The shear band controlled deformation in metallic glass: a perspective from fracture.

G N Yang,Y Shao,K F Yao

doi:10.1038/srep21852

Abstract

Different from the homogenous deformation in conventional crystalline alloys, metallic glasses and other work-softening materials deform discontinuously by localized plastic strain in shear bands. Here by three-point bending test on a typical ductile Pd-Cu-Si metallic glass, we found that the plastic deformed region during fracture didn’t follow the yielding stress distribution as the conventional material mechanics expected. We speculated that such special behavior was because the shear bands in metallic glasses could propagate easily along local shear stress direction once nucleated. Based on a 3D notch tip stress field simulation, we considered a new fracture process in a framework of multiple shear band deformation mechanism instead of conventional materials mechanics, and successfully reproduced the as-observed complicate shear band morphologies. This work clarifies many common misunderstandings on metallic glasses fracture, and might also provide a new insight to the shear band controlled deformation. It suggests that the deformation of metallic glasses is sensitive to local stress condition, and therefore their mechanical properties would depend on not only the material, but also other external factors on stress condition. We hope that start from this work, new methods, criteria, or definitions could be proposed to further study these work-softening materials, especially for metallic glasses.

Highlights

Different from the homogenous deformation in conventional crystalline alloys, metallic glasses and other work-softening materials deform discontinuously by localized plastic strain in shear bands
By comparing it with experiment (Fig. 1a), we found that the real plastic deformed region didn’t follow the plastic zone description in linear elastic fracture mechanics (LEFM). (In this paper we named it as plastic region to distinguish it from the plastic zone concept in LEFM.) It seemed that the stress on the boundary of the plastic region was not constant but would decrease along the shear band
We found 3D stress field to be necessary in prediction such shear band morphology under complicated stress field condition

Summary

Introduction

Different from the homogenous deformation in conventional crystalline alloys, metallic glasses and other work-softening materials deform discontinuously by localized plastic strain in shear bands. By three-point bending test on a typical ductile Pd-Cu-Si metallic glass, we found that the plastic deformed region during fracture didn’t follow the yielding stress distribution as the conventional material mechanics expected We speculated that such special behavior was because the shear bands in metallic glasses could propagate along local shear stress direction once nucleated. As the shear band behavior is sensitive to the local stress condition and distribution, it becomes reasonable to understand the scattered and variable mechanical properties of metallic glasses, and the affection from sample size, shape, loading method and other parameters Based on such new understanding, we suggest that those concepts in classic materials mechanics, such as yielding stress, plasticity and fracture toughness, need to be reconsidered in these work-softening materials

Methods

Results

Conclusion