Determination of the dislocation-based mechanism(s) responsible for the anomalous ductility of a class of B2 intermetallic alloys

Abstract

A class of stoichiometric, ordered MR alloys (where M denotes a metal and R either a rare earth or group IV refractory metal) with B2 (CsCl) crystal structure, were discovered to possess anomalous ductility by researchers at Ames Laboratory in the early 2000’s. Despite active research on the topic for over 15 years, a holistic explanation for the anomalous ductility of these metal alloys has not yet been accepted by the scientific community. The reason for this appears to be a failure to account for all relevant length scales. Researchers either focus on the atomistic length scale relevant to dislocation core structures, the microscopic length scale of dislocations and dislocation ensembles, the mesoscopic length scale of grains and their interactions, or the macroscopic scale of mechanical testing samples with millimetre dimensions or larger. Focused studies at each of these length scales have provided essential, but insufficient information to provide a complete answer to the question. The insight provided by in-situ diffraction studies, along with crystal plasticity modelling as an interpretive tool, and a novel grain-by-grain line profile analysis technique for assessing dislocation densities in polycrystals is highlighted. In addition to the primary <100> dislocations, which are most easily activated in all of these anomalous MR alloys, the results show that dislocations with large Burgers vectors (e.g., <110> and <111>) are present in large numbers beyond a transition in the strain hardening response. The motion of these large Burgers vector dislocations is experimentally observed to occur at stress levels consistent with atomistic modelling predictions of the Peierls resistance and simultaneously provides a satisfying explanation for the anomalous ductility and other experimental results.