Anisotropic martensitic transformation in B2-CuZr-structured crystallites

The remarkable mechanical properties achieved in CuZr-based complex alloys consisting of, or containing, austenitic B2-CuZr-structured crystallites rely on the B2→B19′ martensitic transformation (MT), which results in the well-known transformation-induced plasticity (TRIP) and shape memory effects. However, owing to the anisotropic nature of crystals, the MT and, in turn, the transformation-induced mechanical behaviors essentially depend on the crystal orientation of B2-CuZr, which remains rarely known in the literature. Here, we present a systematic investigation on the crystallographic orientation-dependent MT in B2-CuZr-structured single crystals using micropillar compression tests. Strong anisotropy is revealed, in which, along loading axis, a near-<100>-oriented B2 tends to fracture without MT, whereas a near-<011>/<111>-oriented B2 is prone to MT which leads to extensive plasticity. A critical resolved shear stress (CRSS) is thus determined for the first of its kind, demonstrating that the onset of MT is stress-controlled via a {011}<100>_B2 lattice shear in which {011}_B2 is the shear plane and <100>_B2 is the shear direction. On this basis and using an Eshelby theory-based analysis of triaxial stress fields, we further reveal the anisotropic MT behavior of micrometer-sized monocrystalline B2-CuZr spherulites when embedded in a metallic glass matrix; we show the dominant effect of anisotropic MT on the shear-band/microcrack propagation that eventually leads to fracture in such a B2-containing composite material. Our results shed light on the mechanisms of anisotropic MT with or without matrix confinement, and provide a new perspective for understanding the TRIP-enhanced mechanical properties as well as shape memory/superelastic behaviors of B2-structured or B2-containing complex alloys. © 2025 Acta Materialia Inc.