The precipitation hardening behaviour of two Co-Ni-Cr-Al alloys containing Nb or Ti has been studied. On aging between 873K and 1073K, the main strengthening phase in both the alloys was observed to be a homogenous precipitation of γ' phase which has an ordered Cu³Au type F.C.C. structure. The shape of the γ' phase changed from spherical to cubical in the initial stages of aging, and the cube edges were aligned along the <100> directions. On aging for prolonged periods at 1073K, the particles coalesced to form irregular shapes. The formation of γ' follows the Engel-Brewer theory of the structure of compounds based on electronic considerations.

Carbon, present as an impurity element, was seen to form NbC/TiC precipitate. This precipitate, when present at the grain boundary, gave rise to a precipitate free zone around the boundary. Within the grain, NBC precipitated in association with stacking faults. Discontinuous precipitation of γ' occurs by localized dissolution of grain boundary NBC/TiC precipitate at sharp grain boundaries and seems to be initiated by the process of grain boundary straightening. Occasionally acicular sigma phase was also observed at the grain boundaries. This is in contradiction of predictions by Engel-Brewer and PHACOMP Analyses, and is considered to be due to localised chemical inhomogeneity.

In the small particle size range of 7-20nm diameter deformation of the Co-Ni-Cr-Nb-Al alloy occurs by the shearing of γ' particles by pairs of moving dislocations. Dislocations bypass large particles (over 20nm in diameter) by the Orowan looping mechanism. In the diameter range of 9-20nm a transition stage encompassing a progressive increase of looping activity is seen. The alloy in the underaged state is strengthened by the order hardening mechanism proposed by Brown and Ham. The shear modulus-corrected yield strength values for underaged specimens did not increase with temperature in the range 77 to 373K, thus eliminating coherency hardening as a contributory mechanism to strengthening in the underaged condition. In the overaged alloy, deformation occurs by the Orowan looping process, and the experimental results are in excellent agreement with the mechanisms proposed by Hirsch and Humphreys and Bacon et al. Also, there is an excellent agreement between the observed incremental CRSS and theoretical incremental CRSS predicted by the Ashby mechanism if the average incremental CRSS for edge and screw dislocations is considered.