Dominant shear bands observed in amorphous ZrCuAl nanowires under simulated compression
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Dominant shear bands observed in amorphous ZrCuAl nanowires under simulated compression Qiran Xiao, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 H.W. Sheng, School of Physics, Astronomy and Computational Sciences, George Mason University, Fairfax, Virginia 22030 Yunfeng Shi, Department of Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180 Address all correspondence to Yunfeng Shi at [email protected] (Received 29 September 2011; accepted 21 December 2011)
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 size effect in MG samples.
Metallic glasses (MGs), unlike other forms of glasses, have distinctive mechanical properties, including large elastic strain limit and high tensile strength.[1,2] MGs, however, suffer from catastrophic failure upon free propagating shear bands. The formation of localized shear bands strongly compromises the practical usages of the MGs for load-bearing applications. One promising way to overcome this shortcoming of MGs is to reduce the sample size of MGs using a focused ion beam (FIB)[3] or hot extension.[4] It has been shown that MGs 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 run-away 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[11–13] shear banding behaviors in MG nanorods have been reported in experiments. Moreover, molecular dynamics (MD) simulations on two-dimensional or three-dimensional thin-slab geometry[14,15] show clear shear band formation, while MD simulations on three-dimensional nanowire samples show homogeneous flow.[16,17] Recently, Shi[18] 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-from-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 simu
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