Thermal stress relaxation in Mg composites studied by non-destructive methods

This paper describes changes in the microstructure of Mg composites determi ned by non-destructive methods: dilatometry and acoustic emission during th ermal cycling and internal friction measurements after thermal cycling and heat treatment. Thermal cycles between room and an upper temperature (up to 300 degrees C) were performed. The dimensional changes and the acoustic em ission activity during thermal cycling indicate the creation and motion of dislocations. The strain dependence of the logarithmic decrement measured a fter thermal cycling can be divided in two regions. A region of strain inde pendent damping is followed by a region with a strongly strain dependent da mping. The values of the logarithmic decrement in the strain dependent regi on depend strongly on the upper temperature of thermal cycle. The observed experimental data can be explained assuming that new dislocations are creat ed during thermal cycling and that therefore, the microstructure of the com posites changes.