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

The phase stability of Ti-15 at% Nb alloy has been studied in the temperature range 466-1258 K using inverse drop calorimetry. In the annealed condition, the alloy consists of orthorhombic alpha ''-martensite and bcc beta phases. The calorimetry experiments indicated the occurrence of two phase changes upon heating. They are (i) precipitation of hexagonal omega phase at 582 +/- 2 K followed by (ii) alpha '' -> beta in the temperature domain 836-985 +/- 2 K. The enthalpy change due to omega formation is small; however, the alpha '' -> beta. phase transformation is associated with a measurable enthalpy effect of 57 +/- 3 J g(-1). Since alpha '' -> beta 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 alpha ''-martensite and beta phase mix and another transformational component arising from continuous alpha '' -> beta. phase change. Since the fractional extent of alpha '' -> beta 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 alpha '' + beta region. (C) 2011 Elsevier B.V. All rights reserved.