Structural and mechanical aspects of high temperature deformation of polycrystalline nickel

Abstract

The mechanisms of dynamic recrystallisation of nickel have been analysed by carrying out compression tests over a wide range of temperatures and strain rates and by examining the microstructures using optical and transmission electron microscopy. Under all deformation conditions, the true stress–true strain (σ–ɛ) curves display a typical maximum corresponding to dynamic recrystallisation, followed by a steady state flow. A correlation is shown to exist between the global strain hardening rate during high temperature straining (region of dynamic recrystallisation) and the microstructural features. Before the flow stress peak, commonly assumed to be the result of dynamic recrystallisation, two stages of strain hardening are observed, corresponding to a homogeneous (the higher hardening rate) and a heterogeneous mode of deformation, respectively. It is demonstrated that observable strain localisation takes place through the conversion of deformation bands from micro- to macrodimensions, a process which results in the global softening of nickel at high temperatures. This process leads to the formation and migration of recrystallisation fronts within periodically passive bands of localised deformation.

MST/1494