On the shear-affected zone of shear bands in bulk metallic glasses

https://doi.org/10.1016/j.jallcom.2020.155494Get rights and content

Highlights

  • Strain-induced modifications of the medium-range order (MRO) in the environment of shear bands in bulk metallic glasses.

  • Characteristic material-independent MRO gradients for the tensile and compressive sides.

  • An upper limit of a few microns for the lateral extension of the shear-affected environments of shear bands.

Abstract

Notched bars of bulk metallic glasses, Pd40Ni40P20 and Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105), were deformed under 3-point bending conditions, resulting in the formation of shear bands before failure. The immediate environment of shear bands was investigated in detail using fluctuation electron microscopy to extract information on the strain-induced modification of the medium-range order (MRO) and its lateral extension. Characteristic material-independent gradients were observed for the tensile and compressive sides of the samples indicating the impact of the local stress state on the MRO. Our results based on structural changes of the MRO suggest an upper limit of a few microns for the lateral extension of the shear-affected environments of shear bands.

Introduction

Metallic glasses (MGs) are principally of great interest as structural materials since they exhibit high strength and hardness. However, most MGs lack ductility, especially under tension where zero ductility prevails. However, in recent years progress has been made in developing bulk metallic glasses (BMGs) exhibiting respectable ductility during cold rolling, bending and compression tests. Upon inhomogeneous deformation, that is at low temperatures and high stresses, the plasticity is manifested by a macroscopic sliding along a localized region called a shear band (SB) having a typical thickness of about 15 nm, when tilted upright (edge-on) [[1], [2], [3]]. It has been found that such SBs contain alternating density changes accompanied by structural changes in the medium range order (MRO) [[3], [4], [5], [6]]. The current understanding of how mesoscopic SBs evolve from shear transformation zones (STZs), which are regarded as the main carriers for plasticity in metallic glasses [7], is that alignments of Eshelby-like quadrupolar stress-field perturbations lead to percolation and thus to SB formation [6,8]. The formation of a SB upon deformation creates an interface between the SB and the matrix. To maintain cohesion at the interface atoms need to be rearranged in the matrix, which consequently should also affect the adjacent matrix regions. Indeed, recent publications report on the existence of so-called shear band affected zones (SBAZs) [[9], [10], [11], [12]]. To address the relation between the structure of amorphous materials in terms of MRO and their mechanical behavior in more detail, the immediate environment of SBs in BMGs was investigated using fluctuation electron microscopy (FEM) to extract the local information on MRO [[13], [14], [15]]. FEM contains information about the four-body correlation of atom pairs (pair-pair correlation function) g4(r1,r2,|r|,θ) yielding information about the MRO (cluster size and volume fraction) in amorphous materials [15,16]. The obtained MRO profiles measured across SBs from tensile and compressive sides of 3-point bending tests display the impact of the local stress state on the MRO and thus shed more light on the lateral extension of deformation in SB environments.

Section snippets

Methods

FEM and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were performed with a Thermo Fisher Scientific FEI Themis 300 G3 transmission electron microscope (TEM) operated at 300 kV. Nanobeam diffraction patterns (NBDPs) were acquired with parallel illumination using probe sizes of 1.3 and 1.6 nm at full width half maximum (FWHM) in μProbe-STEM mode operated at spotsize 8 with a 50 μm C2 aperture giving a semi-convergence angle of 0.58–0.8 mrad. The probe size

Results

3-point bending test of notched bars were carried out (see supplementary video in Appendix A). A more detailed description is given in Ref. [23]. During such deformation tests, the area around the notch is dominated by tensile strain whereas the side opposite to the notch is mainly under compressive strain. In this paper these specific regions are referred to as the tensile and compressive sides, respectively. FIB lamellae containing SBs from each side (tensile and compressive) of the deformed

Discussion

In the following we discuss the robustness of this analysis.

Conclusions

Fluctuation electron microscopy revealed a detailed structural picture of the interplay between deformation and MRO structure obtained from two representative BMGs (Pd40Ni40P20 and Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105)) performed under 3-point bending conditions. (i) Prior to deformation, the amount of MRO was observed to be higher for Vit105 than for Pd40Ni40P20. The degree of MRO was reduced after deformation. Profiling the MRO of shear band environments from compressive and tensile sides

CRediT authorship contribution statement

Farnaz A. Davani: Investigation, Data curation, Formal analysis, Writing - original draft. Sven Hilke: Investigation, Formal analysis, Software, Writing - review & editing. Harald Rösner: Conceptualization, Methodology, Writing - review & editing, Validation, Supervision. David Geissler: Methodology, Software, Writing - review & editing. Annett Gebert: Supervision, Writing - review & editing, Funding acquisition. Gerhard Wilde: Writing - review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We gratefully acknowledge financial support by the DFG via SPP 1594 (Topological engineering of ultra-strong glasses, WI 1899/27-2 and GE 1106/11) and WI 1899/29-1 (Coupling of irreversible plastic rearrangements and heterogeneity of the local structure during deformation of metallic glasses, projekt number 325408982). Moreover, we acknowledge funding for our TEM equipment via the DFG Major Research Instrumentation Programme under INST 211/719-1 FUGG.

References (35)

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