Critical Analysis of an FeP Empirical Potential Employed to Study the Fracture of Metallic Glasses

Abstract

An empirical potential that has been widely used to perform molecular dynamics studies on the fracture behavior of FeP metallic glasses is shown to exhibit spinodal decomposition in the composition range commonly studied. The phosphorous segregation induces a transition from ductility to brittleness. During brittle fracture the atomically sharp crack tip propagates along a percolating path with higher P concentration. This embrittlement is observed to occur over a wide range of chemical compositions, and toughness decreases linearly with the degree of compositional segregation over the entire the regime studied. Stable glass forming alloys that can be quenched at low quench rates do not, as a rule, exhibit such thermodynamically unstable behavior near to or above their glass transition temperatures. Hence, the microstructures exhibited in these simulations are unlikely to reflect the actual microstructures or fracture behaviors of the glassy alloys they seek to elucidate.

Observation of cavitation governing fracture in glasses

“…Amazingly, an isolated cavity with a width of 32 nm and a depth of 1.1 nm is found ahead of the sharp crack tip (Fig. 1, E and F), making it clear that plasticity exists ahead of the crack tip (38,42). The three-dimensional (3D) close-up topographies in Fig.…”

“…Specifically, how the unique nanoscale periodic corrugation structures are formed in the "mirror" zones of the brittle fracture surfaces is a pressing problem (24,(32)(33)(34)(35)(36)(37). Recently, numerous calculations and simulations have recognized that cavitation is involved in the fracture of MGs and is believed to play an essential role in the failure of MGs, as well as other glassy materials (3,(36)(37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47). Nonetheless, direct evidence or experimental observation of cavitation in the fracture of glasses is hitherto lacking.…”

Numerical-continuation-enhanced flexible boundary condition scheme applied to mode-I and mode-III fracture

“…The picture is further complicated if the modelled crack propagates, causing an effective shift of origin of the entire strain field, which is often not reflected in the supplied boundary condition. Challenges present in the (quasi-) static modelling of fracture translate directly to more complex simulations of fracture-related phenomenona, as pointed out in a recent study establishing the ill-suitability of a popular empirical potential for molecular dynamics studies of fracture phenomena in FeP metallic glasses [14].…”

“…15 for an accessible overview of the topic. These techniques allow the fracture predictions of a range of candidate interatomic potentials to be efficiently screened, helping to address issues such as that identified in [14]; this approach will be demonstrated here using two potentials for silicon.…”

“…The admissible range of K is now predicted by the simple procedure introduced in Section III A 3, i.e. by finding roots of equation (14), namely f 0 α (K, α) = 0, leading to the predictions shown with the dashed lines in Figure 12, compared against full solution paths computed with pseudo-arclength continuation with flexible boundary. Here, K is found numerically for each value of α in a 200-element grid.…”