Deformation mechanisms and defect structures in Heusler intermetallic MnCu2Al

Intermetallics with crystals derived from body-centered cubic structures are promising materials to enhance the performance and high temperature capability of high-strength alloys both as matrix materials and as strengthening precipitates. Given the ordered nature of these materials, the defect structures that ultimately mediate plasticity can be complex and strongly dependent on processing vis-à-vis the extent of disorder inherited from high temperature phases. Here, we elucidate the elementary characteristics of the defect structures in the Heusler (L21) intermetallic MnCu2Al as investigated by the orientation-dependent strength and compressive plasticity measured from in situ micro-compression experiments and post-mortem transmission electron microscopy. Our experiments reveal single crystal compressive yield strengths as high as 1.2 GPa and a capacity for stable plastic deformation, accompanied by slip characteristics reminiscent of bcc-derived crystals. We study the equilibrium dissociation mechanisms and critical stresses of 〈111〉{110}-type dislocations in the Heusler intermetallic using an ab initio informed phase field dislocation dynamics model. The calculations suggest that the dislocation dissociates into multiple distinct partials, which depend on the character of the dislocation, and are closely spaced to a degree that intervening faults are undetectable using conventional transmission electron microscopy. Critical stresses to initiate glide of these defect structures quantitatively agree with measured yield strengths. Our work details the interplay between the significant degree of elastic anisotropy, stacking fault energies, and glissile defect structures governing mechanical properties and motivates an increased awareness of the role that complex defect structures play for alloy design involving bcc-derived intermetallics.