Study of relaxation of residual internal stress in Mg composites by internal friction

When a metal matrix composite is subjected to temperature changes, thermal stresses arise at the interfaces between the matrix and the reinforcement as a result of the considerable mismatch between the thermal expansion coefficients of the matrix and the reinforcement. Even moderate temperature changes can produce thermal stresses that exceed the matrix yield stress. Consequently, new dislocations are generated at the interfaces causing thermal fatigue (microstructural changes, matrix plastic deformation and irreversible shape changes). The microstructural changes in the matrix can be detected by damping and stress relaxation measurements. The logarithmic decrement and modulus defect were measured after thermal treatment at temperatures between room temperature and temperatures up to 400degreesC. The strain dependence of the logarithmic decrement can be divided into two regions. The values of the logarithmic decrement are also influenced by heat treatment and foreign atoms in the matrix. The results can be explained assuming that straining and thermomechanical treatment produce changes in the micro structure of the composites. The stress relaxation curves were analysed and the modulus defect, relaxation strength and activation energy for dislocation motion were estimated. It is very difficult to draw conclusions on the mechanisms responsible for the stress relaxation but part of the relaxation strength is due to reversible dislocation movement. (C) 2002 Elsevier Science B.V. All rights reserved.