| Abstract |
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This article reviews recent progress in understanding the stress-relaxation mechanisms in metal-matrix composites (MMCs) subjected to thermomechanical fatigue. Mechanical loss, dynamic shear modulus, and permanent torsional-strain measurements have been performed with forced oscillations during thermal cycling. A transient mechanical-loss maximum, which is absent in the monolithic material, appears during cooling. It has been attributed to the development of plastic zones around the reinforcements by dislocation generation and motion, which result from the differential thermal contraction of the matrix and reinforcement. This damping maximum is strongly dependent on both measurement and material parameters. The reversible shear-modulus evolution during thermal cycling suggests that no interfacial debonding occurs. In unalloyed matrices, extended thermal-stress-induced pleasicity occurs, leading to a plateau in the shear modulus, which is recovered at low temperatures by plastic-zone overlapping and matrix strain hardening. Simultaneously measured strain-temperature loops exhibit both reversible and permanent plasticity during thermal cycling (strain ratcheting). |