Abstract
The kinetics of γ′ rafting in CMSX-4 single crystals at 950°C has been studied using scanning and transmission electron microscopies. The crept material was subjected to further heat treatment in the absence of applied stress, in order to deduce the threshold strain ϵ th for rafting to continue; this is shown to be 0.10±0.03%. For strains smaller than ϵ th, rafting occurs exceedingly slowly; once ϵ th is exceeded the kinetics are largely unaffected by the absence of applied stress. The magnitude of ϵ th confers a reduction in γ/ γ′ interfacial coherency and a relaxation of interfacial misfit stresses. It is concluded that the threshold strain is a quantity suitable for distinguishing between rafting occurring in the “elastic” and “plastic” regimes. The results confirm conclusively that plasticization of the horizontal γ-channels by (a/2)〈1 10〉{111} creep dislocations and their subsequent adsorption at the γ/ γ′ interfaces, with a concomitant loss of perfect coherency and reduction in elastic misfit strains, is responsible for providing the kinetic path which enables rafting to occur at a reasonable speed. An argument is put forward to explain the magnitude of ϵ th.
Published Version
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