An analysis of flow mechanisms in high temperature creep and superplasticity

The flow of crystalline solids at elevated temperatures is dependent upon creep mechanisms associated with the movement of dislocations, the relative displacements of adjacent grains and the stress-directed flow of vacancies. It is convenient to express the steady-state creep rate in terms of the dependences of these various creep mechanisms on the applied stress, the testing temperature and the grain size of the material. In practice, however, there are similarities in some of the predicted dependences for different creep processes and this may lead to experimental difficulties in unambiguously identifying the rate-controlling creep mechanism. The difficulties in identifying the creep mechanism become especially significant at low stresses. This paper reviews the characteristics of the various flow processes occurring in simple metallic systems such as pure metals and metallic alloys and describes procedures that may be adopted to provide an unambiguous identification of the rate-controlling flow mechanism.