ICAMS / Interdisciplinary Centre for Advanced Materials Simulation

Crack propagation along interfaces in multiphase TiAl lamellar alloys

There are several potential applications that have been identified for TiAl-based alloys in the aerospace, automotive and turbine power generation markets. TiAl-based alloys with two-phase structures consisting of the major γ (with L10 structure) and minor α2 (with D019 structure) phases are the most intensively studied materials among these aluminides and their alloys. Their low density, strength and modulus retention at high temperatures, some tensile ductility at room temperature, and reasonably good oxidation resistance are very attractive as a new class of light-weight high-temperature materials for structural applications [1]. When γ/α2 two-phase alloys with nearly stoichiometric or Ti-rich compositions are prepared by usual melting-and-casting processes, a polycrystalline lamellar structure is formed. The lamellar microstructures are poor in ductility; however, they are generally superior to the duplex structures in other mechanical properties such as fracture toughness, fatigue resistance and high-temperature creep strength [1-2].


Mechanical properties of the lamellar microstructures in TiAl-based alloys depend on the lamellar orientation with respect to the loading axis and lamellar microstructural variables such as grain size, thickness and spacing of γ and &alpha2 lamellae and γ domain size [3-4]. Moreover, crack propagation in different loading states can occur in both inter- and trans-lamella types [4]. Thus, a precise characterization of each interface in terms of structure, energy and mechanical properties and the comparison to the bulk phases is worthwhile.
The aim of this project is to characterize the material properties and mechanical properties of the lamellar interfaces by DFT based calculation. To have a comprehensive understanding of mechanical behavior, both γ and α2 bulk structures and also different grain boundaries are investigated under tensile and shear deformation.

Shear strength can be obtained from so-called gamma-surface calculation; for one of the γ/γ interfaces the related gamma-surface energy plot is shown in Fig 1. Furthermore, tensile strength of different grain boundaries and also bulk structures is illustrated in Fig1.

References:
[1] M. Yamaguchi, H. Inui, K. Ito, High-temperature structural intermetallics, Act. Mat. 48, 307 (2000).
[2] F. Appel, R. Wagner, Microstructure and deformation of two-phase γ-titanium aluminides, Mat. Sci. Eng. Report 22, 187 (1998).
[3] M. Yamaguchi, H. Inui, TiAl-base Alloy crystals in the polysynthetically pwinned (PST) form, Encyclopedia Mat. Sci Tech., 1 (2005).
[4] A. Chatterjee, H. Mecking, E. Arzt, H. Clemens, Creep behavior of γ-TiAl sheet material with differently spaced fully lamellar microstructures, Mat. Sci. Eng. A 329-331, 840 (2002).
[5] Y.H. Lu, Y.G. Zhang, L.J. Qiao, Y.B. Wang, C.Q. Chen, W.Y. Chu, In-situ TEM study of fracture mechanisms of polysynthetically twinned (PST) crystals of TiAl alloys, Mat. Sci. Eng. A 289. 91 (2000).

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