Dislocation loops in single-crystal NiAl

Abstract

In the conventional viewpoint, dislocation loops of vacancy type in single crystal NiAl can only exist in Al-rich compositions. Their configurations and their changes when deformation occurs, and their effects on the deformation have not been studied thoroughly.In the present study, nickel-rich NiAl single crystals of 48.1∼48.6 at %A1 were annealed at 1315°C for 50 hours, then cooled at 200°C/hr to permit vacancy condensation. Single-crystal samples cut with an [001] axis then underwent high temperature (850°C) tensile and cyclic deformation. TEM specimens were cut from the undeformed, 0.3% tensile strain, ±0.5 cyclic and 30∼40% tensile deformation samples.Two kinds of dislocations, edge and screw, were found in the undeformed NiAl. Edge dislocations consisted of concentric dislocation loops (Fig. 1) and spiral dislocations (dislocation A in Fig. 2). They lie on cubic planes and have cubic line orientations. Invisibility conditions of g.b=0 and g.b x u=0 show that they have <001> type Burgers vectors normal to the loop planes. The shift in image position on changing the sign of g was used to verify that the loops are vacancy in nature.