Precipitation kinetics of Ti3Ni4 and multistage martensitic transformation in an aged Ni–rich Ni–Ti shape memory alloy

Abstract

The precipitation kinetics of Ti3Ni4 in a Ti–51.0 at.% Ni alloy was studied through the martensitic transformations temperatures and the JMAKE model. The influences of different aging temperatures and aging time on martensitic transformations temperatures were investigated using differencial scanning calorimetry and X–ray diffraction. The Ti3Ni4 precipitation is more preferred at aging temperatures between 350 and 450 °C for 1 h. At these aging temperatures, the martensitic transformation occurred in 4 stages due to Ti3Ni4 heterogeneous nucleation. The obtained Avrami exponents suggests that Ti3Ni4 precipitation has unidimensional growth, is diffusion controlled, and presents nucleation site saturation before complete precipitation. Also, heterogeneous nucleation explains why Avrami exponent values were lower than theoretically expected for JMAKE model. The apparent activation energy estimated for Ti3Ni4 precipitation was 108 (± 8) kJ/mol, and the frequency factor 5.1 (± 4.2) 104 s−1. MET and EDS techniques confirmed the presence of Ti3Ni4 precipitates through morphological and chemical characterization. The estimated Ti3Ni4 volumetric fractions at metastable equilibrium were 12.5, 11.4, and 9.2% for 350, 400, and 450 °C, respectively.