Abstract: This paper investigates the temperature and loading rate dependencies of the critical stress intensity factor (K_IC) for dislocation nucleation at crack tips. We develop a new K_IC formula with a generalized form by incorporating the atomistic reaction pathway analysis into Transition State Theory (TST), which captures the K_IC of the first dislocation nucleation event at crack tips and its sensitivity to temperature and loading rates. We use this formula and atomistic modeling information to specifically calculate the K_IC for quasi-two-dimensional crack tips located at various slant twin boundaries in nano-twinned TiAl alloys across a wide range of temperatures and strain rates. Our findings reveal that twinning dislocation nucleation at the crack tip dominates crack propagation when twin boundaries (TBs) are tilted at 15.79° and 29.5°. Conversely, when TBs tilt at 45.29°, 54.74°, and 70.53°, dislocation slip becomes the preferred mode. Additionally, at TB tilts of 29.5° and 70.53°, at higher temperatures above 800 K and typical experimental loading rates, both dislocation nucleation modes can be activated with nearly equal probability. This observation is particularly significant as it highlights scenarios that molecular dynamics simulations, due to their time scale limitations, cannot adequately explore. This insight underscores the importance of analyzing temperature and loading rate dependencies of the K_IC to fully understand the competing mechanisms of dislocation nucleation and their impact on material behavior.

Key words: Crack, Dislocation nucleation, Critical stress intensity factor, Temperature and loading rate sensitivity, Twin boundary, Atomistic simulation, TiAl alloy