Abstract
Anisotropic thermal expansion coefficients of tetragonal -TiAl and hexagonal -Ti3Al phases were calculated using first principles methods. Two approaches with different computational costs and degrees of freedom were proposed. The predicted values were compared with available experimental data showing that for -TiAl, the more computational demanding method with decoupled impact of volume and temperature effects on the cell shape leads to significantly better results than that with only ground-state optimised unit cell geometry. In the case of the -Ti3Al phase, both approaches yielded comparable results. Additionally, heat capacity and bulk modulus were evaluated as functions of temperature for both phases, and were fitted to provide an analytical formula which can be further used.
Highlights
First principles calculations are a widely used and well-established method for complementing experimental materials science research [1]
We start our analysis by comparing the predicted temperature dependence of specific volumes of the α2 -Ti3 Al and γ-TiAl phases using both approaches as described in the Section 2
The gs-cs method yields larger and faster expanding volumes than the to-cs treatment. The former is significantly non-linear, suggesting that the resulting coefficient of volume thermal expansion is strongly increasing at higher temperatures and does not reach the usual near-to-linear behaviour for temperatures above room temperature (RT, ∼298 K)
Summary
First principles calculations are a widely used and well-established method for complementing experimental materials science research [1]. Despite the fact that many recent activities have been directed towards big-data and machine learning [2,3,4], there are still many topics which require individualised treatments. An example of such a problem is the discrepancy between the experimentally and theoretically reported stability and chemistry of the Nb3 Al phase published in this special issue [5]. The there implemented quasi-harmonic approximation (QHA) for calculating thermal properties, such as thermal expansion, bulk modulus or heat capacity, does not include effects of temperature-induced changes in the unit cell geometry in terms of c/a or b/a ratios or lattice angles, as may be the case of systems with lower than cubic symmetry
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