High strength NiTiHf shape memory alloys with tailorable properties

Microstructure of NiTiHf shape memory alloys can be engineered to have high strength and operate at high stress levels for a large temperature window. Nanoprecipitation is well-known method to improve the strength of materials but it can be employed to NiTiHf alloys to substantially alter their phase transformation characteristics (martensite morphology, transformation strain, hysteresis and stress). The martensitic transformation of Ni-rich Ni_51.2Ti_28.8Hf₂₀ was severely suppressed in the solution treated condition (900 °C-3h/water quench) and after aging at low temperatures, while the transformation temperatures were greater than 100 °C after 650 °C-3h aging. Generation of nanosize precipitates (∼20 nm in size) after 3 h aging at 450 °C and 550 °C improved the strength of the material, resulting in a near perfect dimensional stability during isobaric thermal cycling at stress levels of greater than 1500 MPa, with work output of 20–30 J cm⁻³. Superelastic behavior with 4% recoverable strain was demonstrated at low temperatures (−20 to 40 °C) after aging at 450 °C-3h and at elevated temperatures (120–160 °C) after aging at 550 °C-3h, with stresses reaching 2 GPa without the onset of plastic deformation. A clear relationship between thermal treatments, microstructure, mechanical and shape memory properties will be shown.