Tons/ 'lons/ pei' iint square square inch inch 0.83% Misch- 1.88 0.6% nom- )6 hr. at 570 0., oil quench 17.5 19:9 4. 5

metall. inal. 24 hrs. at 175 C'-. 1.25% Neo- 2.5 0.6% nom 1hr.at5300.,waterquench 19.8 21.9 4

dymium 1 inal. 8 hrs. at 200 C. 1.37% New 2. 5 0.6% nom- 1 hr. at 535 0. water quench 23.1 24. 3 2

dymium 1 incl. 16 hrs. at 200 C.

Denotes a rare earth mixture comprising approximately neodymium. This designation is used to distinguish this RE. mixture from didymium which may contain a lower contentof neodymium.

The proof stresses shown for alloys K1, K2, in the above table are particularly noteworthy and are believed The values quoted represent the mean of a number of results.

TABLE IV Properties in 1" test-bars Properties in 4" diameter block 0.1% Ultimate 0.1% Ultimate Alloy Heat-Treatment Proof Tensile Prooi Tensile Stress Stress, Elonga- Stress Stress, Elonga- Tons/ Tons/ tion, Tons/ Tons] tion, square square Percent square square Percent inch inch inch inch Alloy N Mg-2.03%, Ag1.27%, 8 hrs. at 550 C. quench and 11. 3 15. 6 2. 5 9. 9 13. 4 i

1ZV[ischmetall1.57%, Til-0.6% 16 hrs. at 200 0.

r. Alloy OMg4.5%, Zn0.6% Zr 16 hrs. at 200 C 9.1 16. 3 6 7. 3 14. 2 3. 4 Alloy PMg5.5%, Zn1.8%, 2 hrs. 1112330 0. air cool, and 9. 7 17. 9 12 7. 4 13. 5 2.8

Th0.6% Zr. 16 hrs. at 200 C.

to be higher than any so far seen for a magnesium alloy 90 not in the work-hardened condition.

The benefits springing from the addition of silver could not have been anticipated, as silver is recognized as a metal which, even if used in substantial amounts, does not impart marked hardening tendencies to magnesium. Table III shows that the proof stress of a magnesium- The properties in actual sand castings of a Mg2.56% Ag-2.73% Nd0.7% Zr alloy have also been studied and compared with those of sand cast test-bars cast from the same melt.

The result of tensile tests on specimens cut from a heat-treated casting, which included sections up to 3 inches thickness, are as follows:

TABLE V 0.1% Proof Stress, Tons/ Ultimate Tensile Stress, Elongation, Percent square ch Tons/square inch Description lvluimum Mean MinimnfiMmrimnm Mean M'inimnrn Maximum Mean Minimum Test-pieces from casting (mean values based on ssevent resutltlsyuu 11.8 11.3 10.7 16.7 16.1 14.8 4 2.8 1

epara e es ars (Actual values)--- g zirconium alloy containing 2% silver (typical of the amounts used in alloys according to the present invention) but no rare earth metals, is very low even after two-stage heat-treatment and not significantly better than that of a simple magnesium-zirconium alloy. In respect to its hardening and strengthening tendencies silver is much less effective than, say, zinc, yet zinc is surprisingly not found to be a useful constituent of alloys of the present inven- The heat-treatment Was 4 hours at 530 0., water quench, and 8 hours at 200 C.

Alloys of the type with which We are here concerned also have a resistance to creep at temperatures of the order of 200 C. comparable With that of the magnesiummischmetall-zirconium alloys (with or Without zinc addi tions) such as are in general use.

Alloy ZREl (D.T.D. 708) is the most widely used of tion and certainly cannot replace silver. the latter. The following table shows that a Mg-2.5%

TABLE III The Properties of Cast Magnesium-Zirconium Alloys With and Without Silver Composition Mechanical Properties 0.1 Ultimate Alloy Zr Ag Heat-Treatment Proof Tensile Elongapercent percent Stress, Stress, tion,

Tons/ Tons/ percent square square inch inch 0.6 2 hours at 570 (3., water 3.1 10.7 21.5

. quenched. 0. 6 2. l3 Aged 16 hrs. at 200 C 3. 9 12. 6 18 1 Nominal contents.

Alloys of the type described maintain their properties well when cast in heavy section and are superior to existing alloys in this respect. As an example, the properties of a magnesium-silver-mischmetall-thoriurn-zirconium alloy (alloy N), determined in test-bars and in heavy 4" diameter blocks, are compared in Table IV with those of a conventional magnesium-zinc-zirconium alloy (alloy O) and a magnesium-zinc-thorium-zirconium alloy (alloy P cast under the same conditions.

Ag2.2% Nd0.6% Zr alloy possesses a far superior combination of room temperature static strength'and elevated temperature creep strength than the ZREl alloy which has heretofore been used. In addition, the Mg 2.5% Ag2.2% Nd0.6% Zr alloy shows much higher tensile properties than ZREl when tested at temperatures of 200-250 C., i.e. temperatures at which castings in ZREl very frequently operate in service.

TABLE VI Tensile Properties (tons/square Stress to Stress to inch) produce produce 0.1% creep 0.2% creep Heat strain in strain in Alloy Treatment Ulti' Per- 100 hrs. at 500 hrs. at

Tem 0.1% mate cent 200 C 200' C. Proof Tensile Elon- (Tons/ (Tons/ Stress Stress gation square square inch) inch) R E2.2%, 200 4. 5 s 1 29 4.2 3. 7 Zn0.6% Zr 250 3.8 6 9 39 (ZREI). Mg--2.5%, Ag 4 hrs. at

N 530 C. 11.3 16. 3 3 0.6% Zr (Alloy quench, 200 9. 7 12. 2 24 4. 8 4. 3' of present 8 hrs. at 250 7. 0 9.0 33 invention). 200 C. Mg2.1%, Ag- 8 hrs a v 10 s 5 4 1.7%, Thquench, 0.6% Zr (Alloy is 1115. g-E 5 of present at 250 invention). C.

1 Room temperature.

Rare-earth metals constitute the principal hardening agent in these alloys and the content of these will generally be of the same order as the maximum solubilities of these metals in solid magnesium, so that the maximum response to heat-treatment may be obtained. More or less of these metals may, however, be used depending on the form in'which the alloy is to be used and the combination of properties sought. Thus for example it might be advantageous to use a lower total rare earth content in wrought'than in cast alloys. Again, where a high hardness was desired it might be appropriate to use a greater amount of rare earth metals than can be taken into solid solution and in other cases to sacrifice proof stress (by using. a reduced rare earth content as in alloy B, Table I) in the interests of an improved ductility.

The silver content is most advantageously put around 23%.

The temperature and periods of treatment will in general be as follows:

For the solution or homogenising treatment the temperature may range from 100 C. below the temperature of first fusion up to about the said temperature and the period from half an hour up to 12 hours or more.

For the precipitation treatment the temperature may be from 150 C. to 250 C. from 1 to 16 hours or more.

In all alloys having a composition within the scope of the present invention this double heat treatment will produce a proof stress of at least 8.0 tons per square inch.

The precise temperatures and periods will depend upon the particular compositions and some tests may be desirable to establish the optimum.

We claim:

1. A heat-treatable magnesium base alloy consisting of the following (other than iron and other impurities):

Rare earth metals 0.5 to 3.5 Silver 1.5 to 3.5

together with a proportion of zirconium not exceeding 1.0 percent; the rare earth metals containing a major proportion of at least one element selected from the group consisting of neodymium and praseodymium and less than percent of lanthanum and cerium together;

2. A magnesium base alloy as claimed in claim 1 wherein the rare earth metals consist of at least 60 percent by weight neodymium.

3. An alloy as claimed in claim 1 which contains manganese not exceeding 2.0 percent; the sum of the zirconium and manganese being at least 0.4 percent; the maximum permissible quantity of each of these two'elements being limited by the quantity of the other. 7

4. A method of producing a casting or wrought article which consists in submitting an alloy of the composition claimed in claim 1 to a heat treatment consisting of (1) a solution treatment at a temperature of from 100 C. below the temperature offirst fusion up to about the said temperature for at least half an hour; (2) quenching and (3)v a precipitation treatment at a temperature of from 150 C. to 250 C. for at least one hour.

5. An alloy as claimed in claim 1, containing also at least one of the following:

Percent by weight Zinc Up to 0.5 Thorium Up to 5.0 6. A heat-treatable magnesium base alloy consisting of the following (apart from iron and other impurities): Percent by weight 7. A heat-treatable magnesium base alloy consisting of the following (apart from iron and other imIpu-ri ties):

Percent by weight Silver From 1.5 to 3.0 Rare earth metals (of which the major proportion is at least one element selected from the group consisting of neodymium and praseodymium and of which lanthanum and cerium together are less than 25 percent by weight) From 0.5 to 2.0 Thorium From 1.0 to 2.5 Zirconium From 0.4- to 1.0 Magnesium Remainder from the group consisting of neodymium and praseodymium and of which lan thanum and cerium together are less than 25 percent by weight) 0.5 to 3.5 Zirconium 0.4' to 1.0

Silver 1.5 to 3.5

9 the quantity of rare earth elements and silver together being at least 3.0 percent; the article being in the physical condition produced by solution heat treatment followed by precipitation heat treatment whereby the article has a minimum 0.1 percent proof stress of 8.0 tons/ square inch.

9. An article as claimed in claim 8, containing also at least one of the following:

Percent by weight Zinc Up to 0.5 Thorium Up to 2.5

References Cited in the file of this patent UNITED STATES PATENTS Gann Nov. 7, 1939 Murphy et a1. Feb. 8, 1949 FOREIGN PATENTS Great Britain Jan. 9, 1957 France Mar. 25, 1957

Claims (1)

1. A HEAT-TREATABLE MAGNESIUM BASE ALLOY CONSISTING OF THE FOLLOWING (OTHER THAN IRON AND OTHER IMPURITIES) TOGETHER WITH A PROPORTION OF ZIRCONIUM NOT EXCEEDING 1.0 PERCENT; THE RARE EARTH METALS CONTAINING A MAJOR PROPORTION OF AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF NEODYMIUM AND PRASEODYMIUM AND LESS THAN 25 PERCENT OF LANTHANUM AND CERIUM TOGETHER.