Influence of microstructure on the shape memory properties of two Ti-lean, Ni-Ti-Pt high temperature shape memory alloys

Abstract

Because of NiTi's superior properties (work output, strength, ductility, recoverable strain, etc.) it is the base system of choice for development of derivative high-temperature shape memory alloys (HTSMAs). Ternary additions of Hf, Zr, Pt, Pd, and Au can be made, in quantities greater than [approximate] 10 at.%, to increase the transformation temperature of Ni-Ti based SMAs. Pt as an alloying addition is attractive because of (1) its efficiency in raising the martensitic transformation temperature, (2) the relatively stable properties during thermal cycling of Pt-containing Nitinol (NiTi), and (3) the high work outputs of Ni-Ti-Pt alloys relative to other HTSMAs. Platinum containing samples of NiTi were thermally processed to explore the utility of Ti-lean precipitates for matrix strengthening and stabilization of shape memory properties during thermomechanical cycling. Two alloys, Ti48.5Ni30.5Pt21 and Ti49.5Ni29.5Pt21 were heat treated for 1, 5, 24 and 100h at 500, 550, 600, 650, and 700 degrees C and examined using SEM, EDS, DTA, XRD and TEM techniques. Two relevant precipitate phases, the PL and Ti2(Ni,Pt)3 phases, were identified, characterized and the thermodynamic stability and relevant behavior during thermal processing determined. Samples were then subjected to thermomechanical testing that consisted of two parts, (1) two thermal cycles (75 degrees C to 500 degrees C to 75 degrees C) each at stresses of 0, 50, 100, 150, 200, 250, and 300MPa, and (2) 100 thermal cycles at 200MPa. With this combination of systematic microstructural characterization and isobaric thermal cycling, the link between microstructure and shape memory performance was made. The influence the PL and Ti2(Ni,Pt)3 phases have on properties such as martensitic transformation temperatures, transformation strain, and accumulated irrecoverable strain are explained and discussed. Specifically, it was found that the PL-phase suppresses transformation temperatures and strains through a matrix strengthening effect, but also stabilizes property evolution, while the Ti2(Ni,Pt)3 phase plays a dominant role in modifying matrix chemistry, and subsequent transformation temperatures.