A first-principles study of site occupancy and interfacial energetics of an H-doped TiAl-Ti3Al alloy

Abstract

We investigate the site occupancy and the interfacial energetics of TiAl-Ti 3 Al binary-phase system with H using a first-principles method. H energetically prefers to occupy the Ti-rich octahedral interstitial site because H prefers to bond with Ti rather than with Al. The occupancy tendency of H in the binary phase TiAl-Ti 3 Al alloy from high to low is  2 -Ti 3 Al to   / 2 interface and -TiAl, because the decrease of the Ti local concentration is in the same order. We demonstrate that H can largely affect the mechanical properties of the TiAl-Ti 3 Al system. On the one hand, H at the interface reduces the interface energy with the H 2 molecule as a reference, implying the TiAl/Ti 3 Al interface is stabilized. On the other hand, the ratio between the cleavage energy and the unstable stacking fault energy decreases after H-doping, indicating H will reduce the ductility of the TiAl/Ti 3 Al interface. Consequently, the mechanical property variation of TiAl alloy due to the presence of H not only depends on the amount of TiAl/Ti 3 Al interfaces but also is related to the H concentration in the alloy.TiAl alloys have great potential for aerospace applications because of their high melting temperature, low density, high modulus, good oxidation and creep resistance. However, their low ductility and toughness at room temperature limit the applications [1]. Research currently focuses on improving ductility and toughness at room temperature of TiAl alloys while maintaining its attractive high temperature characteristics. As a second phase of the -TiAl based alloys, the addition of  2 -Ti 3 Al is capable of improving the ductility of TiAl [2][3][4][5]. The interest in the binary-phase TiAlTi 3 Al alloys is further encouraged because of their superior specific strength and oxidation resistance [6,7]. It is significant to reveal the mechanical property of the TiAl/Ti 3 Al interface to find a way to further improve the ductility of the TiAl alloys.It is generally accepted that a trace amount (ppm) of impurity will result in an obvious variation of mechanical properties of materials [8][9][10][11][12]. H is a typical impurity that degrades the mechanical properties of metals. For example, H-induced embrittlement of metals has been studied from early last century, but the embrittlement mechanism is still not fully understood. The H-induced embrittlement has been explained by various mechanisms [13], such as precipitation of H 2 gas, formation of hydrides, deformation localization, and reduction of cohesion across the grain boundary.Experimentally, H has been demonstrated to embrittle the TiAl alloy [14,15]. On the one hand, several hydrides were reported in TiAl intermetallics [14], and the formation of such hydrides results in an increase in yield stress and a decrease in plasticity and fracture toughness because hydride is a brittle phase. On the other hand, H atoms can be