The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy du
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Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on ./' grain boundary precipitate morphology in an alloy consisting of y' (Ni3Al) precipitates in a y (nickel solid solution) matrix. We find that the uniform distribution of 7' precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of y' precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of 7' precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that HerringNabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations. NICKEL-BASE superalloys are only one example of the increasing number of alloy systems which have high temperature applications under load. It is therefore prudent to study the influence of applied stress on transformations in solids. Accordingly, Tien and Copley have determined the effects of applied stress on the coarsening characteristics of coherent y ' ~recipitates in nickel-base superalloy single crystals. '~ The purpose of the present investigation is to study the influence of applied stress on the morphology of the same type of precipitates at the grain boundaries of a polycrystalline nickel-base superalloy, a subject which is relevant to the creep behavior of this type of alloy. It has been established that grain boundary sliding and cavitation usually cause the failure of polycrystalline nickel-base superalloys during creep at elevated temperatures.3-5 In order to suppress or delay this mode of intergranular failure, the grain boundaries are usually strengthened with Mz3C6 type carbide particles, which are normally chromium-rich. For further strengthening the grain boundary, carbides are enveloped by Y' precipitates. This duplex structure is sometimes unstable, however, especially in alloys having lower precipitate volume fractions, with the result that carbide growth and y ' precipitate dissolution at transverse grain boundaries occur simultaneously during tensile creep, s'~ The nature of the mechan
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