Experimental Evidences of Thermo-Elasto-Plastic Strain Incompatibilities in Microheterogeneous Metal Based Materials

Abstract

Microheterogeneous materials exhibit homogeneous macroscopic properties, but contain small volumes (size of the order of several micrometers) whose properties differ from those of the matrix. As a consequence, in metal based microheterogeneous materials such as metal matrix composites (containing reinforcing ceramic particles) or metallic alloys (containing coarse precipitates), strain incompatibilities arise during the thermo-elasto-plastic loading of the material. For example, the thermal expansion mismatch between matrix and particles induces a thermal stress field when the system is cooled. This stress field may be partially relaxed if plastic strain of the matrix occurs around the reinforcement. The corresponding thermally induced dislocations (TID) lead to a strong forest effect and contribute to the strengthening observed for the composite compared to the unreinforced material [1]. Several models have been proposed in the past for describing this process [2]. They are based on the assumption that the emitted dislocations are distributed randomly in the matrix, i.e. that the yield properties of the matrix are homogeneous. The present work aims at giving a new insight into this phenomenon on the experimental point of view. Firstly, thermal treatments during in situ TEM experiments show the emission and the motion of dislocations from the interface and through the matrix.