Aspects of residual thermal stress/strain in particle reinforced metal matrix composites

The influence of inclusion geometry and thermal residual stresses and strai ns on the mechanical behaviour of a 20 vol% Al2O3 particulate reinforced 60 61-T0 Al alloy metal matrix composite is investigated through finite elemen t analysis. The introduction of residual thermal stresses/strains prior to external loading leads to a decrease of the proportional limit, 0.2% offset yield stress and the apparent stiffness. The residual stresses/strains are shown to have a greater effect on the composite behaviour under compressiv e loading than tensile loading. The residual stresses/strains have little e ffect on the cyclic behaviour of the composite. In only the second cycle, t he difference between the cyclic curves, with and without a thermal history , was 2 MPa. Use of a cube shaped particle, with sharp corners and edges, i n the unit cell model led to much greater initial hardening behaviour than spherical inclusions, and therefore a greater 0.2% offset yield stress due to stress/strain localisation at the particle corners and edges. This resul ts in regions of constrained plasticity and high stress triaxiality in the matrix around the particle, producing improved load transfer in the composi te. It is shown that inclusion aspect ratio, in the range of 0.5-2.0, has a n impact on the yield stress. A minimum yield stress occurred at an aspect ratio of approximately 0.9 with significant increases on either side of thi s point. The influence of residual stress/strain had a similar effect throu ghout the aspect ratio range except tensile loading, following thermal trea tment, on unit cells with inclusion aspect ratios greater than 1.5 resulted in the highest yield stresses. (C) 2000 Elsevier Science Ltd. All rights r eserved.