SiC粒子/7075Al合金複合材料の高温圧縮における応力-ひずみ速度特性に及ぼすSiC粒子体積率の影響

Abstract

The effect of SiC particulate volume fraction on hot deformation of a mechanically alloyed 7075Al alloy (MA7075) and three 7075Al alloy composites reinforced with SiC particulate (MA7075+5SiC, +10SiC, +15SiC composites) has been studied by the compression test. The samples were compressed to about 65% at a temperature of 793 K and in an initial strain rate range from 10−3 s−1 to 2×10 s−1.The strain rate sensitivity exponent m was larger than 0.33 at high strain rates (Region 2) and, in contrast, less than 0.13 at low strain rates (Region 1). Grain boundary sliding occurred more remarkably in Region 2. With increasing of the SiC particulate content, the transition in strain rate between Region 1 and Region 2 moved to be higher, and the compressive stress increased in the whole strain rate range. The increase in flow stress was more remarkably in Region 1.Threshold stresses (σth) for each composites increased with increasing of the SiC particulate content. A double logarithmic plot of strain rate and effective stress, σe, (=σ−σth) can be approximated by a straight line with a slope of 0.5 irrespective of Regions 1 and 2 for MA7075+SiC composites. This strain rate sensitivity exponent of 0.5 is explained by the value predicted from the grain boundary sliding model accommodated by dislocation slip.It was suggested that the presence of SiC particulates on grain boundaries seemed to resist the grain boundary sliding, thus resulting the threshold stress. The transition from region 1 to region 2 was considered to be not necessarily reflect the change in deformation process but to be arose from the effect of the threshold stress on the superplastic model with grain boundary sliding descrived above.