US3334998A

US3334998A - Magnesium base alloys

Info

Publication number: US3334998A
Application number: US409584A
Authority: US
Inventors: Fisher Philip Andrew
Original assignee: Magnesium Elektron Ltd
Current assignee: Magnesium Elektron Ltd
Priority date: 1963-11-15
Filing date: 1964-11-06
Publication date: 1967-08-08
Anticipated expiration: 1984-08-08
Also published as: DE1248306B; GB1035260A; SE307677B; BE655735A; GB1075010A; NL6413107A; NL143280B; CH436734A

Description

United States Patent Claims priority, application Great Britain, Nov. 15, 1963,

45,142/ 63 15 Claims. (Cl. 75-168) This invention relates to magnesium base alloys containing at least 80 percent magnesium. It is a common experience in magnesium base alloys that a grain boundary phase may have an embrittling or other deleterious effect on the alloy and the object of the present invention is to reduce this harmful effect.

As an instance, the addition of rare earth metals to magnesium results, when the rare earth metal exceeds a certain percentage, in a brittle grain boundary phase. Providing that the addition of rare earth metal does not exceed a known percentage, the grain boundary phase may be taken into solution by heat treatment in known manner and the embrittling effect is thus reduced. If the amount of rare earth metals exceeds the limit of solid solubility, however, complete solution of the phase by heat treatment in known manner is impossible. Further, it is known that if the magnesium base alloy also contains zinc, the grain boundary phase resulting from addition of rare earth metal is very stable and cannot be dissolved by heat treatment in known manner.

It is further known that the addition of certain elements such, for instance, as rare earth metals and thorium to magnesium alloys, particularly those containing zinc, results in technical advantages, e.g., improved soundness in castings, and improved ability to work the material by plastic deformation. The use of additions of rare metals or thorium is presently limited by the embrittling effects which result.

According to one aspect of this invention a magnesium alloy is provided with an alloying constituent consisting of at least one of the elements rare earth metals and thorium, some at least of which is incorporated in a grain boundary phase, and the alloy is then heated in the presence of hydrogen to effect reaction of hydrogen with said constituent and to cause part at least of the grain boundary phase to diffuse into the base metal.

The term rare earth metals for the purpose of the I present invention includes yttrium. I

For the purpose of this invention the alloying element which is to react with hydrogen may be termed the active constituent.

The hydrogen may, for example, react with the active constituent in the grain boundaries to form hydride thus attacking the grain boundary phase and liberating one or more of its constituents to go into solution in the mag nesium. Or, for further example, the hydrogen may react with the active element already dissolved in the alpha phase, permitting solution from the grain boundary phase with further progressive attack on the dissolved active constituent.

A particular application of the invention is in magnesium base alloys containing zinc. For example, a magnesium base alloy having high room temperature strength contains zinc 6% and zirconium 0.6 to 0.9%. This alloy, however, has a marked tendency to exhibit microporosity in the cast form, such porosity resulting in considerable reduction of strength. This tendency to exhibit microporosity has restricted commercial exploitation of the alloy. It is further well known that the tendency to microporosity of this alloy may be improved by addition of rare earth metals, e.g., such as cerium mischmetal or 3,334,998 Patented Aug. 8, 1967 didymium mischmetal which are two forms of commercially available rare earth metal. Such additions, however, reduce the tensile strength of the alloy and render it unattractive commercially. In accordance with one example of the present invention, however, one or more rare earth metals may be included in the alloy and the alloy is then heat treated in the presence of hydrogen so that the hydrogen reacts with one or more of the rare earth metals. The results given in the following table illustrate the improvement in properties which can be obtained by using the novel procedure described in this application:

MAGNESIUM-BASE ALLOY CONTAINING NOMINALLY 6% ZINC AND 0.7% ZIRCONIUM 0.1% Proof Tensile Properties Heat Treatment Atmosphere Stress 0.2% U.T.S. E.,

'ISl P.S. TSl percent 'ISl S0, (Normal atmosphere for mag. based alloys) 11. l 12. l 17. 5 3 Hydrogen 10. 7 11.7 16. 4 2

MAGNESIUM-BASE ALLOY CONTAINING NOMINALLY 6% ZINC, 2% R E METALS, 0.7% ZIRGQNIUM SO; 5. 3 6. 0 ll. 3 4 Hydrogen 10, 4 11. 7 19. 2 10. 5

The above results were obtained from cast test bars made in accordance with British Standards Specification L101, FIG. No. 1. The heat treatment in each case was for 24 hours at 500 C. in S0 or hydrogen atmosphere followed by 64 hours at 125 C. in air.

In the case of the alloy detailed in the above example the improvement in mechanical properties resulting from application of this novel principle is associated with the modification of a metallurgical phase present in the grain boundaries of the alloy. The presence of this phase is both responsible for the improvement in respect of microporosity and also the resultant loss in tensile properties. The phase is normally very stable and cannot readily be dissolved by heat treatment in previously known manner. It is believed that the introduction of hydrogen during heat treatment converts the rare metal content to hydride, resulting in breakdown of the original phase and thus removing the deleterious eifect on strength. The probability of an improvement in strength when using any given active constituent can, therefore, be assessed by comparative metallographic examination of specimens heat treated in previously known manner with those heat treated according to the present application.

By such comparative metallographic examination it has been found that suitable phase changes occur in magnesium alloys containing thorium so that rare earth metals may be partly or wholly replaced by thorium. Yttrium may also be used as an active constituent.

The present invention, therefore, includes a magnesium base alloy consisting apart from impurities of:

RE percent by Weight 0.2-6 Zinc d0 0.25-10 H cc./ g 1 At least 50 Zirconium percent by weight 0-1 Manganese d0 0-2.5

Approx. 0.005% by weight.

In alloys containing both zirconium and manganese, if either is at least 0.3% the other will not exceed 0.2%.

In alloys which do not contain zirconium and Where iron is included in contents of e.g. 0.03 to 0.1%, the silicon content will be less than 0.05%

If desired, one or more of the following elements may be included in the alloy:

Percent by weight The following more limited range of composition is preferred:

RE metals percent by weight 0.75-4 Zn do 3-8 Zr do 0.3-1.0 Ag do 0.25 Hydrogen cc./ 100 g At least 50 For high ductility combined with a satisfactory proof stress the RE and zinc contents may be restricted to the following ranges:

Percent by weight RE 0.75-1.5 Zn 3.5-5

The following RE and zinc ranges provide high proof stress combined with good ductility:

Percent by weight RE l.75-4.0

For the highest proof stress values the following composition is preferred:

Percent by weight RE 1.75-4 Zn 5-8 Zr 0.3-1.0 Ag 0.5-5

Since the proportion of certain of these elements (par ticularly zinc) is in practice limited by their unfavourable effect on such characteristics as porosity, tendency to crack during solidification, weldability, etc., and since the addition of the constituents listed as active in this application alleviates such tendency, the use of the principles disclosed in this application make it possible to increase the permissible amounts of certain of these alloying constituents beyond the levels found in present practice and thus make available alloys of higher strength.

Since addition of the active constituents is known to suppress the tendency of a magnesium-base alloy to crack when cooled rapidly (e.g. by quenching in water or oil) from a high temperature, it is expected that the use of the principle herein disclosed will permit such rapid cooling in alloy systems in which it is not now practicable. Such rapid cooling would be expected to still further im prove the strength of the alloy.

In respect of silver as an alloying element it is known that the addition of silver, e.g. up to 6%, to the magnesium base alloy containing 6% zinc and 0.7% zirconium re sults in significant improvement to tensile strength. Such addition, however, results in no benefit to the marked tendency of this alloy to exhibit microporosity in the cast form and hence the silver containing alloys are also restricted in commercial exploitation by virtue of this characteristic. It has been found that the application of the principle herein disclosed also provides for significant improvement in freedom from porosity of the silver containing alloy with subsequent recovery of properties by the heat treatment in hydrogen. The addition of silver appears to reduce the rate at which attack of the grain boundary phase by hydrogen occurs but complete or substantially complete conversion of the phase may be effected by somewhat more prolonged treatment as compared with the silver free alloy.

In respect of thorium used as an active alloying element it is known that the grain boundary phase formed, particularly when zinc is also present, has a lower embrittling effect than in the case of rare earth metals although for many applications the embrittling effect is too severe. It may therefore be advantageous to use thorium wholly or partly to replace rare earth metals as the active constituent particularly where only partial conversion of the grain boundary is desired. For this purpose the thorium content may be 0.5-2.5 and the RE content 0.75-2.5%. If desired rare earth metals may be used in which the lanthanum content has been diminished, e.g. didymium.

It is well known that magnesium alloys for use at elevated temperatures may contain rare earth metals or thorium as an essential alloying addition. Alloys of this type when heat treated in the manner described herein may suffer a loss of high temperature strength owing to conversion of the essential alloying addition to a form unsuitable for the development of high temperature strength. The conversion of the active element may be halted before completion, e.g. by suitable choice of time and temperature, leaving the remainder of the active element to perform its other function of providing high temperature strength. In this manner alloys of improved castability and room temperature strength but still retaining high temperature strength may be produced.

A further advantage relates to alloys having high damping capacity. It is known that a binary alloy of magnesium with nominally 0.6% zirconium has a high damping capacity, and this alloy is used commercially for this purpose. This alloy, however, has very poor casting characteristics and its commercial use for high damping purposes is limited by the inability to cast complex shapes. The addition of the active constituents herein listed, together preferably with an addition of zinc, substantially improves the castability of this magnesium-zirconium binary alloy but that the addition of these active metals reduce the damping capacity. Similarly pure magnesium and magnesium alloys containing manganese are known to have high damping capacity but are not used in practice owing to the inability to cast complex shapes. The castability of such alloys may also be improved by addition of the active constituents herein listed but such additions in turn reduce the damping capacity. The damping capacity of magnesium alloys containing active elements may be improved by heat treatment in hydrogen. Preferred compositions afiording good damping capacity combined with good castability are as follows:

RE Percent by weight 0.2-4 Zn do 0-3 Zr do 0.3-1.0 H cc./ g.-- At least 50 When the allow does not contain zirconium, the RE content is preferably 0.25-3%, with zinc 3.5-8% and manganese 0.15-2.5%, the silver being in the range 0- 0.25%, or 0.25-5% where improved properties are desired. These compositions are also well suited to plastic working by known processes.

The temperature of heat treatment of the magnesium alloy may be from C. to the immediate neighborhood of the solidus. Temperatures of at least 450 C. will normally be used for the hydriding step and this will normally be followed by a precipitation treatment at a temperature not exceeding 250 C.

In performing the heat treatment disclosed herein it is essential that the allow be permitted to absorb a significant quantity of hydrogen. The hydrogen content of the magnesium alloy will be at least 50 ccs. per 100 grams of the alloy and may be at least 80 ccs. The heat treatment may be carried out in an atmosphere of hydrogen or one rich in hydrogen, e.g. ammonia, hydrocarbon gases, etc., and also in an atmosphere containing moisture such that the magnesium alloy reduces the moisture to liberate hydrogen. The heat treatment may also be carried out in any other medium, e.g. a salt bath, providing that this is suited to the heat treatment of magnesiumbase alloys, e.g. nitrate baths are usually considered unsuitable owing to the risk of explosive reaction between magnesimum and molten nitrates, but chloride baths could be used without such risk; and providing that hydrogen is made accessible to the magnesium alloy, e.g. by addition to the bath of hydroxides or unstable hydrides such as sodium hydride. The access of hydrogen in such a heat treatment may also be by an electrolytic process.

In carrying out the heat treatment it would be expected that the rate of absorption of hydrogen would increase if the pressure of the hydrogen-containing medium surrounding the magnesium was increased. It would further be expected that partial or complete ionisation of the hydrogen containing medium would increase the rate of hydrogen absorption. This is in accordance with the known principles of gas absorption. It has further been found that the magnesium alloy surface. may be beneficially treated to increase the rate of hydrogen absorption. Such treatments include shot blasting, providing a chromate film on the magnesium alloy surface, and applying salt solution to the magnesium alloy surface.

It has further been found that the absorption of hydrogen may be decreased or inhibited by other surface treatments. Magnesium surfaces cleaned by anodising in a solution of ammonium bifluoride tend to resist hydrogen absorption; magnesium surfaces coated with fused boric acid may be made to completely resist hydrogen penetration and similarly a magnesium alloy surface closely covered by a steel sheath has been shown to resist hydrogen penetration. It is therefore possible to produce magnesium alloy articles in which specific mechanical properties are developed in selected parts of the article, the other parts having specific properties of a different nature. Thus by way of example a cast article could be made in a magnesium-base alloy containing nominally 3% rare earth metals, 3% zinc and 0.6% zirconium. This alloy has good strength at elevated temperatures and has very good castability but has a somewhat low ductility, of the order of 5% elongation in a tensile test. By use of the invention the ductility can be increased to a value of about 15% elongation, although in this specific alloy the new principle of treatment would be expected to result in some loss of high temperature strength. By suitably masking selected areas of the cast article, the hydriding treatment can be restricted to the unmasked areas, providing a cast article with good creep resistance in selected parts and good ductility in other selected parts.

It has further been found that by suitable choice of time, temperature, heat treatment medium, etc., the rate of progress of conversion of the active element may be controlled to provide modified properties to a restricted depth below the surface of the article being treated. The invention can, therefore, be utilised to produce magnesium-base articles having strength characteristics significantly different at their surfaces compared with the characteristics towards the centre of their cross section.

It is further known that the addition of of the active metals listed herein improves the deformation characteristics of magnesium alloys but their use is limited by their effect in reducing strength. The alloys and methods disclosed herein may therefore also be applied to wrought forms.

The invention enables castings to be produced possessing a minimum 0.1% proof stress of 9.0 t.s.i. combined with a minimum elongation value of at least 7%, these figures relating to specimens cut from the casting.

I claim:

1. A magnesium base alloy comprising apart from impurities RE metals percent by weight 0.2-6 Zinc do 0.25-10 Hydrogen ccs./ g. of alloy At least 50 Magnesium Balance 2. The alloy of claim 1 including a maximum of approximately 1 percent by weight of zirconium.

3. The alloy of claim 1 including a maximum of approximately 2.5 weight percent manganese.

4. The alloy of claim 1 including a maximum of approximately 1 percent by weight zirconium and about 2.5 percent by weight manganese, the zirconium and manganese being so related that if either is at least 0.3 percent the other will not exceed 0.2 percent.

5. A magnesium base alloy comprising the following composition:

RE metals percent by weight 0.25-3 Zinc do 3.5-8 Manganese do 0.15-2.5 Hydrogen ccs./ 100 g. of alloy At least 50 Magnesium Balance 6. The alloy of claim 5 including a maximum of about 0.25 percent by weight silver.

7. An alloy as claimed in claim 1, together with one or more of the following elements in the amounts stated:

Percent by weight 8. A magnesium .base alloy having the following composition:

RE metals percent by weight-.. 0.75-4 Zinc do 3-8 Zirconium do 0.3-1.0 Silver do 0*025 Hydrogen ccs./ 100 g. of alloy At least 50 Magnesium Balance 9. A magnesium base alloy in accordance with claim 1 wherein the RE and zinc contents are as follows:

Percent by weight RE metals 0.75-1.5 Zinc 3.5-5

10. A magnesium base alloy in accordance with claim 1 wherein the RE and zinc contents are as follows:

Percent by weight RE metals l.754.0 Zinc 5.5-7.5

11. A magnesium base alloy having the following composition:

7 12. A magnesium base alloy having the following composition:

Percent by weight RE metals 0.752.5 Zinc 5-8 Zirconium 0.3-1.0 Silver 0.5-5.0 Thorium 0.5-2.5 Hydrogen ccs./ 100 g. of alloy At least 50 Magnesium Balance 13. A magnesium 'base alloy having the following composition:

RE metals --percent by weight 0.25-3 Zinc do 3.5-8 Manganese do 0.15-2.5 Silver do 0.25-5 Hydrogen ccs./100 g. of alloy At least 50 Magnesium Balance 14. A sand casting in accordance with claim I possess- References Cited UNITED STATES PATENTS 3,101,269 8/1963 Emley 75l68 3,157,496 11/1964 Foerster 75l68 3,167,425 1/1965 Petch et al 75l68 3,183,083 5/1965 Foerster 75l68 DAVID L. RECK, Primary Examiner.

CHARLES N. LOVELL, Examiner.

Claims

11. A MAGNESIUM BASE ALLOY HAVING THE FOLLOWING COMPOSITION: