Dynamic indentation response of ZrHf-based bulk metallic glasses

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Hongwen Zhang Department of Materials Science & Engineering, Michigan Technological University, Houghton, Michigan 49931 (Received 13 August 2006; accepted 23 October 2006)

Static and dynamic Vickers indentations were performed on ZrHf-based bulk amorphous alloys. A decrease in indentation hardness was observed at higher strain rates compared with static indentation hardness. For equivalent loads, dynamic indentations produced more severe deformation features on the loading surface than static indentations. Using bonded interface technique, the induced shear band patterns beneath the indentations were studied. In static indentations, the majority of the deformation was primarily accommodated by closely spaced semicircular shear bands surrounding the indentation. In dynamic indentations two sets of widely spaced semicircular shear bands with two different curvatures were observed. The observed shear band patterns and softening in hardness were rationalized based on the variations in the confinement pressure, strain rate, and temperature within the indentation region during dynamic indentations. It is also proposed that free volume migration and formation of nano-voids leading to cracking are favored due to adiabatic heating and consequently cause the observed softening at high strain rates. I. INTRODUCTION

The exceptional mechanical properties, such as high Young’s modulus (87∼110 GPa), high compressive yield strength (1500–2200 MPa), and large elastic strain (2∼3%) in bulk metallic glasses (BMGs) have made them attractive structural materials for military and civil applications.1–3 Localized shear bands evolution in the form of slip steps is widely accepted as the primary mechanism to accommodate plastic deformation in BMGs.4–6 However, direct observation of shear bands evolution in traditional compression and tension tests is difficult because shear bands develop quickly and can be identified only when they grow and leave distinguishable slip steps or even crack patterns on the specimen surface. To study this inhomogeneous deformation, indentation tests have been widely used due to two distinct advantages: (i) indentation tests do not require large specimen size and are easy to perform and (ii) the deformation zone is constrained locally around the indent and allows for large strain to be accumulated, which facilitates the

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Address all correspondence to this author. Present address: Mechanical and Aerospace Engineering, University of Florida, Gainesville, Florida 32611. e-mail: [email protected] DOI: 10.1557/JMR.2007.0058 478 J. Mater. Res., Vol. 22, No. 2, Feb 2007 http://journals.cambridge.org Downloaded: 04 Apr 2015

study of plastic flow surrounding and underneath the indentation. However, only a few shear bands have been observed on the top surface of indentation. This small number of shear bands cannot account for large inelastic strain in indentation tests.7–10 Therefore, using bonded split specimens shear band patterns beneath Vickers indentations was studied.8–10 These observations verified that numer