Frictional shear stress-induced incomplete martensitic transformation in mono-crystalline B2-CuZr spherulites

An austenitic B2-CuZr phase, when deformed, can undergo a martensitic transformation (MT) that produces a martensitic B19′-CuZr phase, resulting in remarkable transformation-induced plasticity (TRIP) and shape memory effects in B2-structured alloys. However, the shear mechanism as well as the crystallographic characteristics, which underlie the MT and, in turn, the TRIP and shape memory effects, remain unclear and somewhat controversial. Here we report that a single shear on {110} _B2<001> _B2, along with {1 1 5.5} _B2 being the habit plane, are responsible for the MT in B2-CuZr, where {110} _B2 and <001> _B2 are the shear plane and the shear direction, respectively. These were revealed by determining unambiguously the orientation relationship between B2 and B19′, co-existing in individual spherulites contained in a B2-CuZr-containing bulk metal glass-matrix composite (BMGC). This exotic microstructure, featuring a core-shell like distribution in each single spherulite in which all the 12 possible martensite (B19′) variants constitute the core and the parent B2 forms the shell, was made from an incomplete MT in originally mono-crystalline B2 spherulites of the as-cast BMGC when subjected to moderately applied frictional shear stresses upon mechanical grinding. Finite element analysis suggests that a core-shell like shear stress distribution inside a B2 spherulite, caused by the confinement of the stiffer glass matrix, may account for it. These results provide direct experimental evidence to elucidate the B2→B19′ MT and its variant selections under various stress states, and pave the way for developing ductile B2-structured alloys, including but not limited to, shape memory intermetallics, high entropy alloys and B2-containing composite materials.