Stress-Induced Two-Step Phase Transformation in NiTiCu Alloys

Deformation behavior of NiTiCu alloys with Cu levels of 5, 10, and 20 at.% was studied by in-situ synchrotron X-ray diffraction tensile testing. It was found that residual work from cold-forming and stress-relieving processes promotes stress-induced B2→B19→B19′ two-step phase transformation in NiTiCu alloys due to its strong effects on material strength and martensite phase stabilities. Regardless of their wide range of Cu-content, all alloys experienced reversible two-step phase transformation during deformation, which produced large recoverable strains upon unloading. The second-phase transformation happened after B19 martensite already experienced severe lattice distortion, which accommodated most of the transformation strain and resulted in a relatively smoother transition to B19′. All alloys displayed similar texture evolution during the two-step transformation: from a strong near-{111}_B2 fiber to a near-{103}_B19 and finally a strong near-{13¯0}_B19′ textures following their crystal orientation relationship. Despite the negative effect of Cu on B2→B19 transformation strain, the total transformation strain appears approximately constant across Cu-contents, giving all three alloys the capability of large strain recovery. © ASM International 2025.