Atomistic underpinnings for dislocation emission behaviors at the crack tips in FCC metals in light of thermo-kinetic synergy

Dislocation emission at the crack tips in faced-centered cubic (FCC) metals generally involves multiple mechanisms of dislocation nucleation dominated by the motion behaviors of atoms near the crack tips. Taking FCC aluminum as a representative, in this work, the emission behaviors and nucleation mechanism of dislocations at the crack tips in FCC metals are investigated in light of the continuum mechanics models within the framework of the anisotropic linear elastic fracture mechanics. The system energy evolution and motion behaviors of atoms near the crack tips under Mode I loading conditions are obtained first by performing molecular dynamics simulations. The underlying thermo-kinetic origins of dislocation emission are clarified based on driving force U and energy barrier Q<sub>k</sub> for dislocation emission at the crack tips acquired from system energy variations. As the applied load rises, the value of U increases while that of Q<sub>k</sub> decreases, accelerating the dislocation emission process. The magnitude of Q<sub>k</sub> is proportional to the dislocation nucleation energy Q, which depends on the extrema of the generalized stacking fault (SF) energy curve along the dislocation emission direction (including the unstable SF energy γ<sub>usf</sub>, intrinsic SF energy γ<sub>isf</sub>, and unstable twinning fault energy γ<sub>utf</sub>). When abnormal fluctuations appear in the system energy evolution curve, the energy barrier Q<sub>k</sub> for dislocation emission undergoes a sudden change, signifying the transition of the dislocation nucleation mechanism. Accordingly, a thermo-kinetic criterion for the mechanism transition of dislocation nucleation at the crack tips in FCC metals is proposed. Our investigation provides an innovative thermo-kinetic perspective to understand the dislocation emission behaviors and the critical conditions for maintaining stability at the crack tips of FCC metals.