The influence of heterogeneity in grain boundary sliding resistance on the constitutive behavior of AA5083 during high-temperature deformation

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 degrees C. Previous simulations and experiments have demonstrated that under these conditions, grain boundary sliding (GBS) is the dominant deformation mechanism at strain rates below 0.001 s(-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 x 10(-5) s(-1) to 10(-3) s(-1) as f is increased from 20% to 100%. (d) 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(-4) s(-1) (GBS regime) increases from 6 MPa to 14 MPa as f varies from 100% to 0%; in contrast the flow stress at strain rate of 0.01 s(-1) increases from 33 MPa to 42 MPa (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. (C) 2008 Elsevier B.V. All rights reserved.