Martensitic transformation and microstructure of (Ni, Cu, Pd)-rich Ti49.5Ni39-xCu11.5Pdx alloys with near-zero hysteresis and excellent thermal stability

Abstract

The martensitic transformation behavior and microstructure evolutions of the (Ni, Cu, Pd)-rich Ti49.5Ni39-xCu11.5Pdx alloys along with the change of Pd content were investigated. The Ti49.5Ni39-xCu11.5Pdx alloys are composed of orthorhombic B19 martensite and a small amount of cubic Ti2Ni-type Ti2(Ni, Cu, Pd) phase and hexagonal TiNi2-type Ti(Ni, Cu, Pd)2 phase. The λ2 is affected by the increasing Pd contents, presenting a trending towards to 1. The martensitic transformations in all the Ti49.5Ni39-xCu11.5Pdx alloys are one-step of B2–B19 transformation. There presents a downward trend of the transformation temperatures with the increase of Pd content. The thermal hysteresis has been significantly reduced by the addition of Pd. The minimum thermal hysteresis is found in the Ti49.5Ni33.5Cu11.5Pd5.5 alloy, showing a value of 4.9 °C with the scan rate of 20 °C/min. It also exhibits an excellent thermal stability that only an increment of 0.04 °C in Mp and a decrement of 0.23 °C in Ap appear through 100 thermal cycles. The effect of Pd content on the martensite transformation is decided by the combination of strengthening the matrix and improving the crystallographic compatibility of the martensite and austenite phases in the (Ni, Cu, Pd)-rich Ti49.5Ni39-xCu11.5Pdx alloys.