Martensitic phase transformation and shape memory properties of the as-cast NiCuTiHf and NiCuTiHfZr alloys

•Transformation enthalpy in the NixTi85‐xHf5Zr5Cu5 alloys decreased by increasing Ni content from 44.8 to 45.5 at.%.•Substituting 5 at.% Zr for Ti in the Ni44.8Ti45.2Hf5Cu5 alloy lowered martensitic transformation temperatures.•As-cast Ni44.8Cu5Ti45.2Hf5 and Ni44.8Cu5Ti40.2Hf5Zr5 alloys showed shape memory strains of 6.25% and 5%, respectively. This study investigates the structure, martensitic phase transformation behavior, and shape memory properties of the quaternary Ni44.8Cu5Ti45.2Hf5 and quinary Ni44.8Cu5Ti40.2Hf5Zr5 and Ni45.5Cu5Ti39.5Hf5Zr5 shape memory alloys (SMAs) in the as-cast state. The alloys exhibited a dual-phase microstructure composed of matrix phase (austenite or martensite depending on the chemical composition) and Ti2Ni-type phase. The experimental results revealed that the alloys underwent a one-step B2↔B19′ transformation. The Ni44.8Cu5Ti45.2Hf5 alloy showed a maximum shape memory strain of 6.2% with a recovery ratio of 81% upon thermal cycling under a tensile stress of 300 MPa, while in the Ni44.8Cu5Ti40.2Hf5Zr5 alloy, a maximum shape memory strain of about 5% was obtained under 500 MPa with a recovery ratio of 86%. The as-cast Ni44.8Cu5Ti45.2Hf5 alloy showed superelastic response with 0.3% maximum transformation strain at 125℃. On the other hand, the Ni44.8Cu5Ti45.2Hf5 alloy showed the best superelastic response at 65℃ with a recoverable strain of about 1.9%. Two-way shape memory strains of 0.8% and 3.3% were obtained in the Ni44.8Cu5Ti45.2Hf5 and Ni44.8Cu5Ti40.2Hf5Zr5 alloys, respectively. The Ni45.5Cu5Ti39.5Hf5Zr5 alloy, however, did not reveal the shape memory effect. Results obtained from low-temperature X-ray diffraction (XRD) analysis indicated that the B2→B19′ transformation was considerably suppressed in this alloy and the volume fraction of B19′martensite did not exceed 8% even at an ultra-low temperature of − 150 °C.