Experimental measurements are used to characterize the anisotropy of flow stress in extruded magnesium alloy AZ31 sheet during uniaxial tension tests at temperatures between 350°C and 450°C, and strain rates ranging from 10 -5 to 10 -2 s -1 . The sheet exhibits lower flow stress and higher tensile ductility when loaded with the tensile axis perpendicular to the extrusion direction compared to when it is loaded parallel to the extrusion direction. This anisotropy is found to be grain size, strain rate, and temperature dependent, but is only weakly dependent on texture. A microstructure based model (D. E. Cipoletti, A. F. Bower, P. E. Krajewski, Scr. Mater., 64 (2011) 931–934) is used to explain the origin of the anisotropic behavior. In contrast to room temperature behavior, where anisotropy is principally a consequence of the low resistance to slip on the basal slip system, elevated temperature anisotropy is found to be caused by the grain structure of extruded sheet. The grains are elongated parallel to the extrusion direction, leading to a lower effective grain size perpendicular to the extrusion direction. As a result, grain boundary sliding occurs more readily if the material is loaded perpendicular to the extrusion direction.

We show that the variation of flow stress with strain rate and grain size in a magnesium alloy deformed at a constant strain rate and 450 degrees C can be predicted by a crystal plasticity model that includes grain boundary sliding and diffusion. The model predicts the grain size dependence of the critical strain rate that will cause a transition in deformation mechanism from dislocation creep to grain boundary sliding, and yields estimates for grain boundary fluidity and diffusivity. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Continuum finite element simulations are used to investigate the influence of heterogeneity in grain boundary sliding resistance on the creep response of aluminum alloy AA5083 when deformed at 450°C. Previous simulations and experiments have demonstrated that under these conditions, grain boundary sliding (GBS) is the dominant deformation mechanism at strain rates below 0.001s−1, and dislocation creep (DC) is the dominant mechanism for higher strain rates. These simulations assumed a uniform resistance to sliding on all grain boundaries. Molecular dynamic simulations indicate that sliding resistance is strongly sensitive to the character of the boundary: high angle boundaries have resistance up to an order of magnitude lower than low angle boundaries. To investigate the effects of this heterogeneity, finite element simulations are used to compute the influence of the fraction of freely sliding boundaries f in a polycrystal on its creep response and operative deformation mechanisms. Our computations show that (i) The critical strain rate at which the deformation mechanism transitions from GBS to DC varies from 2×10−5s−1 to 10−3s−1 as f is increased from 20% to 100%. (ii) The stress exponent in the GBS regime decreases from approximately 3.25 to 1.5 as f increases from 0% to 100%. The stress exponent in the DC regime is less sensitive. (iii) The flow stress at a strain rate of 10−4s−1 (GBS regime) increases from 6MPa to 14MPa as f varies from 100% to 0%; in contrast the flow stress at strain rate of 0.01s−1 increases from 33MPa to 42MPa (DC regime). (iv) A sudden increase in GBS and a corresponding reduction in flow stress occur as f is increased from 67% to 77%, suggesting the presence of a percolation threshold. (v) A second (but smaller) increase in grain boundary sliding occurs when f is increased from 39% to 46%. Microstructures with f=39% appear to contain at least one grain which is completely surrounded by sliding resistant boundaries; microstructures with f=46% do not. We speculate that the blocked grains may act as reinforcing particles. (vi) The flow stress and deformation mechanism are determined by the fraction of freely sliding grains, and are not sensitive to the detailed spatial distribution of sliding resistance in the polycrystal.