Martensitic transformation and shape memory effect in NiGaCr polycrystalline alloys

Body-centered cubic (BCC)-derived ordered alloys show a potential utilization as SME alloys. Two kinds of B2-based alloys, Ni 50-x Ga 50-x Cr 2x (x=0-6, ENG group) and Ni 50-x (Ni 15-0.3x Ga 35-0.7x )Cr 2x (x=0-6, NENG group), were designed based on the Ni-Ga-Cr ternary phase diagram and the related composition-dependent VEC (valence electron concentration) diagram. The microstructure, crystal structure, and phase transformation of the NiGaCr alloys were systematically investigated through FE-SEM (field-emission scanning electron microscopy), FIB (focused-ion-beam) tomography, TEM (transmission electron microscopy), and DSC (differential scanning calorimetry). Martensitic microstructures were observed in the Ni-rich NENG group rather than the ENG group. In Cr-free alloy (NENG0), single-stage , martensitic phase transformation accompanied by brittle fracture behavior was observed. Contrastingly, Cr-containing alloys (NENG4, NENG8, NENG12) demonstrated a two-step martensitic transformation ( ) and enabled successful fabrication of polycrystalline strips suitable for shape memory effect (SME) evaluation. The martensitic transformation temperature (Ms) was found to correlate positively with the VEC value. The ω phase was found to precipitate within the B2 matrix in NENG4, NENG8, and NENG12 alloys, impeding the martensitic transformation. Additionally, the microstructural characterization revealed a ∼20 nm-scale metastable chromium-enriched phase in the NENG12 alloy. This work establishes the first experimental evidence of temperature-induced shape recovery in NiGaCr alloys, elucidating the chemical composition design for SME alloy using the VEC rule.