430 BELL SYSTEM TECHNICAL JOURNAL 



Hydrides can be formed of many metals, those of titanium, zirconium, 

 thorium, hafnium, columbium, and tantalum being of particular interest since 

 they are reported to be stable at room temperature. They are produced 

 in 300 mesh size or finer, have the appearance of metal, and begin to dissoci- 

 ate into hydrogen and the pure metal in vacuum or non-oxidizing atmos- 

 pheres above 350° C. The hydrides can be mixed with other metal powders, 

 and, when compacted and sintered, slowly release hydrogen which creates 

 a protective atmosphere around the metal particles and sometimes acts 

 to remove oxide films already present. 



Despite the number of methods known for producing metal powders, 

 the bulk of the powders used on a large scale are produced by only three 

 methods-^: electrolytic deposition, atomization, and reduction of metal 

 salts by gases. The carbonyl process produces a specialty product as 

 does the hydride process, and, while both have their uses, the amount 

 consumed is probably small in relation to that prepared by the other methods. 



The Powder Metallurgy Process 



As has been indicated in the introduction, there are a number of definite 

 steps in the powder metallurgy process which may be summarized as 

 follows: 



1. Selection of the powder or powders best suited for production of the 

 part under consideration. 



2. Proper mixing. (If more than one type of powder is being used) 



3. Pressing. (Sometimes followed by pre-sintering) 



4. Sintering. (Sometimes followed by an impregnating operation) 



5. Coining or Sizing operation if necessary. 



Each of these important operations is discussed in somewhat more detail 

 below: 



1. Selection of Powder 



Wlien the actual metal or alloy composition has been decided upon, 

 there are a number of factors which must be considered in the selection 

 of the type of powder itself. An essential characteristic is purity-^ because 

 in the powder metallurg>' process impurities cannot be slagged off as in 

 most melting processes, and may interfere with pressing and sintering oper- 

 ations. Oxide films, for example, may prevent good contact between metal 

 particles. Clean surfaces are essential if ductility, and high tensile and 

 shear strength are required in the finished article. In most cases, there 

 is a definite hmit set for objectionable impurities in a given powder, but 

 in some instances materials normally classed as impurities are deliberately 

 added to obtain a desired result. An example is the addition of thorium 



