THE EFFECT OF DIFFUSION-INDUCED POROSITY IN NICHROME ALLOY ON THE SINTERING OF NICKEL AND CHROMIUM POWDERS

Mixtures of Cr and Ni powders were cold-pressed and the compacts with porosities of 10 to 15, 25 to 30, and 40 tc 45% were sintered in a hydrogen stream for 8 hours. Study of the X-ray diffraction pictures showed a sudden change of the lattice constants and a loss of sharpness of the lines. This behavior is attributed to the change of the previously uniform character of the system: submicroscopic pores are formed in the solid solutions, reaching diameters of several hundreds of Angstrom units. The lack of uniformity is caused by closure of these pores, resulting in an intenference with the diffusion of Cr the partial diffusion coefficient of which is considerably higher than that of nickel in the nichrome system. This heterogeneous nature of the sintered specimens is responsible for the lack of sharpness of the diffraction lines. After the Cr concen tration becomes more even, the number of vacancies increases until a certain amount of Cr is removed from the system. (TTT)