Thermo-kinetic insights into the crack-tip dislocation emission behaviors of FCC metals with different stacking fault characteristics

Face-centered-cubic (FCC) metals have multiple dislocation slip paths due to their abundant slip systems, which enable the crack tips to relieve stress concentration effectively via dislocation emission behaviors, as governed intrinsically by the stacking fault energy (SFE). Taking FCC Al, Cu, Ni, Ag, Au, and Pt with different SFE characteristics as representatives, in this work, the regularities and distinctions for the crack-tip dislocation emission behaviors of FCC metals are investigated from the perspective of thermo-kinetic synergy. The variations in structural defects during the crack-tip dislocation emission are obtained by performing molecular dynamics simulations, together with the system energy evolution with respect to the driving force and energy barrier for dislocation emission. It is found that the evolution of structural defects near the crack tips in FCC metals undergoes the same three stages: (i) the initial dislocation emission accompanied by the formation of surface steps, (ii) the twinning formation or extended dislocation emission, and (iii) the twinning extension or extended dislocation multiplication. The distinctions lie in the ease of crack-tip dislocation emission, as reflected by the magnitude of their critical stress intensity factor (KIe) for the initial dislocation emission, i.e., KIe = 0.278, 0.462, 0.780, 0.296, 0.225, and 0.454 MPa m1/2 for FCC Al, Cu, Ni, Ag, Au, Pt, respectively. The reasons are attributed to the differences in their elastic moduli and SFE characteristics. Although the varying trend of generalized SFE curves for FCC Al, Ni, Pt (featured by relatively high SFE) remains similar, the lower elastic moduli for FCC Al correspond to its smaller energy barrier for the crack-tip dislocation emission, which can facilitate crack instability. The higher unstable SFE and shear modulus for FCC Ni correspond to larger energy barrier for dislocation emission, which can suppress crack propagation. In contrast, there are apparent differences between the unstable SFE and intrinsic SFE of FCC Cu, Ag, Au (featured by relatively low SFE), which signifies that more system energy is released during crack-tip dislocation emission and more strain energy consumption is required for subsequent dislocation emission, thus delaying crack propagation by enhancing the capability for crack-tip plastic deformation. Hence, both the higher values of characteristic SFE and their larger variations are beneficial for the crack-tip stability, based on which two pathways are proposed for improving the fracture toughness of FCC metals. Our investigations establish a physical landscape linking the structural defects and SFE characteristics of FCC metals to their crack-tip dislocation emission behaviors and provide an insightful guidance for regulating the fracture toughness of metallic alloys with the FCC structure.