High strength NiTiHf shape memory alloys with tailorable properties

Abstract

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 Ni51.2Ti28.8Hf20 was severely suppressed in the solution treated condition (900 degrees C-3h/water quench) and after aging at low temperatures, while the transformation temperatures were greater than 100 degrees C after 650 degrees C-3h aging. Generation of nanosize precipitates (similar to 20 nm in size) after 3 h aging at 450 degrees C and 550 degrees 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 degrees C) after aging at 450 degrees C-3h and at elevated temperatures (120-160 degrees C) after aging at 550 degrees 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. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.