Metastable phases and defect microstructures in melt-spun ribbons of Nb3Al

This paper reports on the phases and defects produced in a binary alloy, with nominal composition Nb-19at.% Al, studied using transmission electron microscopy (TEM). In addition to the equilibrium A15 phase (Nb3Al) a previously unreported Nb-rich B2 phase is present. A significant number of grow-in dislocations were observed in the A15 phase, these have b=a <100> and are predominantly screw on character. They are dissociated into two partial dislocations which bound a stacking fault according to the reaction: left-angle 100 right-angle → 1/2 left-angle 100 right-angle + SF + 1/2 left-angle 100 right-angle. The habit plane of the fault is shown to be (012). These observations are compared with previous studies performed on the isostructual compound V3Si. The consequences of the observed issociation for the plastic deformation of Nb3Al are discussed