Influence of Dislocation Mobility on the Yield in Silicon-Iron

Abstract

This thesis presents the results of an experimental investigation of the yielding phenomenon during the transition from elastic to plastic behavior in 3 per cent silicon-iron alloy. Static load tests, constant strain-rate tests and square wave rapid loading tests were conducted on single crystal. and polycrystalline specimens in order to determine the dislocation configurations at various stages in the yielding process. An electrolytic etching technique was used to reveal dislocation intersections with the observation surface. Grain boundaries were found to be the primary source of dislocations for slip band formation in fully annealed material. The velocity of mobile dislocations was found to be an extremely sensitive function of the applied stress. A true static upper yield point was found to be consistent with theories of the upper yield point based upon dislocation kinetics. The delay-time for yielding at constant applied stress was found to depend upon the dynamics of dislocation motion, the anisotropic stress concentration, the stress concentration due to relaxation of individual grains and the Schmid factor of the least favorably oriented grains. Dislocation dynamics and dislocation configurations at various stages of plastic deformation were capable of accounting for a true upper yield point, the shape of the strain-time curve at constant stress and the Luder's band propagation velocity in polycrystalline specimens.