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# Thermodynamic characterization of Ti-15at.%Nb alloy by drop calorimetry

J. Prabha1, R. Subramanian2, J. Balakrishnan2, A. Rai2, M. Behera2, M. Vijayalakshmi2 and I. Johnson3

1Bishop Heber College, Trichy, India
2Indira Gandhi Centre for Atomic Research, Kalpakkam, India
3St. Joseph's College, Trichy, India

Keywords: calorimetry, phase transformation
property: enthalpy, specific heat
material: Ti-15at.%Nb

Ti-Nb alloys due to their exemplary bio-corrosion resistance, high strength to weight ratio and tailorable elastic modulus are well known in the world of bio and aerospace materials. The Ti-15at.% Nb alloy is a two phase $\displaystyle\alpha$ ”+ $\displaystyle\beta$ alloy at room temperature with an estimated volume percentage of $\displaystyle\beta$ about 34% [1, 2]. In view of the paucity of thermodynamic data on this alloy, the enthalpy increment (HT-H298.15) for this alloy with a starting microstructure comprising of $\displaystyle\alpha$ ”-orthorhombic martensite, $\displaystyle\beta$ -bcc and a small extent of $\displaystyle\omega$ -hexagonal phase has been measured in the temperature range 466-1258 K. The enthalpy increment as a function of temperature indicated the occurrence of two distinct phase changes. These are: (i) $\displaystyle\alpha$ ”→ $\displaystyle\omega$ phase transformation at 582 K attended by a small enthalpy change and (ii) $\displaystyle\alpha$ ”→ $\displaystyle\beta$ phase transformation in the temperature domain 836-985 K which is associated with a significant change in enthalpy of about 57 J g-1 [3].

The enthalpy variation in $\displaystyle\alpha$ ”→ $\displaystyle\beta$ transformation region has been modelled using Kolmogorov-Johnson-Mehl-Avrami formalism for diffusive phase transformation kinetics. The microhardness of samples quenched from the transformation zone showed an increase with the temperature of quench, due to the increasing extent of $\displaystyle\alpha$ ”→ $\displaystyle\beta$ transformation. This is also supported by microstructure of quenched-in samples.

The measured enthalpy data have been appropriately modelled in $\displaystyle\alpha$ ”+ $\displaystyle\beta$ , $\displaystyle\alpha$ ”→ $\displaystyle\beta$ and $\displaystyle\beta$ regions to obtain the temperature dependent heat capacity in these domains. The results of high temperature heat capacity obtained in this study are combined with low temperature estimated specific heat values evaluated using Einstein model to obtain a comprehensive thermodynamic description. The specific heat capacity and entropy thus estimated at 298.15 K are found to be 440 and 528 J kg-1 K-1 respectively. The change in specific heat due to $\displaystyle\alpha$ ”→ $\displaystyle\beta$ phase transformation is found to be 384 J kg-1 K-1.

References
1. T. Ahmed and H. J. Rack, J. Mat. Sc., 31, 4267 (1996).

2. D. Raabe, B. Sander, M. Fraik, D. Ma and J. Neugebauer, Acta Mater., 55, 4475 (2007).

3. A. Josephine Prabha, S. Raju, B. Jeyaganesh, Arun Kumar Rai and I. Johnson, Proceedings of the 17th National Symposium on Thermal Analysis, (eds. P. C. Kalsi, Rajesh V. Pai, Mirnal R. Pai, S. Bharadwaj and V. Venugopal) Kurukshetra, India, March 9-11, (2010) pp. 42-45.