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Thermal stability and thermal property studies on Ti-5Ta-1.8Nb (mass %) alloy


M. Behera1, R. Subramanian1, J. Balakrishnan1, R. Mythili1 and S. Saibaba1

1Indira Gandhi Centre for Atomic Research, Kalpakkam, India

Keywords: drop calorimetry, differential scanning calorimetry
property: enthalpy increment, specific heat, phase transformation enthalpy
material: Ti - 5 mass% Ta- 1.8 mass% Nb alloy

Titanium alloys having transition elements like Ta, Nb, Mo, V etc., are finding increased use in diverse biomedical and corrosion resistant applications, owing to the advantageous combination of mechanical, physical and chemical properties. In this regard, a Ti alloy containing 5Ta and 1.8 Nb (mass %) has been recently developed, characterized and corrosion tested for possible applications in strongly oxidizing nitric acid environment. The basic physical metallurgy aspects of this `alpha` +`beta`  alloy have been extensively characterized. In the present study, an attempt is made to analyze the high temperature thermal stability and measure the high temperature thermodynamic properties of this alloy using drop and differential scanning calorimetry.

In this study, enthalpy increment values (HT-H298.15) for solution annealed samples have been accurately measured as a function of temperature using drop calorimetry in the temperature range 463–1457 K. The temperature range of measurement covers the stable `alpha` (hcp), the `alpha` `|->` β phase transformation domain and the high temperature single phase `beta` (bcc) regions. The behavior of the enthalpy curve in low temperature `alpha` +`beta` and high temperature `beta` domains is rather smoothly increasing with temperature, indicating thereby a steadily rising specific heat. However, in the intervening `alpha` `->` β transformation domain, the measured enthalpy reflected a marked deviation from that of extrapolated smooth variation. The drop calorimetry results indicated that the transformation domain is spread in the temperature range, 1072 –1156 K [1]. This is further confirmed by differential scanning calorimetry experiment.

The enthalpy results are modelled separately for the three domains. In the transformation domain, it is assumed that the measured enthalpy contains two components: (i) enthalpy from `alpha` +`beta` phases with varying mass percent of `beta` as a function of temperature; (ii) transformational enthalpy due to `alpha``beta` phase change. In the pre and post transformation regions, the enthalpy is governed by the thermochemistry of `alpha` and `beta` phases respectively. An important aspect of modelling is the realistic estimation of the fraction of high temperature`beta` phase using the Kolomogorov-Johnson-Mehl-Avrami model of diffusional phase change. The results of this transformational model are suitably adopted to estimate the transformational component of enthalpy.

References
  1. Madhusmita Behera, B. Jeyaganesh, S. Raju, R. Mythili, Arun Kumar Rai, and S. Saroja, Proceedings of the 17th National Symposium on Thermal Analysis (THERMANS-2010), p.46.

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