MECHANISMS OF CYCLIC STRAIN-HARDENING IN NI3AL+B SINGLE-CRYSTALS

The evolution of dislocation substructures and their correlation with stress response in Ni3Al + B single crystals fatigued at room temperature has been studied. Fatigue was conducted at total-strain amplitudes of 0.05-0.2%. Cyclic strain hardening and a tension/compression flow stress asymmetry were observed. The magnitude of stress asymmetry was found to depend on the applied cyclic strain. A dislocation structure composed of jogged superdislocations and superdislocation dipoles was observed. The dislocation dipoles were mainly formed by nonconservative motion of jogged superdislocations. Dragging of jogs, interaction between dislocations, and impedance of dislocation motion by dislocation dipoles (point defect clusters) are considered to be the major contributors to cyclic strain hardening in N3Al + B single crystals. The separation between superpartial dislocations of a paired superdislocation was found to fluctuate away from the equilibrium spacing during cyclic straining. The extent of the fluctuation became more pronounced as the applied cyclic strain increased. This phenomenon was intimately related to the cyclic-strain dependence of tension/compression flow stress asymmetry found in fatigued Ni3Al + B single crystals.