434 BELL SYSTEM TECHNICAL JOURNAL 



general case for pressed powder compacts, or "green compacts" as they 

 are designated. There is frequently a surprising strength associated with 

 such pressed parts but, on the whole, a heat treatment is necessary to produce 

 a material approaching the strength and solidity of a cast or wrought metal 

 part. 



The heating of pressed powder briquettes is usually done in an inert, 

 reducing, or neutral atmosphere, or in vacuum. The temperature used 

 is determined by the metal powders comprising the compact, and by the 

 properties desired in the final product. The melting point is not exceeded 

 for any of the components of the mixture except in those instances where 

 such fusion of a minor constituent is desired, as, for example, in the pro- 

 duction of cemented carbides. No definite temperature may be set for 

 the heat treatment, but general practice is to treat at a temperature about 

 two-thirds that of the melting point of the metal or alloy being fabricated. 

 Higher temperatures are frequently used, however, and may be only slightly 

 below the melting point. 



The effect of heat is possibly that of causing increased surface diffusion 

 and plasticity. The atoms on the surface of metal particles possess consid- 

 erable mobility far below the melting point, and the surface energy at 

 elevated temperatures may be appreciable. Where particles are in contact 

 surrounding a void, flow of metal is in such a direction as to increase the 

 area of contact. 



When the sintering temperature is within the recrystallization range of 

 the metal or metal alloy powder being treated, marked structural changes 

 may occur. Recrystallization takes place at sites of plastic strain. Since 

 these sites are regions of contact between particles, new crystallites form 

 and grow into the adjacent particles so that a new series of grain boundaries 

 is formed. The numerous cavities or voids present in the structure are 

 not completely filled in or sealed in this operation. This could not occur 

 without change of overall dimensions of the compressed mass. The voids 

 may be present at the new boundaries or even enclosed in the crystalhtes, 

 and produce a non-homogeneous sintered metal of relatively weak struc- 

 ture susceptible to sudden shock. By a high temperature treatment just 

 below the melting point, or by alternate working and annealing, the voids 

 can be closed and the metal consolidated to a dense, strong mass. 



Surface oxide films which interfere with the sintering operation may 

 sometimes be destroyed by treatment of the powder compact in a reducing 

 atmosphere. If the oxide cannot be reduced in this manner, the pure 

 metal can only be obtained by sintering operations if the oxide has a higher 

 vapor pressure than the metaP^. 



Gases, either adsorbed, dissolved, entrapped, chemically bound, or 



