SOME ASPECTS OF POWDER METALLURGY 443 



5. Heavy Alloy 



"Heavy alloy" is the name applied to a group of alloys composed of tung- 

 sten, copper, and nickel having a density of 16 grams per cc. or greater^"'^^ 

 They were originally developed for fabricating the containers and nozzles 

 for radium units, but have such interesting properties that a number of 

 other uses have become evident. Specific properties depend upon the 

 composition, but generally the tungsten comprises about 90 per cent of 

 the alloy. 



One of the best compositions claimed is that of 90 tungsten-7.5 nickel-2.5 

 copper wJiiich has properties as listed below: 



Tensile Strength 90,000 psi 



Yield Point 83,000 psi 



Elongation in 1 inch 4% 



Elastic modulus 32 X 10^ psi 



Brinell hardness 250-290 



Density 16.3-17.0 gms. per cc. 



Coefficient of expansion 5.6 X 10"^ 



Thermal conductivity 0.25 c.g.s. units 



The alloys are prepared by mixing the metal powders dry, adding a 

 small amount of wax, in benzol solution, mixing until the solvent has evap- 

 orated, and then pressing to shape. The compact is heated slowly to 

 about 1000° C. and then sintered at a higher temperature at which the nickel 

 and copper particles fuse, and the tungsten is not only wet by the liquid, 

 but actually dissolved. The fine particles are thus dissolved, but tungsten 

 is reprecipitated on certain nuclei to develop large rounded grains. The 

 solution and redeposition continue until the original fine tungsten particles 

 are replaced by grains approximately 100 times the original particle diam- 

 eter. The alloy thus consists of tungsten particles in a cementing phase 

 of copper-nickel-tungsten. 



There is a shrinkage of up to 20 per cent, and the resulting compact is 

 relatively free of porosity. 



The alloy has good machining properties and can be treated much like 

 many cast alloys. It has good corrosion resistance and can take a variety 

 of surface finishes. 



In addition to its use in X-ray and radium work, its high density and 

 strength make it attractive for use as a counterweight material in high- 

 speed motor setups of many types. 



6. Electrical contacts and electrode materials 



Powder metallurgy can be utilized to fabricate material composed of 

 two or more metals without any appreciable alloying so that the character- 

 istics of each of the components may be retained to a large degree. This 

 has opened a large field for electrical contacts and welding electrodes made 

 by using compositions where the refractory nature of materials such as 



