Local microstructure evolution at shear bands in metallic glasses with nanoscale phase separation

Jie He,Jiuzhou Zhao,Ivan Kaban,Do Hyang Kim,Jürgen Eckert,Kaikai Song,A Lindsay Greer,Norbert Mattern,Baoan Sun

doi:10.1038/srep25832

Abstract

At room temperature, plastic flow of metallic glasses (MGs) is sharply localized in shear bands, which are a key feature of the plastic deformation in MGs. Despite their clear importance and decades of study, the conditions for formation of shear bands, their structural evolution and multiplication mechanism are still under debate. In this work, we investigate the local conditions at shear bands in new phase-separated bulk MGs containing glassy nanospheres and exhibiting exceptional plasticity under compression. It is found that the glassy nanospheres within the shear band dissolve through mechanical mixing driven by the sharp strain localization there, while those nearby in the matrix coarsen by Ostwald ripening due to the increased atomic mobility. The experimental evidence demonstrates that there exists an affected zone around the shear band. This zone may arise from low-strain plastic deformation in the matrix between the bands. These results suggest that measured property changes originate not only from the shear bands themselves, but also from the affected zones in the adjacent matrix. This work sheds light on direct visualization of deformation-related effects, in particular increased atomic mobility, in the region around shear bands.

Highlights

Slab[10], cut to include the shear band in the central plane, shows an energy of cold work of 1.7 kJ mol–1; if all of this were attributed to a 20-nm-thick band, the stored energy in the shear band would be ~6.8 MJ mol–1, nearly 103 times a typical heat of fusion
We report the synthesis of new phase-separated bulk Metallic glasses (MGs) containing glassy nanospheres that are on a scale fine enough to act as a local internal recorder of the flow pattern near a shear band
Glassy structure is further confirmed by X-ray diffraction (XRD) patterns (Fig. S1(a)) as well as by clear glass transitions and crystallization exotherms seen in differential scanning calorimetry (Fig. S1(b))

Summary

Introduction

Slab[10], cut to include the shear band in the central plane, shows an energy of cold work of 1.7 kJ mol–1; if all of this were attributed to a 20-nm-thick band, the stored energy in the shear band would be ~6.8 MJ mol–1, nearly 103 times a typical heat of fusion. The profiles of reduced hardness suggest a shear-band thickness that is roughly equal to the shear offset on the band[10]. This makes it easier to interpret the enthalpy and volume increases resulting from deformation, but how this effective thickness relates to the flow pattern in shear-band operation is still has remained obscure. We report the synthesis of new phase-separated bulk MGs containing glassy nanospheres that are on a scale fine enough to act as a local internal recorder of the flow pattern near a shear band. The nanospheres are smaller, with diameters less than the shear-band thickness, and are glassy not crystalline

Methods

Results

Conclusion