Influence of grain size on the constitutive response and substructure evolution of MONEL 400

The influence of grain size on the constitutive behavior (strain-rate and temperature dependence of the yield stress and strain hardening) and substructure evolution of MONEL 400 was investigated. Increasing the grain size from 9.5 to 202 μm was seen to reduce the quasi-static yield strength from 290 to 115 MPa, while having a minimal effect on the work-hardening response. Increasing the strain rate from quasi-static to dynamic strain rates (3000 s^-1) was seen to increase the yield and overall flow-stress levels, but has no effect on the strong grain-size dependency exhibited by this alloy. The persistent influence of grain size to large strains is inconsistent with previous d^{- 1/2} pileup grain-size modeling in the literature, which predicts convergence at large strains. Substructure evolution differences between the grain interiors and adjacent to grain boundaries supports differential defect storage processes which are consistent with previously published work-hardening d^-1 modeling arguments for grain size-dependent strengthening in polycrystals. The integration of grain-size dependency into constitutive modeling using the mechanical threshold stress (MTS) model is discussed. The MTS model is shown to provide a robust constitutive description capturing yielding, large-strain work hardening, and grain-size effects simultaneously. The MTS model is, additionally, shown to satisfactorily address the experimentally observed transients due to strain-rate or temperature-path dependency.