Free-volume dependent pressure sensitivity of Zr-based bulk metallic glass
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K. Eswar Prasad Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India
Rejin Raghavan EMPA Materials Science and Technology, 3602 Thun, Switzerland; and Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India
Jo¨rg F. Lo¨ffler Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
Johann Michler EMPA Materials Science and Technology, 3602 Thun, Switzerland
Upadrasta Ramamurtya) Department of Materials Engineering, Indian Institute of Science, Bangalore-560012, India (Received 5 March 2009; accepted 4 May 2009)
Instrumented indentation experiments on a Zr-based bulk metallic glass (BMG) in as-cast, shot-peened and structurally relaxed conditions were conducted to examine the dependence of plastic deformation on its structural state. Results show significant differences in hardness, H, with structural relaxation increasing it and shot peening markedly reducing it, and slightly changed morphology of shear bands around the indents. This is in contrast to uniaxial compressive yield strength, sy, which remains invariant with the change in the structural state of the alloys investigated. The plastic constraint factor, C = H/sy, of the relaxed BMG increases compared with that of the as-cast glass, indicating enhanced pressure sensitivity upon annealing. In contrast, C of the shot-peened layer was found to be similar to that observed in crystalline metals, indicating that severe plastic deformation could eliminate pressure sensitivity. Microscopic origins for this result, in terms of shear transformation zones and free volume, are discussed.
I. INTRODUCTION
Experimental assessment of plasticity in amorphous alloys is relatively complicated, compared with that of crystalline metals and alloys. This difference is because of the inhomogeneous nature of deformation in metallic glasses through shear banding as well as the mechanical instability associated with the strain softening postyielding. Pressure sensitivity of plastic flow complicates the matter further, imparting tension/compression asymmetry. In uniaxial tension, fracture ensues almost immediately after yield due to the mechanical instability. Hence, only the yield strength can be measured reliably by tension tests. Flexure loading, which is popular for testing ribbon forms of rapidly quenched glasses,1,2 is a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0304 J. Mater. Res., Vol. 24, No. 8, Aug 2009
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not suitable for thicker plates of bulk metallic glasses (BMGs). This is because shear bands become shear cracks, once they are allowed to traverse certain minimum distance, and the large elastic strain energy stored in the specimen gets released upon the initiation of shear cracks, leading to immediate fracture.1 In addition, accurate determination of the stress state under flexure loading is complicated by the tension-compression asy
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