Grain Boundary Sliding and Strain Rate Sensitivity of Coarse and Fine/Ultrafine Grained 5083 Aluminum Alloys
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ULTRAFINE grained (UFG) metals or alloys generally exhibit a substantially higher flow stress than their coarse grained (CG) counterparts, at room temperature. Their fracture strain is most often lower than that of CG materials at room temperature. However, it tends to increase as the strain rate decreases due to of a modification in the plastic strain distribution leading to a lot of diffuse—and thus harmless—micro-bands, instead of a few sharp and intense shear bands observed at higher rates.[1–3] Regarding the high-temperature behavior of these materials, most investigations focused on the superplastic regime, characterized by a strain rate sensitivity (SRS) close to 0.5, which is controlled by grain boundary sliding (GBS)[4,5] and is observed at comparatively lower temperatures than in CG materials.[6–9] The intermediate range between room temperature and
A. GOYAL and V. DOQUET are with the Laboratoire de Me´canique des Solides, UMR 7659 CNRS, Ecole Polytechnique, 91128, Palaiseau, France. Contact e-mail: [email protected] A. POUYA is with the Laboratoire Navier (IFSTTAR, CNRS, ENPC), Paris-Est University, 77455 Champs sur Marne, France. Manuscript submitted August 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
superplastic regime has, comparatively, received much less attention. However, it has been shown by Kapoor et al.[10] that above a strain rate-dependent temperature, instead of GBS-induced hardening, GBS-induced softening occurs in Al-1.5Mg,. Since many potential applications of UFG alloys involve heating above ambient temperature, it is necessary to determine the temperature and strain rate domains in which a beneficial effect of the ultrafine microstructure can be expected. This is one of the purposes of this study on Al 5083 alloy. Since the evolutions in ductility with strain rate mentioned above seem to correlate with the degree of strain localization, and thus to the SRS, this parameter was measured as a function of temperature and strain rate, through stress relaxation tests in CG and UFG Al 5083 alloys. The plastic strain fields in these materials were measured by digital image correlation (DIC) before and after tensile tests run at room or high temperature. The discontinuities induced by GBS along the micro-grids laid on the surface for digital image correlation (DIC) were used to estimate the contribution of GBS to the overall deformation, which appeared to explain most of the temperature and strain rate-dependence of the SRS. In the seventies, Raj and Ashby[11] proposed an analytical model of grain boundary sliding accommodated either elastically, or by diffusion. They formulated a linear, threshold-free relation between the GB sliding
rate and the resolved shear stress, in which viscosity increased with the roughness of the GB, and with the presence of second phase particles along its length. Ashby and Verral[12] introduced a GBS-induced neighboring grains switching mechanism, without any elongation or change in crystal orientation. They predicted a grain size-dependent evol
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