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

摘要

The influence of inclusion geometry and thermal residual stresses and strains on the mechanical behaviour of a 20 vol/100 Al_2O_3 particulate reinforced 606l-TO Al alloy metal matrix composite is investigated through finite element analysis. The introduction of residual thermal stresses/strains prior to external loading leads to a decrease of the proportional limit, 0.2/100 offset yield stress and the apparent stiffness. The residual stresses/strains are shown to have a greater effect on the composite behaviour under compressive loading than tensile loading. The residual stresses/strains have little effect on the cyclic behaviour of the composite. In only the second cycle, the 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, in the unit cell model led to much greater initial hardening behaviour than spherical inclusions, and therefore a greater 0.2/100 offset yield stress due to stress/strain localisation at the particle corners and edges. This results in regions of constrained plasticity and high stress triaxiality in the matrix around the particle, producing improved load transfer in the composite. It is shown that inclusion aspect ratio, in the range of 0.5-2.0, has an 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 this point. The influence of residual stress/strain had a similar effect throughout the aspect ratio range except tensile loading, following thermal treatment, on unit cells with inclusion aspect ratios greater than l.5 resulted in the highest yield stresses.