Serrated yielding in the B2-ordered Nb–15Al–20V alloy

Serrated yielding is usually caused by the interaction of mobile dislocations with solute atoms or fine precipitates and the critical strain for commencing serration is a function of strain rate and temperature. However, serrated flow in the single B2 phase Nb–15Al–20V (atomic per cent) alloy is unusual. The serration occurs immediately after yielding over the whole range of strain rate from 10−2 to 10−4/s. In this paper, these serrations in flow stress are explained as a result of the interaction of planar slip systems. When a superdislocation sweeps across antiphase domain boundaries, additional antiphase boundary (APB) is left on the slip plane. An easy-slip channel is then created for the following superdislocations. Planar slip is thus preferred. If a planar slip band is intersected by another planar slip band, the existing planar slip channel will be destroyed. Superdislocations will then be temporarily trapped at the intersection. To maintain a constant strain rate, an increase of applied stress to release these trapped superdislocations by creating a new slip channel is therefore necessary. This repeated trapping and un-trapping of superdislocations leads to microstructural instability and serrated yielding.