Cyclic Deformation in Metallic Glasses

Abstract

Despite the utmost importance and decades of experimental studies on fatigue in metallic glasses (MGs), there has been so far little or no atomic-level understanding of the mechanisms involved. Here we perform molecular dynamics simulations of tension-compression fatigue in Cu50Zr50 MGs under strain-controlled cyclic loading. It is shown that the shear band (SB) initiation under cyclic loading is distinctly different from that under monotonic loading. Under cyclic loading, SB initiation takes place when aggregates of shear transformation zones (STZs) accumulating at the MG surface reach a critical size comparable to the SB width, and the accumulation of STZs follows a power law with rate depending on the applied strain. It is further shown that almost the entire fatigue life of nanoscale MGs under low cycle fatigue is spent in the SB-initiation stage, similar to that of crystalline materials. Furthermore, a qualitative investigation of the effect of cycling frequency on the fatigue behavior of MGs suggests that higher cycling frequency leads to more cycles to failure. The present study sheds light on the fundamental fatigue mechanisms of MGs that could be useful in developing strategies for their engineering applications.

Numerical Modeling of Cyclic Deformation in Bulk Metallic Glasses

“…The free volume evolution can be fitted by a power law function in the form of f (x) = ax n and plotted with a blue line, as shown in Figure 3. This rule is just identical to that given in Sha's work [16]. A sequence of snapshots monitoring the shear-band evolution was also associated.…”

“…Shear banding is found to be the main fatigue mechanism for BMG under low-cycle fatigue. Similar to Sha's MD simulations [16], the shear banding evolution is mainly comprised of shear-band initiation and propagation processes. the corresponding snapshot of shear banding evolution, a dominate shear band runs through the whole BMG sample.…”

“…Shear banding is found to be the main fatigue mechanism for BMG under low-cycle fatigue. Similar to Sha's MD simulations [16], the shear banding evolution is mainly comprised of shear-band initiation and propagation processes. Wang et al [23] investigated the evolution processes from shear banding to fatigue cracking and fracture in Zr52.5Cu17.9Ni14.6Al10Ti5 BMGs by a quasi-in situ cyclic compression-compression experiment.…”

“…As BMG is in a small size scale, it can deform under cyclic loading in a semi-homogeneous manner without the pronounced mature shear-bands. Sha et al [16] performed MD simulations of tension-compression fatigue in Cu 50 Zr 50 BMGs under strain-controlled cyclic loading, and confirmed that the accumulation of shear transformation zone follows a power law with rate depending on the applied strain. Unfortunately, to the author's knowledge, there is no numerical modeling that has yet been conducted based on the finite element method (FEM).…”

“…Their results showed that the crack propagation, rather than the crack initiation, dominates the fatigue life at high stress levels. On the other hand, Sha et al [16] pointed out that the fatigue life mainly comprises of the shear band formation stage, not their propagation.…”