Thermodynamics of α″→β phase transformation and heat capacity measurements in Ti–15 at% Nb alloy

Abstract

The phase stability of Ti–15at% Nb alloy has been studied in the temperature range 466–1258K using inverse drop calorimetry. In the annealed condition, the alloy consists of orthorhombic α″-martensite and bcc β phases. The calorimetry experiments indicated the occurrence of two phase changes upon heating. They are (i) precipitation of hexagonal ω phase at 582±2K followed by (ii) α″→β in the temperature domain 836–985±2K. The enthalpy change due to ω formation is small; however, the α″→β phase transformation is associated with a measurable enthalpy effect of 57±3Jg−1. Since α″→β phase change occurs over a temperature interval, the measured enthalpy in the transformation domain derives from two principal contributions, namely, the matrix contribution coming from untransformed α″-martensite and β phase mix and another transformational component arising from continuous α″→β phase change. Since the fractional extent of α″→β transformation varies continuously with temperature the transformation enthalpy also exhibits similar temperature dependence. This aspect is modeled using Kolmogorov–Johnson–Mehl–Avrami formalism for treating the diffusional transformation kinetics. Further, the measured enthalpy variation with temperature has been subjected to standard analytical treatment to obtain a comprehensive thermodynamic description of entire α″+β region.