A Study on Thermal Properties and α(hcp) → β(bcc) Phase Transformation Energetics in Ti–5 mass% Ta–1.8 mass% Nb Alloy Using Inverse Drop Calorimetry

Abstract

Accurate measurements of enthalpy increment (H T − H 298. 15 ) values have been made on a Ti-5 mass% Ta-1.8 mass% Nb alloy using the inverse drop calorimetry technique in the temperature range from 463 K to 1457 K. The measured enthalpy increment values show a steady increase with temperature in both α-hcp and β-bcc solid solution regions. It is found that both the onset as well the completion of the α → β phase change are demonstrated by a marked deviation of the enthalpy increment behavior from the otherwise smooth variation encountered in the respective low-temperature α-and high-temperature β-phase domains. The transformation start (T s ) and finish (T f ) temperatures of the α → β phase change are found to be (1072 ± 10) K and (1156 ± 10) K, respectively. In the actual α → β phase transformation region, the variation of the enthalpy with the progress of transformation is found to follow a sigmoidal shape which is in line with the diffusive nature of the phase transformation. An estimation of the total enthalpy change associated with the α → β phase transformation ( • H tr ) has been made by assuming a simple diffusion limited kinetic model for the phase change. The net enthalpy change for the α → β transformation is found to be 76 J · g −1 . The measured temperature variation of the enthalpy increment in both α-and β-phase regimes are fitted to simple analytical functional forms to obtain temperature-dependent estimates of the specific heat, C P . The total specific heat change associated with the α → β phase transformation • C α P →β is estimated to be 904 J · kg −1 · K −1 .