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Abstract

Nano-scale coherent twin boundaries can be an effective way in overcoming the strength-ductility trade-off of metals and their alloys. In this sense also twin boundaries in nano-lamellar lightweight Ti-Al alloys promise a great potential. Furthermore, the existence of three types of these twin boundaries with different misorientation and coherency state at the interface provide an excellent opportunity to study the effect of exactly these parameters in a realistic model system. To this end, we carried out molecular statics simulations to characterize the crack advancement at and across internal true-twins (TTs), rotational boundaries (RBs), and pseudo-twins (PTs) in lamellar γ-TiAl alloys, at α2-Ti3Al/γ interfaces, and in fully lamellar structures. We used these systematic studies not only to reveal material properties, but also to shed light on the effect of the simulation simulation setup, i.e. mainly on how the shape of the crack tip and the crack orientation, affects the predicted behavior. It was confirmed that both, interface type and spacing, affect the fracture toughness and crack growth resistance. Elastic energy, which is stored at interfaces with a lattice misfit, can be released during deformation [1] and fracture. The semi-coherent γ-pseudo twin interface is the strongest barrier for the crack propagation while the coherent true twin interface is the weakest. The analysis of the contributing factors shows, that the crack orientation has more influence on the crack evolution than the crack aspect ratio. Regardless of the crack configurations, the coherent γ-true twin interface appears to be the most effective interface in terms of shielding the stress at the crack tip. The crack tip mechanisms exhibit prominent sensitivity to the crack system and crystallographic directions. For trans-lamellar cracks the tip shows plastic deformation and toughening at all interfaces [2]. The overall fracture initiation toughness in a microstructure of TTs exhibits an increasing trend with decreasing lamellar size down to a critical thickness, below which the fracture toughness drops again [3]. These and more phenomena and their origins will be discussed in the presentation. References [1] A. Chauniyal, R. Janisch, Materials Science & Engineering A, 2020, 796, 140053. [2] A. Neogi, R. Janisch, Acta Materialia, 2021, 213, 116924. [3] A. Neogi, R. Janisch, Acta Materialia, 2022, 227, 117698.