MICROMECHANICS EFFECTS IN CREEP OF METAL-MATRIX COMPOSITES

The creep of metal-matrix composites is analyzed by finite element tec hniques. An axisymmetric unit-cell model with spherical reinforcing pa rticles is used. Parameters appropriate to TIC particles in a precipit ation-hardened (2219) Al matrix are chosen. The effects of matrix plas ticity and residual stresses on the creep of the composite are calcula ted. We confirm (1) that the steady-state rate is independent of the p article elastic moduli and the matrix elastic and plastic properties, (2) that the ratio of composite to matrix steady-state rates depends o nly on the volume fraction and geometry of the reinforcing phase, and (3) that this ratio can be determined from a calculation of the stress -strain relation for the geometrically identical composite (same phase volume and geometry) with rigid particles in the appropriate power-la w hardening matrix. The values of steady-state creep are compared to e xperimental ones (Krajewski et al.). Continuum mechanics predictions g ive a larger reduction of the composite creep relative to the unreinfo rced material than measured, suggesting that the effective creep rate of the matrix is larger than in unreinforced precipitation-hardened Al due to changes in microstructure, dislocation density, or creep mecha nism. Changes in matrix creep properties are also suggested by the com parison of calculated and measured creep strain rates in the primary c reep regime, where significantly different time dependencies are found . It is found that creep calculations performed for a time-independent matrix creep law can be transformed to obtain the creep for a time-de pendent creep law.