Effect of Solution Treatment Temperature on Microstructures and Mechanical Properties of (35.5-x)Ti-49.5Ni-15Hf-xNb (x = 3, 10) (at. %) High Temperature Shape Memory Alloys

Abstract

In this study, the effects of solution treatment temperature on the microstructures and mechanical properties of (35.5-x)Ti-49.5Ni-15Hf-xNb (x = 3, 10) (at.%) high-temperature shape memory alloys were studied by means of scanning electron microscopy, differential scanning calorimetry, and tensile tests. The microstructures of the solution treated with a 1123 K Ti-49.5Ni-15Hf-3Nb alloy consisted of (Ti, Hf, Nb)2Ni and Nb-rich phases and a B19′ martensite matrix. The solution treated at 1123 K Ti-49.5Ni-15Hf-10Nb alloy consisted of the B19′ martensite matrix and Nb-rich, Hf-rich, and Ni-rich phases. The Ni-rich phases were eliminated by increasing the solution treatment temperature. When the solution treatment temperature was increased from 1123 K to 1273 K, the fracture strain increased from 8.4% to 10.4% and from 11.2% to 22.1% for the Ti-49.5Ni-15Hf-3Nb and Ti-49.5Ni-15Hf-10Nb alloys, respectively, which was ascribed to the elimination of the Ni-rich phase. Ms (B2-B19′ transformation start temperature) also increased from 421.3 K to 439.7 K and from 192.6 K to 299.2 K for the 3Nb and 10Nb alloys, respectively. Superelastic recovery ratios of 62.7% and 78.7% were observed in the 3Nb and 10Nb alloy solutions treated at 1123 K, respectively. The superelastic recovery ratio increased with an increase in solution treatment temperature from 1123 K to 1273 K for both the 3Nb and 10Nb alloys.