DISLOCATION-STRUCTURES AND MECHANISMS OF STRAIN-HARDENING IN CYCLICALLY DEFORMED 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%. H loops showing cyclic strain hardening and tension/compression flow stress asymmetry were recorded. The magnitude of stress asymmetry was dependent on the applied cyclic strain. A dislocation structure composed of jogged superdislocations and superdislocation dipoles was observed. The dislocation dipoles were mainly formed by non-conservative motion of jogged superdislocations during the cyclic hardening stage. The dragging of jogs, the interaction between dislocations and the impedance of dislocation motion by dislocation dipoles (point defect clusters) are the major contributors to cyclic strain hardening in Ni3Al + B single crystals. The separation between superpartial dislocations in a paired super-dislocation was observed to fluctuate away from the equilibrium spacing during cyclic straining. The extent of the fluctuation became even more pronounced as the applied cyclic strain increased. This phenomenon may explain the cyclic strain dependence of tension-compression flow stress asymmetry found in fatigued Ni3Al + B single crystals.