Roles of nanoclusters in shear banding and plastic deformation of bulk metallic glasses

PDF Version Also Available for Download.

Description

During the course of this research we published 33 papers in various physics/material journals. We select four representing papers in this report and their results are summarized as follows. I. To study shear banding process, it is pertinent to know the intrinsic shear strain rate within a propagating shear band. To this aim, we used nanoindentation technique to probe the mechanical response of a Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass in locality and found notable pop-in events associated with shear band emission. Using a free volume model and under the situation when temperature and stress/hardness are fixed result in an equation, which ... continued below

Creation Information

Nieh, T.G. July 31, 2012.

Context

This report is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided by UNT Libraries Government Documents Department to Digital Library, a digital repository hosted by the UNT Libraries. More information about this report can be viewed below.

Who

People and organizations associated with either the creation of this report or its content.

Author

Publisher

Provided By

UNT Libraries Government Documents Department

Serving as both a federal and a state depository library, the UNT Libraries Government Documents Department maintains millions of items in a variety of formats. The department is a member of the FDLP Content Partnerships Program and an Affiliated Archive of the National Archives.

Contact Us

What

Descriptive information to help identify this report. Follow the links below to find similar items on the Digital Library.

Description

During the course of this research we published 33 papers in various physics/material journals. We select four representing papers in this report and their results are summarized as follows. I. To study shear banding process, it is pertinent to know the intrinsic shear strain rate within a propagating shear band. To this aim, we used nanoindentation technique to probe the mechanical response of a Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass in locality and found notable pop-in events associated with shear band emission. Using a free volume model and under the situation when temperature and stress/hardness are fixed result in an equation, which predicts that hardness serration caused by pop-in decreases exponentially with the strain rate. Our data are in good agreement with the prediction. The result also predicts that, when strain rate is higher than a critical strain rate of 1700 s^-1, there will be no hardness serration, thereby no pop-in. In other words, multiple shear bandings will take place and material will flow homogeneously. The critical strain rate of 1700 s^-1 can be treated as the intrinsic strain rate within a shear band. We subsequently carried out a simulation study and showed that, if the imposed strain rate was over , the shear band spacing would become so small that the entire sample would virtually behave like one major shear band. Using the datum strain rate =1700 s^-1 and based on a shear band nucleation model proposed by us, the size of a shear-band nucleus in Au-BMG was estimated to be 3 × 10^6 atoms, or a sphere of ~30 nm in diameter. II. Inspired by the peculiar result published in a Science article “Super Plastic Bulk Metallic Glasses at Room Temperature”, we synthesized the Zr-based bulk metallic glass with a composition identical to that in the paper (Zr64.13Cu15.75Ni10.12Al10) and, subsequently, tested in compression at the same slow strain rate (~10^-4 s^-1). We found that the dominant deformation mode is always single shear. The stress-strain curve exhibited serrated pattern in the plastic region, which conventionally has been attributed to individual shear band propagation. The scanning electron micrographs taken from the deformed sample surface revealed regularly spaced striations. Analysis indicates that the observed stress-strain serrations are intimately related to the striations on the shear surface, suggesting the serrations were actually caused slip-and-stick shear along the principal shear plane. We further use video camera to conduct in situ compression experiments to unambiguously confirm the one-to-one temporal and spatial correspondence between the intermittent sliding and flow serration. This preferential shear band formation along the principal shear plane is, in fact, a natural consequence of Mode II crack, independent of strain softening or hardening, usually claimed in the literature. III. Flow serration in compression of metallic glasses is caused by the formation and propagation of localized shear bands. These shear bands propagate at an extremely high speed, so high that a load cell and load frame were unable to capture the details of the dynamic event. To subdue this problem, we conducted uniaxial compression on Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass using a high-speed camera to capture the sample image and also high-sensitivity strain gauges attached to the test samples to directly measure the strain. The displacement-time curves obtained from the test and a magnified version of the displacement burst reveals clearly a three-step (acceleration, steady-state, and deceleration) process during shear band propagation. The fastest propagating speed occurring at the steady state is calculated as 8×10^2 µm/s. This speed is about 1,000 times faster than the crosshead speed. This explains the gradual disappearance of flow serration at higher strain rates previously reported during compression of BMGs. IV. Shear banding is associated with a local viscosity drop, which may be associated with a temperature rise in the shear band or dynamic stability caused by excessive applied stress. Molecular dynamic simulations recently have shown that applied stress and temperature are equivalent and shear banding is essentially a stress-induced glass transition process. To validate this, we carried out a series compression tests on the binary Cu50Zr50 metallic glass in a temperature range below glass transition temperature where deformation mode was inhomogeneous (T = 0.40-0.65 Tg); the results are shown in the left figure below. The yield strength (onset of plastic flow) was found to decrease monotonically with the increase of test temperature. The strength-temperature relation for the binary glass, as well as several other metallic glass systems, can be well correlated. This result supports the proposed concept of stress-induced glass transition.

Language

Item Type

Identifier

Unique identifying numbers for this report in the Digital Library or other systems.

  • Report No.: DOE/ER?46338-4
  • Grant Number: FG02-06ER46338
  • DOI: 10.2172/1047044 | External Link
  • Office of Scientific & Technical Information Report Number: 1047044
  • Archival Resource Key: ark:/67531/metadc828578

Collections

This report is part of the following collection of related materials.

Office of Scientific & Technical Information Technical Reports

Reports, articles and other documents harvested from the Office of Scientific and Technical Information.

Office of Scientific and Technical Information (OSTI) is the Department of Energy (DOE) office that collects, preserves, and disseminates DOE-sponsored research and development (R&D) results that are the outcomes of R&D projects or other funded activities at DOE labs and facilities nationwide and grantees at universities and other institutions.

What responsibilities do I have when using this report?

When

Dates and time periods associated with this report.

Creation Date

  • July 31, 2012

Added to The UNT Digital Library

  • May 19, 2016, 9:45 a.m.

Description Last Updated

  • June 20, 2016, 12:38 p.m.

Usage Statistics

When was this report last used?

Yesterday: 0
Past 30 days: 2
Total Uses: 6

Interact With This Report

Here are some suggestions for what to do next.

Start Reading

PDF Version Also Available for Download.

Citations, Rights, Re-Use

Nieh, T.G. Roles of nanoclusters in shear banding and plastic deformation of bulk metallic glasses, report, July 31, 2012; United States. (digital.library.unt.edu/ark:/67531/metadc828578/: accessed December 17, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.