A UNIFIED DETERMINATION OF CREEP AND STRAIN-RATE SENSITIVITY OF POLYCRYSTALS FROM THE PROPERTIES OF CONSTITUENT GRAINS

On the basis of the observation that both creep and plasticity are fun damentally rate processes, a unified micromechanical constitutive theo ry is developed to predict both the strain rate sensitivity and the ti me-dependent creep of a polycrystal. The theory is established within the small-strain range for power-law creep and can provide the evoluti on of the microstress-strain relation and creep strain of the constitu ent grains. The distinctly different responses of a constituent grain and a free crystal of the same orientation are also examined under bot h constant-strain-rate and constant-stress creep conditions. It is als o demonstrated that the higher creep rate associated with a higher str ess automatically translates into the commonly observed strain rate ef fect on the stress-strain curves of polycrystals and that under a cons tant total strain rate the stress-strain curve would lead to a saturat ion stress. When applied to a 304 stainless steel under a thermal cycl ing between 600 and 650-degrees-C, the theory also yields reasonably a ccurate creep strains for the polycrystal. Its predictive capability i s 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.