A continuum polycrystal plasticity model was used to estimate the influence of a threshold stress for grain boundary sliding on the relationship between macroscopic flow stress and strain rate for the aluminium alloy AA5083 when subjected to plane strain uniaxial tension at 450C. Under these conditions, AA5083 deformed by dislocation glide at strain rates exceeding 0.001/s, and by grain boundary sliding at lower strain rates. The stress–strain rate response could be approximated by strain-rate = Aσn, where A and n depended upon the grain size and strain rate. It was found that a threshold stress less or equal to 4MPa had only a small influence upon flow stress and stress exponent n in the dislocation creep regime (a threshold stress of 2MPa increased n from 4.2 to 4.5), but substantially increased both flow stress and stress exponent in the grain boundary sliding regime (a threshold stress of 2MPa increased n from 1.5 to 2.7). In addition, when the threshold stress was included, the model predicted stress versus strain-rate behavior that was in good agreement with experimental measurements.

The Influence of a Threshold Stress for Grain Boundary Sliding on Constitutive Response of Polycrystalline Al During High Temperature Deformation. N.Du, A.F.Bower, P.E.Krajewski, E.M.Taleff: Materials Science and Engineering A, 2008, 494[1-2], 86-91