Implications from pre-straining experiments on emerging kink-based models for anomalous flow in L1₂ alloys

Abstract

We re-examine an experiment on the transient behaviour of Ni₃Al which demonstrates that the anomalous flow stress is thermally reversible in the small-strain yielding regime. Further, we discuss the phenomenon within emerging kink-based models for the flow-stress anomaly in an effort to highlight critical aspects of these models which need further attention. Specimens from a Ni-22.9 at. % Al single crystal, having an orientation near (001), were pre-strained at 873 K and additionally strained at 300 K. The flow properties of this crystal were compared to those from a crystal strained at 300 K alone. Both crystals were examined using transmission electron microscopy. In the small-strain flow regime, the flow stress anomaly is reversible. Pre-straining at a higher temperature before room-temperature deformation shows little memory of the pre-straining during the yield transient, even though TEM observations show that many Kear-Wilsdorf locks are retained in the substructure. Microscopic examination also reveals the presence of a second dislocation structure, characteristic of the low-temperature deformation, in selected regions of thin foils from the pre-strained sample. This observation demands that some part of the substructure recovers extremely rapidly, if not instantly upon changing temperature and (or) unloading. We suggest that future models must consider the full behaviour of all dislocation loop characters and character junctions in accomplishing the thermally reversible transition from the elastic regime, through the microstrain regime, to flow.