Enhancement of plasticity in Zr-based bulk metallic glasses

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K. Saksl and H. Franz HASYLAB am DESY, D-22603 Hamburg, Germany

H-J. Fecht Department of Materials, Faculty of Engineering, University of Ulm, D-89081 Ulm, Germany; and Forschungszentrum Karlsruhe, Institut für Nanotechnologie, Karlsruhe, Germany

Y.G. Liu and H.S. Xian Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, People’s Republic of China (Received 30 December 2006; accepted 14 March 2007)

We present evidence that a minor adjustment in Zr/Ni concentration ratio can dramatically enhance the plasticity of monolithic Zr-based bulk metallic glasses (BMGs) from about 2.2% for Zr65Al8Ni10Cu17 BMG to 14% for Zr62Al8Ni13Cu17 BMG. No deformation-induced nanocrystallization appears in a 55% strained Zr62Al8Ni13Cu17 BMG without catastrophic failure while pre-existing nanocrystals in Zr65Al8Ni10Cu17 BMG result in its limited plasticity. Also note that the stability of Zr62Al8Ni13Cu17 BMG against crystallization upon deformation is somewhat higher than that of Zr65Al8Ni10Cu17 BMG. As determined by x-ray diffraction using synchrotron radiation, the enhanced plasticity of Zr62Al8Ni13Cu17 BMG seems to be related to the relative homogeneity of the amorphous structure. I. INTRODUCTION

As it is for traditional oxide glasses, brittleness is a critical drawback of bulk metallic glasses (BMGs) for their widespread application, even under compression conditions.1 The plastic flow generally allows only a few shear bands to be active, resulting in catastrophic failure of the sample. To improve the ductility of monolithic BMGs, so-called BMG composites have been widely developed by introducing nanometer-scale precipitates2–4 or micrometer-scale ductile crystalline phases5–7 that can act as barriers to the propagation of shear bands or microcracks. Actually, the mechanism by which the shear bands can be induced in monolithic BMGs is still unclear. In ductile crystalline materials, such as Cu, Al, etc., the large amount of slip systems consisting of closepacked planes and directions allows dislocations to be easily initiated and plastic strain to homogeneously accumulate upon loading. Due to dislocation interaction, high energy is required to move atoms, which is called work hardening. The slip bands in crystalline materials are the steps formed by the dislocations when they escape

Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2007.0324 2454 J. Mater. Res., Vol. 22, No. 9, Sep 2007 http://journals.cambridge.org Downloaded: 18 Mar 2015

to the surface under shear stress. However, in dislocation-free BMGs, the yield strength approaches the theoretical strength, and their catastrophic failures are thought to be associated with the local changes in shear bands (strain softening), including the free volume coalescence8,9 and the localized instability by adiabatic heating.10,11 Recently, ductile monolithic BMGs have been reported.12–14 Nanometer-scale heterogeneities15,16 in the amorphous phase were proposed to enhance plasticity because t