A unified determination of creep and strain rate sensitivity of polycrystals from the properties of constituent grains

Abstract

On the basis of the observation that both creep and plasticity are fundamentally rate processes, a unified micromechanical constitutive theory is developed to predict both strain rate sensitivity and the time-dependent creep of polycrystal. The theory is established within the small-strain range for power-law creep and can provide the evolution of the microstress-strain relation and creep strain of the constituent grains. The distinctly different responses of a constituent grain and a free crystal of the same orientation are also examined under both constant-strain-rate and constant-stress creep conditions. It is also demonstrated that the higher creep rate associated with a higher stress automatically translates into the commonly observed strain rate effect on the stress-strain curves of polycrystals and that under a constant total strain rate the stress-strain curve would lead to a saturation stress. When applied to a 304 stainless steel under a thermal cycling between 600 and 650°C, the theory also yields reasonably accurate creep strains for the polycrystal. Its predictive capability is further confirmed by comparison with experiments on the development of creep strain and the strain rate effect on the stress-strain curves for this material.