Theoretical Prediction of Transition Metal Alloying Effects on the Lightweight TiAl Intermetallic
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ODUCTION
THE TiAl-based intermetallics have received extensive attention due to their high melting point, high specific strength and stiffness, low density, and excellent oxidation resistance.[1–3] Because of their attractive properties, they have played important roles in aerospace, automotive, and energy production industries.[4–8] In fact, the good specific mechanical properties of titanium aluminide intermetallics push the development of these materials. Because of their ordered structure, intermetallics have high strength at high temperature.[9] Despite that, the room-temperature brittleness is an important factor restricting their large-scale applications. As far as mechanical properties are concerned, the addition of ternary elements such as V, Cr, Nb, W, and Mo can improve mechanical properties of alloys.[10–12] For instance, Cr and V reduce the grain size and improve ductility. Nowadays, a theory-guided materials design (TGMD)[13,14] method is used instead of classical metallurgical method. This method combines quantum-mechanical calculations of thermodynamic stability and single-crystalline elastic properties of different phases with microstructure-specific homogenization methods which are the combinations of theories to find suited compositions with regard to the thermodynamic
SHUAI LIU and CHENGHUANG TANG, Masters, and YONGZHONG ZHAN, Professor, are with the College of Materials Science and Engineering, Guangxi University, Nanning 530004, Guangxi, P.R. China. Contact e-mail: [email protected] Manuscript submitted December 3, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
stabilization of the desired phase.[14] By using the TGMD method, the macroscopic experimentally detectable elastic parameters of new materials can be predicted. For example, site substitution of transition metal elements in c-TiAl intermetallics has been investigated with different theoretical calculation methods.[15–18] The results show that transition metal elements which have few d electrons, such as Y, Zr, Nb, and Mo, mainly substitute for the Ti site in c-TiAl. Both experiments and theoretical calculations have shown that the properties of c-TiAl depend strongly on crystal structure and elemental composition.[19–21] For some of the c-TiAl intermetallics, however, little information is available on microscopic mechanism which governs the strength and ductility. In this paper, we perform a systematic investigation of the structural and mechanical properties of the Ti7Al8X (X = Sc, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W) intermetallics by the first-principle calculations in order to preselect the promising alloying elements.
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METHODOLOGY
The TiAl binary compound crystallizes in the L10 superlattice structure with space group P4/MMM (No.123) consisting of two aluminum atoms and two titanium atoms in per conventional cell (Figure 1(a)). The TiAl unit cell with CuAu tetragonal structure, which can be considered as a succession of two simple tetragonal sublattices, has been used to study the anisotropy of theoretical strength of TiAl in
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