On the measurement of strain-rate sensitivity for deformation mechanism in conventional and ultra-fine grain alloys

Strain-rate sensitivity (SRS) of flow stress is an important parameter for deformation mechanism of materials. Definition of SRS is based on incremental changes in strain rate during tests performed at a fixed temperature and fixed microstructure, to determine corresponding changes in flow stress. In the creep literature, a steady state is assumed to be characterized by “constant” structure, but that “steady structure” varies with strain rate and temperature. Strain hardening causes structure to develop during primary creep, and also during low temperature deformation. Incremental tests for SRS minimize effects of changes in structure, temperature and strain hardening during testing, and are used to determine relation between dislocation velocity and stress at constant structure. High strain rate tests used for simulating ballistic applications incorporates multiple deformation mechanisms in a single test. The purpose of those tests has been overextended in recent times to conclude on deformation mechanism in various metals including ultra-fine grain metals. A reexamination of such work and fundamentals of deformation process show that higher SRS values than those for incremental tests are expected at high strain rate due to concurrent strain hardening.