Dislocation dynamics in the copper-tin system

Stress relaxation and strain-rate change experiments have been used to investigate the dynamics of dislocation motion in a series of copper-tin solid solution alloys. The magnitude of the thermal component of the flow stress and exponent m* of the Gilman-Johnston velocity-stress relationship were observed to vary in a complex manner with solute content and the degree of plastic strain. For dilute solid solutions the thermal and athermal components of the flow stress remain proportional and the value of m* increases continuously with plastic strain. In more concentrated solid solutions the magnitude of both m* and the thermal component are essentially constant. The results are rationalized in terms of the waiting time necessary for a dislocation to penetrate an array of random obstacles of different strengths. It is concluded that the parameters used to describe the dynamics of dislocation motion are dependent both on the strength and on the concentration of local obstacles in a manner analogous to the effect of the obstacles on the flow stress. In addition, it is concluded that the value of m* is not in general characteristic of the nature of the local obstacle but reflects the complex manner in which the time needed for a dislocation to move through an array of obstacles is divided between the time spent in the vicinity of obstacles and its free flight time. The authors are grateful to the National Research Council of Canada and the Noranda Research Centre for financial support.