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Abstract
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 ordered (gamma)' phase, Ni(,3) (Al, Nb, Ti). On aging for prolonged periods at 1073K, the initially spherical or cubical particles coalesced to form irregular shapes. The formation of (gamma)' follows the Engel-Brewer theory of the structure of compounds.
Carbon, present as an impurity element, was seen to form grain boundary NbC/TiC precipitate, resulting in denudation. Within the grain, NbC precipitated in association with stacking faults. Discontinuous precipitationof (gamma)' occurs by localized dissolution of grain boundary carbide at sharp grain boundaries and initiated by the process of grain boundary straightening. Formation of brittle sigma phase at grain boundary contradicts Engel-Brewer and PHACOMP Analyses, and is attributed 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 (gamma)' particles by pairs of moving dislocations. Dislocations bypass large particles (over 20nm in diameter) by the Orowan looping mechanism. 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 contribution from coherency hardening. 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, Bacon et al and Ashby.
