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Title: Softening Caused by Profuse Shear Banding in a Bulk Metallic Glass

Abstract

By controlling the specimen aspect ratio and strain rate, compressive strains as high as 80% were obtained in an otherwise brittle metallic glass. Physical and mechanical properties were measured after deformation, and a systematic strain-induced softening was observed which contrasts sharply with the hardening typically observed in crystalline metals. If the deformed glass is treated as a composite of hard amorphous grains surrounded by soft shear-band boundaries, analogous to nanocrystalline materials that exhibit inverse Hall-Petch behavior, the correct functional form for the dependence of hardness on shear-band spacing is obtained. Deformation-induced softening leads naturally to shear localization and brittle fracture.

Authors:
;  [1];  [2];  [1]
  1. Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
20778805
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review Letters; Journal Volume: 96; Journal Issue: 10; Other Information: DOI: 10.1103/PhysRevLett.96.105503; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; AMORPHOUS STATE; ASPECT RATIO; CRYSTALLOGRAPHY; CRYSTALS; DEFORMATION; FRACTURES; GRAIN BOUNDARIES; HARDENING; HARDNESS; METALLIC GLASSES; NANOSTRUCTURES; PLASTICITY; STRAIN RATE

Citation Formats

Bei, H., George, E.P., Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Xie, S.. Softening Caused by Profuse Shear Banding in a Bulk Metallic Glass. United States: N. p., 2006. Web. doi:10.1103/PHYSREVLETT.96.1.
Bei, H., George, E.P., Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, & Xie, S.. Softening Caused by Profuse Shear Banding in a Bulk Metallic Glass. United States. doi:10.1103/PHYSREVLETT.96.1.
Bei, H., George, E.P., Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, and Xie, S.. Fri . "Softening Caused by Profuse Shear Banding in a Bulk Metallic Glass". United States. doi:10.1103/PHYSREVLETT.96.1.
@article{osti_20778805,
title = {Softening Caused by Profuse Shear Banding in a Bulk Metallic Glass},
author = {Bei, H. and George, E.P. and Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 and Xie, S.},
abstractNote = {By controlling the specimen aspect ratio and strain rate, compressive strains as high as 80% were obtained in an otherwise brittle metallic glass. Physical and mechanical properties were measured after deformation, and a systematic strain-induced softening was observed which contrasts sharply with the hardening typically observed in crystalline metals. If the deformed glass is treated as a composite of hard amorphous grains surrounded by soft shear-band boundaries, analogous to nanocrystalline materials that exhibit inverse Hall-Petch behavior, the correct functional form for the dependence of hardness on shear-band spacing is obtained. Deformation-induced softening leads naturally to shear localization and brittle fracture.},
doi = {10.1103/PHYSREVLETT.96.1},
journal = {Physical Review Letters},
number = 10,
volume = 96,
place = {United States},
year = {Fri Mar 17 00:00:00 EST 2006},
month = {Fri Mar 17 00:00:00 EST 2006}
}
  • By controlling the specimen aspect ratio and strain rate, compressive strains as high as 80% were obtined in an otherwise brittle metallic glass. Physical and mechanical properties were measured after deformation, and a systematic strain-induced softening was obseerved which contrasts sharply with the hardening typically observed in crystalline metals.
  • Traditionally, strain localization in metallic glasses is related to the thickness of the shear defect, which is confined to the nanometer scale. In this study, using site-specific x-ray photon correlation spectroscopy (XPCS), we reveal significantly accelerated relaxation dynamics around a shear band in a metallic glass at a length scale that is orders of magnitude larger than the defect itself. The relaxation time in the shear-band vicinity is up to ten-times smaller compared to the as-cast matrix, and the relaxation dynamics occurs in a characteristic three-stage aging response that manifests itself in the temperature-dependent shape parameter known from classical stretchedmore » exponential relaxation dynamics of disordered materials. We demonstrate that the time-dependent correlation functions describing the aging at different temperatures can be captured and collapsed using simple scaling functions. Finally, these insights highlight how an ubiquitous nano-scale strain-localization mechanism in metallic glasses leads to a fundamental change of the relaxation dynamics at the mesoscale.« less
  • Cited by 3
  • Contrary to the cooling-rate induced hardening observed in crystalline metals, the authors report here an unexpected surface softening in rapidly solidified Zr{sub 50}Cu{sub 50} bulk metallic glass. A soft layer {approx}500 {micro}m thick was detected near the surface with both hardness and elastic modulus increasing from the surface to the interior. To understand the reason for this, a correlation between cooling rate and defect concentration was derived. Defect concentration was found to increase as the cooling rate increased, suggesting that surface softening may be the result of freezing-in of excess defects, induced by a faster cooling rate near the surfacemore » compared to the interior.« less