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

Abstract

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 m 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 Ϫ1 ) 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 Ϫ1/2 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 Ϫ1 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.

Constitutive description of dynamic deformation: physically-based mechanisms

“…Mecking [68] developed the conceptual linkage of the various regions within a polycrystalline aggregate and there is a considerable experimental support for his concepts. Recently, Gray et al [69] observed a higher dislocation density in the grain-boundary regions of plastically deformed brass, supporting the ideas discussed here. Meyers and Ashworth [68] considered the polycrystalline aggregate as a composite comprised of a grain interior with flow stress s fG , and the grain boundary regions with flow stress s fGB .…”

“…Nevertheless, there are other effects of great importance. Four principal factors contribute to grainboundary strengthening: (a) the grain boundaries act as barriers to plastic flow (the original pile-up mechanism [57][58][59][60]); (b) the grain boundaries act as dislocation sources [61]; (c) elastic anisotropy causes additional stresses in grain-boundary surroundings [65,66]; and (d) multislip is activated in the grain-boundary regions, whereas grain interiors are initially dominated by single slip, if properly oriented [64][65][66]69]. As a result, the regions adjoining grain boundaries harden at a rate much higher than grain interiors.…”

Shock-induced defects in bulk materials

“…The observation that the measured reload yield strength of the sample prestrained at 7 GPa lies initially below the equivalent-strain low-rate flow stress may additionally reflect a small kinematic hardening, Bauschinger effect [43], on the post-shock mechanical response. Overall, the lack of an enhanced-shock-hardening response in the Ti stresses to 5 GPa is thought to be fully consistent with the previously discussed dominant role of the Peierls stress limiting dislocation storage during shock prestraining [44]. However, when the peak shock pressure imposed exceeds the a-co phase transition, as in the 11 GPa shock case, then a greatly increased deformation twin density is generated in response to the transformation-induced local shears in the lattice.…”

Investigation in Hardsurfacing a Nickel-Copper Alloy (Monel400).

“…Since the grain size [14] also plays a role in mechanical property dependence, work hardening and strain rate effects on grain boundaries in Alloy 400 should be evaluated. The purchase specification for manufacture of this material should speci~the smallest grain size possible to obtain the minimum mechanical and weldability properties.…”