CARBIDE PRECIPITATION IN CERTAIN ALLOYS OF THE CO-NI-CR-C SYSTEM

An investigation is reported of structural and hardness changes during the precipitation of Cr23C6 from f c c supersaturated solid solution in various alloys of the Co-Ni-Cr-C system containing 25 wt % Cr and 0.15 to 0.25 wt % C. The work extends results previously reported on a Co-25.3 wt % Cr-0.26 wt % C alloy, by studying the effect of adding nickel to replace cobalt (either wholly or completely) and hence changing the stacking fault energy. The addition of 10% nickel was found to increase the nucleation rate of matrix precipitation; precipitate particles also nucleated on partial dislocations with associated stacking fault formation. With approximately 55% nickel and 0.15% carbon, there was little matrix precipitation; instead precipitation occurred predominantly on dislocations, as rods growing along 〈1 1 0〉 directions; the main climb morphology was the formation of dislocation dipoles. Substantial precipitation hardening effects were obtained, particularly in the cobalt-rich alloys with 0.25% carbon. In a ternary Ni-25 wt % Cr-0.25 wt % C alloy, a non-uniform precipitate dispersion formed involving nucleation in the matrix and on dislocations. An investigation was also made of the effect of prior strain (1 to 10%) on the ageing of the Co-25.3 wt % Cr-0.26 wt % C alloy. Deformation of the f c c solution-treated material produced faulting and also the formation of h c p ∈ phase. On ageing, Cr23C6 precipitation occurred within the ∈ lamellae; at long ageing times after 10% deformation carbide particle coarsening, and matrix recrystallization occurred concurrently, and the recrystallization was accompanied by the transformation of the f c c matrix to h c p ∈ phase. © 1979 Chapman and Hall Ltd.