Precipitation in Ni-Co-Al alloys

Abstract

Discontinuous precipitation of theγ′ phase in Ni-Co-Al alloys has been studied in the temperature range 673 to 973 K using optical microscopy and transmission electron microscopy. Discontinuous precipitation was observed to occur in all the alloys to some degree, the extent of the precipitation decreasing with decreasing cobalt content of the alloy. Only in high cobalt alloys (greater than 38 at%) did the discontinuous transformation go to completion. The discontinuous transformation product consisted of fine straight coherentγ′ lamellae with a specific cube-cube orientation relationship to theγ matrix at low ageing temperatures and short times. At higher temperatures and longer times, continuous and discontinuous coarsening processes produced a coarse, less regular structure with a plate spacing ten times that of the regular structure. The overall kinetics of the discontinuous transformation were affected by the prior continuous precipitation ofγ′ particles ahead of the discontinuous reaction fronts. The coarsening of these continuous particles as a function of time stopped the transformation in low cobalt alloys and continuously slowed the rate of transformation in high cobalt alloys resulting in Avrami exponents,n, less than 1. Reversion experiments and experiments on prestrained specimens yielded Avrami exponents ofn=1 lending support to the above suggestion. The major effect of cobalt in these alloys was to increase their propensity to transform discontinuously. This effect was largely due to the decrease in the rate of continuous precipitation and coarsening as a result of the increased partitioning of cobalt betweenγ andγ′ in high cobalt alloys.