Interfacial structure and strain accommodation in two-phase NbCo1.2Sn Heusler intermetallics

Abstract

Multiphasic intermetallic materials are attractive candidates for functional applications where the constituent phases alone do not meet the property targets. The interfaces at the phase boundaries in these materials are central to their macroscopic properties, with the details of the atomic structure underpinning local thermomechanical, electrical, and magnetic behavior. Here, we characterize the interface structure in the biphasic Nb-Co-Sn system consisting of $\mathrm{Nb}{\mathrm{Co}}_{2}\mathrm{Sn}$ full Heusler (FH) precipitates embedded in a NbCoSn half Heusler (HH) matrix, which exhibits semicoherent interfaces with a 3.3% lattice parameter misfit that determines the constitution of the FH/HH interface on the nanometer scale. We use detailed transmission electron microscopy (TEM) and atomistic calculations to precisely determine the dislocation content and structure at the semicoherent interface, which modulates the strain fields in a semiregular pattern. We find that the interface forms regularly spaced paired partial dislocations with a joint Burgers vector of $a/2\ensuremath{\langle}110\ensuremath{\rangle}$, favored by misfit energy relief and chemical ordering. The interface exhibits numerous interface steps (disconnections) which in turn determine the precipitate morphology. Overall, the two phases show full coherency except in the vicinity of the misfit dislocation cores located every 11 nm, leading to a modulated strain field parallel to the interface, and no long-range strain fields as the interfacial dislocations accommodate the misfit strain.