Dominant shear bands observed in amorphous ZrCuAl nanowires under simulated compression

Abstract

We observed the formation of dominant shear bands in model ZrCuAl metallic glass (MG) nanowires (18-nm-long) in molecular dynamics simulations, which implies size-independent incipient plasticity in MG materials. The MG nanowires were prepared using the simulated casting technique to ensure proper relaxation of sample surfaces. Under uniaxial compression, shear bands initiate at the surfaces and lead to reduced icosahedral short-range order. The shear band formation is sensitive to sample thermal-history, which calls for careful consideration of sample preparation effects in both experimental and numerical studies of sizeeffect in MG samples.Metallic glasses (MG), unlike other forms of glasses, have distinctive mechanical properties including large elastic strain limit and high tensile strength [1,2]. Metallic glasses, however, suffer from catastrophic failure upon free propagating shear bands. The formation of localized shear bands strongly compromises the practical usages of the metallic glasses for load-bearing applications. One promising way to overcome this shortcoming of metallic glasses is to reduce the sample size of MGs using Focused Ion Beam (FIB) [3] or hot extension [4]. It has been shown that MG in the form of nanowires can exhibit large tensile strain [5,6]. This is thought to be due to the limited spatial scale of nanowires which is insufficient for shear band propagation and maturation into runaway cracks [1].Although shear band propagation and maturation into cracks are likely size-dependent [1], it is still not clear whether the initiation of shear bands (incipient plasticity) is size-dependent. Both size-dependent [3,7-10] and size-independent [12-14] shear banding behaviors in MG nanorods have been reported in experiments. Moreover, MD simulations on two-dimensional or threedimensional thin-slab geometry [15,16] show clear shear band formation, while MD simulations on three-dimensional nanowire samples show homogeneous flow [17,18]. Recently, Shi [11] proposed a new numerical sample preparation procedure termed "simulated casting", during which a molten liquid confined in a cylindrical container is quenched directly into a glassy nanowire. This sample preparation technique is advantageous in comparison to the conventional cut-form-bulk procedure to obtain nanowire samples in computer simulation. Surfaces resulting from the traditional cut-from-bulk procedure are usually not sufficiently relaxed at the processing temperature, whereas in the simulated casting technique, surfaces are created from the melt directly and atoms on the surfaces are fully relaxed. For MG nanowires obtained from the cut-frombulk procedure, homogeneous deformation followed by necking was observed owing to unrelaxed surfaces [11]. By contrast, it was shown that MG nanowires obtained from the simulated casting, thus with relaxed surfaces, exhibit shear banding with a sample length longer than 13 nm, as a result of competition between elastic energy and shear band energy [11]. Interestingly, this critical length va...

Influence of grain size and composition, topology and excess free volume on the deformation behavior of Cu–Zr nanoglasses

“…Basically the NG structure resemble the one of a soft BMG. Similar, a transition in the deformation mechanism of a BMG form a localized deformation in shear bands to a homogeneous plastic flow was observed when increasing the casting cooling rate [26]. Here the softening of the glassy structure is due to the faster quenching, while in our case the NGs with very small grains get softer due to the high fraction of interfaces.…”

Molecular dynamic simulations of the mechanical properties of crystalline/crystalline and crystalline/amorphous nanolayered pillars

“…Low quenching rate models have much higher yield stress but more obvious softening than high quenching rate C/A models. Such findings are consistent with the results in [41] which reported that amorphous nanowires with low quenching rates exhibit dominant shear bands and high yield stress while nanowires with high quenching rates undergo almost homogeneous deformation with low yield stress. However, both the two types of C/A nanolayered pillars in Fig.…”

Suppression of shear banding in amorphous ZrCuAl nanopillars by irradiation

“…17,18 A recent MD study shows that ZrCuAl MG nanopillars obtained from simulated casting, 19 as opposed to traditional cutting from a glassy bulk, exhibited dominant shear banding upon simulated compression test. 20 It was reasoned that unrelaxed surfaces prepared by "cutting" lead to homogeneous flow while relaxed surfaces prepared by "casting" lead to shear banding. Furthermore, consistent with bulk MG samples, 21 MG nanopillars cooled at high quenching rate deform homogeneously while MG nanopillars cooled at low quenching rate exhibit dominant shear banding.…”

“…Furthermore, consistent with bulk MG samples, 21 MG nanopillars cooled at high quenching rate deform homogeneously while MG nanopillars cooled at low quenching rate exhibit dominant shear banding. 20 As MG nanopillars with high quenching rate are relatively unrelaxed structurally, it appears that the deformation mode of MG nanopillars is sensitive to the glassy state of MG nanopillars, which is in turn sensitive to the processing conditions.…”