Temperature dependence of serrated flows in compression in a bulk-metallic glassW. H. Jiang, F. Jiang, F. X. Liu, H. Choo, P. K. Liaw, and K. Q. Qiu Citation: Applied Physics Letters 89, 261909 (2006); doi: 10.1063/1.2424275 View online: http://dx.doi.org/10.1063/1.2424275 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/26?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Serrated flow behaviors of a Zr-based bulk metallic glass by nanoindentation J. Appl. Phys. 115, 084907 (2014); 10.1063/1.4866874 Influence of friction on the residual morphology, the penetration load and the residual stress distribution of a Zr-based bulk metallic glass AIP Advances 3, 042116 (2013); 10.1063/1.4802835 Exponential decay of shearing stress during jerky flows in a Zr-based bulk metallic glass AIP Advances 3, 032105 (2013); 10.1063/1.4794725 Dynamics of serrated flow in a bulk metallic glass AIP Advances 1, 032158 (2011); 10.1063/1.3643218 Rate-dependent inhomogeneous-to-homogeneous transition of plastic flows during nanoindentation of bulkmetallic glasses: Fact or artifact? Appl. Phys. Lett. 90, 211906 (2007); 10.1063/1.2742286

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Temperature dependence of serrated flows in compressionin a bulk-metallic glass

W. H. Jiang,a� F. Jiang, F. X. Liu, H. Choo, and P. K. LiawDepartment of Materials Science and Engineering, The University of Tennessee, Knoxville,Tennessee 37996

K. Q. QiuSchool of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110023,People’s Republic of China

�Received 18 October 2006; accepted 20 November 2006; published online 27 December 2006�

The authors investigated the effect of temperature on the plastic-flow and shear-banding behaviorsof a Zr-based bulk-metallic glass in compression. The results indicate that at a subambienttemperature �194.5 K� the plastic deformation of the bulk-metallic glass is still inhomogeneous andis characterized by the serrated flow and shear banding. But, compared to the ambient-temperatureresults, the subambient temperature moderates serrations in the plastic flow and leads to theformation of fewer shear bands. In view of the spatiotemporality of shear banding, the authorscorrelate the plastic-flow behaviors to shear-band patterns. © 2006 American Institute of Physics.�DOI: 10.1063/1.2424275�

At low temperatures �e.g., room temperature� and highstrain rates, the plastic deformation of metallic glasses isknown to be inhomogeneous and localized to narrow shearbands.1,2 The excessive propagation of individual shearbands is attributed to poor plasticity and premature fracture,which prevent metallic glasses from structural applications.The formation of the shear bands affects the macroscopicplastic-flow behavior substantially. The serrated plasticflow during the inhomogeneous deformation in metallicglasses was widely observed in various loading modes,such as compression,3–9 bending,10–12 tearing,13,14 andnanoindentation.3,15–21 Extensive investigations demon-strated that the serrated flow in metallic glasses is highlydependent on strain rates.4,5,7,9,10,15–21 Like strain rates, tem-perature also affects the plastic-flow behavior of metallicglasses. However, little work has been done in the regime ofcryogenic temperatures. In this letter, we report the effect oftemperature �ambient and subambient� on the plastic-flowbehavior of a Zr-based bulk-metallic glass �BMG� at variousstrain rates.

The Zr52.5Cu17.9Ni14.6Al10.0Ti5.0 �at.%� BMG �Vitreloy105� was used in this work. It was prepared by arc-meltingpure elements in a purified argon atmosphere, followed bycasting in a water-cooled copper mold. The amorphous struc-ture was confirmed by x-ray diffraction with Cu K� radia-tion. The compression-test specimens have a dimension of1.8 mm in height �h� and 3 mm in diameter �d�, with an h /dratio of 0.6. The low h /d ratio can constrain excessive propa-gation of individual shear bands that may cause the prema-ture fracture, facilitating an investigation on the plastic flowin compression.9,22–24 The compression tests were performedat ambient �298 K� and subambient �in dry ice, 194.5 K�temperatures, respectively. An MTS servohydraulic mechani-cal testing machine was used for the compression at strainrates of 1.87�10−1, 2.55�10−2, and 2.34�10−3 s−1. Thedeformation was terminated at a total strain of 20%. Strainwas recorded using an extensometer. The side surfaces of the

postdeformed specimens were observed using scanning elec-tron microscopy �SEM�.

Figure 1 shows the amplified plastic-flow portions of theengineering stress-strain curves of the BMG specimens at theambient and subambient temperatures and at various strainrates. The serrations can be observed for all the specimenssubjected to the compression at various conditions. At thesubambient temperature, the serrated plastic-flow behaviorsexhibit the same strain-rate dependence as those at the am-bient temperature. That is, the serrations tend to decreasewith increasing strain rates, which was widely observed inother loading modes.3–5,7,8,10–13,16–21 However, the serrationsat the subambient temperature become less obvious thanthose at the ambient temperature.

The temporal intervals and the stress variations betweenthe serrations are measured from the nominal stress versustime curves during the plastic flows, whose portions areshown in Fig. 2. The results are listed in Table I. Evidently,for the plastic flows at the ambient and subambient tempera-tures, the temporal intervals between the stress peaks and the

a�Electronic mail: wjiang5@utk.edu

FIG. 1. �Color online� Amplified engineering stress-strain curves of theBMG specimens at the ambient ��a1�–�a3�� and subambient ��b1�–�b3�� tem-peratures at the strain rates of 1.87�10−1 s−1 ��a1� and �b1��, 2.55�10−2 s−1 ��a2� and �b2��, and 2.34�10−3 s−1 ��a3� and �b3��.

APPLIED PHYSICS LETTERS 89, 261909 �2006�

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stress variations are closely related to the strain rates: thehigher the strain rates, the shorter the temporal intervals andthe smaller the stress variations. It can be seen that the de-crease in temperature substantially reduces the temporal in-tervals between the stress peaks and the stress variations.

SEM observations on the side surfaces of the postde-formed specimens show that shear bands form for all theconditions, and the number of shear bands decreases withincreasing strain rates for both the ambient and subambienttemperatures. At the lower strain rates, many fine shearbands are produced, while at higher strain rates, a few coarseshear bands form �Fig. 3�. This trend was observed before inquasistatic uniaxial compression tests of geometrically con-strained BMG specimens,9 which is apparently differentfrom the nanoindentation studies.20

The present work demonstrates that the deformation ofthe BMG at the subambient temperature is also inhomoge-neous and characterized by shear banding and strain-rate de-pendent, serrated, plastic flows. But the temperature signifi-cantly affects the plastic flow: the decrease in temperaturemoderates the serrations. This result indicates that the tem-perature has an impact on a shear-banding operation. How-ever, it seems to be difficult to correlate the serrations �Fig. 2and Table I� to the shear-band patterns �Fig. 3� observed exsitu on the postdeformed specimens using SEM.

Shear banding is a spatiotemporal process. A shear-bandpattern observed ex situ only presents its spatial characteris-tic feature, while a plastic-flow curve �stress versus strain orstress versus time� in uniaxial loading records its temporalprocess. Recently, using an infrared camera, we observed insitu the dynamic shear-banding processes in the quasistaticuniaxial compression of the BMG specimens.9 The infrared-camera records show that at lower strain rates more simulta-neous shear-banding operations occur intermittently, and athigher strain rates fewer simultaneous shear-banding opera-

tions happen successively. The temporal intervals betweenshear-banding events measured from the video records arecoincident with the intervals between the stress peaks in theplastic-flow curve, indicating that the serration is a result ofshear-banding operations. The ex situ observations on theshear-band patterns on the side surface of the postdeformedspecimens demonstrate the same trend as the present work:with increasing strain rates, shear bands become fewer andcoarser.9 The investigations on the spatiotemporality of shearbanding show that in uniaxial compression, with increasingstrain rates, the plastic deformation of the BMG tends to behomogeneous only in time but not in space.

In view of the spatiotemporality of shear-banding opera-tions, we can explain the effect of the temperature on theserrated flow and shear banding. At the subambient tempera-ture the serrations become less obvious. That is, the temporalintervals between the stress peaks become shorter and thestress variations become smaller. This plastic-flow behaviorshows the temporality of shear banding, indicating thatshear-banding operations tend to be temporally successiveand not simultaneous. SEM observations show the spatialityof shear banding. The observed fewer and coarser shearbands �Fig. 3� indicate that shear banding tends to more in-homogeneous in space with decreasing temperatures. Theobserved individual shear bands must be a result of repeateddisplacement bursts at a single shear band, as the number ofdynamic shear-banding operations observed in situ is muchlarger than that of shear bands observed ex situ.9 The re-peated arrest and reactivation of shear bands have not beenexplained conclusively. We think that the preferential occur-rence of shear banding at a preexisting shear band may berelated to strength. A preexisting shear band contains exces-sive free volume introduced by predeformation and, thus, hasa lower strength than undeformed surroundings. Being aweak link in an amorphous matrix, a preexisting shear band

FIG. 2. Amplified engineering stress vs time curves of the BMG specimensat the ambient ��a1�–�a3�� and subambient ��b1�–�b3�� temperatures at thestrain rates of 1.87�10−1 s−1 ��a1� and �b1��, 2.55�10−2 s−1 ��a2� and �b2��,and 2.34�10−3 s−1 ��a3� and �b3��.

TABLE I. Average time intervals between the stress peaks and the stress variations.

Temperatures Ambient �298 K� Subambient �194.5 K�

Strain rates �s−1� 1.87�10−1 2.55�10−2 2.34�10−3 1.87�10−1 2.55�10−2 2.34�10−3

Time intervals �s� 0.005 0.032 0.264 0.003 0.029 0.160Stress variations �MPa� 7.9 8.6 15.5 1.7 5.0 12.9

FIG. 3. SEM images showing shear bands on the side surfaces of the BMGspecimens deformed to a plastic strain of 18% at the ambient ��a1� and �a2��and subambient temperatures ��b1� and �b2�� at the strain rates of 1.87�10−1 s−1 ��a1� and �b1�� and 2.34�10−3 s−1 ��a2� and �b2��. The arrowsindicate the loading direction.

261909-2 Jiang et al. Appl. Phys. Lett. 89, 261909 �2006�

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may become a preferential site for subsequent deformation.Recently, an increased strength was observed at the liquid-nitrogen temperature.25,26 At subambient temperature the in-crease in strength may make the formation of shear bandsmore difficult. On the other hand, since the kinetics of anni-hilation of free volume become smaller at the subambienttemperature,27,28 a larger amount of excessive free volumeremains in a preexisting shear band. Therefore, compared tothe undeformed matrix, a preexisting shear band may have anegligible increase in strength at subambient temperature,and the difference in strength between a preexisting shearband and the undeformed matrix may become larger. As aresult, the tendency of shear banding at a preexisting shearband increases, and the deformation becomes more inhomo-geneous in space.

In summary, like strain rates, temperature significantlyaffects the plastic-flow and shear-banding behaviors of theBMG. Decreasing temperature causes the plastic deforma-tion to be more homogeneous in time but more inhomoge-neous in space. The understanding of the temperature depen-dence of the serrated flow and shear banding may haveimportant implications on the research, development, andstructural applications of bulk-metallic glasses.

This work was supported by the National Science Foun-dation �NSF� International Materials Institutes �IMI� Pro-gram �DMR-0231320�, with C. Huber as the ProgramDirector.

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