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 (Δ°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 ${\Delta^{\circ}{C_{P}^{\alpha}}^{\rightarrow{\beta}}}$ is estimated to be 904 J · kg⁻¹ · K⁻¹.