Microstructure and Shape Memory Behavior of quaternary Ni 50 x Pt Pd x Ti 50 y Hf Zr y alloys

Abstract

Shape memory alloys (SMA) are those class of alloys that can cause shape memory, newlinepseudoelastcity and two-way memory effects due to reversible austenite to martensite phase transformations. The NiTi shape memory alloys exhibit large recovery strains, however, the transformation temperatures of these alloys are less than 100 and#61616;C and these alloys exhibit low detwinning stresses/transformation stresses. Detwinning stress is the stress at which detwinning of martensite variants is initiated and transformation stress is the stress at which newlineaustenite to martensite transformation starts to occur at any temperature. Ternary additions newlinesuch as Hf, Zr, Pt, Pd, Au etc. increase the transformation temperatures and detwinning newlinestresses/transformation stresses of NiTi alloys. Though Hf and Zr are cheaper than the rest, NiTiHf alloys exhibit brittleness, large thermal hysteresis, poor shape memory and superelastic behaviour. The poor shape memory behaviour of Ti rich NiTiHf alloys has been attributed tothe presence of (001)B19 compound twins in its substructure, low matrix strength and occurrence of plastic deformation along with detwinning of martensite variants/stress induced martensite transformation. It has been reported that among the ternary elements Hf, Zr, Pt, Pd and Au, Ni substitution by Pt results in highest transformation temperatures in NiTi alloys.The effect of Pt additions on the microstructure and shape memory behaviour of ternary NiTiHfalloys has therefore been studied in this thesis, along with some compositions with Zr/Pd additions... newline