The role of impurities during creep and superplasticity at very low stresses

Abstract

It is well documented that impurities play an important role in the deformation and fracture of polycrystalline materials. For example, the results of a number of studies have demonstrated that the presence of a very small of amount of impurities in polycrystalline materials can explain many phenomena such as temper embrittlement in steels, creep embrittlement, and enhancement of ductility in the intermetallic compound Ni3Al. This article reviews the details of two high-temperature deformation phenomena whose characteristics are, according to very recent experimental evidence, influenced or controlled by impurities. The first phenomenon, micrograin superplasticity, deals with the ability of fine-grained materials (d<10 µm, where d is the grain size) to exhibit extensive neck-free elongations during deformation at elevated temperatures above 0.5 Tm, where Tm is the melting point. The second phenomenon, Harper-Dom creep, refers to the anomalous creep behavior of large-grained materials at very low stresses and temperatures near the melting point. It is shown that while these two phenomena are different in terms of the conditions of occurrence and the characteristics of deformation, they share three common features: (1) stresses applied to produce deformation are very small; (2) impurities control the deformation characteristics such as the shape of the creep curve, the value of the stress exponent, and the details of the substructure; and (3) boundaries play a key role during deformation.