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Atomistic simulation of fracture in lamellar TiAl microstructures

Date: 05.08.2019
Place: ISAM4-2019: The fourth International Symposium on Atomistic and Multiscale Modeling of Mechanics and Multiphysics (IIS UTokyo SYMPOSIUM No. 101), August 5-8, 2019, Friedrich-Alexander Universität Erlangen-Nürnberg (FAU), Germany

Anupam Neogi
Alexander Hartmaier
Rebecca Janisch

Excellent combination of high specific strength, low density, and good structural stability makes titanium aluminides (TiAl alloys) promising candidates for aerospace structural materials even at high temperature. Despite their high strength, however, TiAl alloys often exhibit in-service brittle failure behavior due to their limited ductility. The underlying microstructure plays a critical role in determining their strength and mode of fracture under different loading conditions. Currently, lamellar TiAl alloys, consisting of the tetragonal -TiAl and the hexagonal 2-Ti3Al phase, seem to provide the best combination of strength and deformability. Systematic and fundamental understanding of the controlling role of the lamella spacing and orientation on the fracture behavior could pave the way towards designing of novel, improved TiAl microstructures for structural applications. Large-scale atomistic simulation is a suitable approach to investigate the underlying events during fracture at the atomic level. We perform such large-scale atomistic simulations using the embedded atom method (EAM) potential of Zope and Mishin [1]. To evaluate the potential, we have studied the behavior of semi-infinite cracks under mode I loading for different orientations of crack plane and direction in single crystal γ-TiAl and α_2-Ti3Al. The critical stress intensity factors (KIC) predicted by our simulations are in good agreement with the existing literature. The mechanism of crack propagation is sensitive to the choice of crack plane and direction. For instance, in -TiAl, a crack in the system [100] (010), propagates in a purely brittle manner. The emission of dislocation from the crack tip and onset of ductile fracture is observed for crack propagation along [-211] in the (111) plane, while complete crack blunting followed by nucleation of a dislocation from the crack tip occurs for a [111] (1-10) crack. These different crack propagation mechanisms lead to a different interaction of the crack tip with the interfaces in lamellar microstructures. The consequences will be discussed in the presentation. Zope, R.R. and Mishin, Y., 2003. Interatomic potentials for atomistic simulations of the Ti-Al system. Physical Review B, 68(2), p.024102.

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