Influence of the loading rate on the indentation response of Ti-based metallic glass
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Fornell Departament de Fı´sica, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Spain
W. Li School of Materials Science and Engineering, Anhui University of Technology, 243002 Maanshan Anhui, China
S. Surin˜ach and M.D. Baro´ Departament de Fı´sica, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Spain (Received 30 July 2008; accepted 27 October 2008)
The mechanical behavior of Ti-based metallic glass has been investigated by means of indentation experiments at different loading rates. Contrary to many crystalline materials, an increase of the loading rate causes a reduction of hardness, i.e., a mechanical softening. This effect is ascribed to deformation-induced creation of excess free volume, which is more pronounced for higher strain rates. The decrease of hardness is accompanied with an increase of the contact stiffness and a reduction of the reduced elastic modulus. Finite element simulations reveal that the mechanical response of this material can be described using the Mohr-Coulomb yield criterion. The changes in the nanoindentation curves with the increase of loading rate are well reproduced by decreasing the value of the Mohr-Coulomb cohesive stress. This result is consistent with the presumed enhancement of free volume. I. INTRODUCTION
During the last decades, the great advancement in the lithography methods to fabricate structures with sizes in the sub-micrometer range has prompted the development of new techniques to characterize the different types of properties (e.g., electric, magnetic, or mechanical) of materials. Depth-sensing nanoindentation appears to be a powerful method for testing the mechanical behavior of materials at the micrometer and sub-micrometer scale.1–5 This method is not restricted to systems with small dimensions but is particularly suitable to characterize bulk materials that exhibit premature failure during macroscopic tensile tests. For this reason, nanoindentation can be used to study the deformation mechanisms of metallic glasses (MGs), which generally show little plasticity during conventional compression tests.6–9 Nanoindentation load-displacement curves of MGs, measured at room temperature (i.e., far below the glass transition), are typically serrated (i.e., exhibit displacement bursts or pop-ins), an effect which is ascribed to a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0117
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J. Mater. Res., Vol. 24, No. 3, Mar 2009
the propagation of shear bands that are formed as a result of coalescence of excess free volume. The influence of the loading rate on the amount and size of these serrations has been extensively investigated during the last few years.10–12 It has been claimed that higher loading rates cause a gradual disappearance of the pop-in events due to the simultaneous operation of multiple shear bands that do not have sufficient time to fully nucleate (and propagate) and, therefore, do not give rise to serrated flow. Another interesting feature of the deformation behavior of MGs is that plastic flow
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