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تعداد صفحات این فایل: ۱۸ صفحه

بخشی از مقاله انگلیسیعنوان انگلیسی:Indentation size effects in the nano- and micro-hardness of a Fe-based bulk metallic glass~~en~~

Abstract

Hardness of a Fe-based bulk metallic glass (BMG) was evaluated by both atomic force microscopy (AFM) nanoindentation (nano-hardness) and instrumented indentation with a traditional indenter setup (micro-hardness) under different maximum loads at room temperature. The nano-hardness and the micro-hardness were found to be comparable. For both of the indentation methods, indentation size effect (ISE) is detected as increase in hardness with decrease in indentation peak load. It is proposed that strain rate dependent softening, loading history and the lag between free volume creation and mechanical softening should be responsible for the ISE in this BMG. Furthermore, ISE is found to be more significant in AFM nanoindentation than in instrumented indentation. This can be explained by taking into account the effect of exerted peak load and the face angle of the indenter in a qualitative manner.

۱ Introduction

In the last decade, mechanical properties of bulk metallic glasses (BMGs) on nano-scale have been motivating extensive researches [1–۱۰]. Recently, microscopic structural and mechanical heterogeneities have been found in CuZr [4], Ti [5], Ni [6], Pd [7] and Zr [8–۱۰] based BMGs. Based on the spatial nano-hardness tests, Wang et al. [11] have quantified the mechanical heterogeneity of Zr64.13Cu15.75Ni10.12Al10 BMG, whose plastic strain was reported to be over 160% at room temperature [9], and the extraordinary plasticity was attributed to distinctive micrometersized structural heterogeneity (i.e., soft and hard regions). On the other hand, nanoindentation hardness of individual shear bands in BMGs has been investigated by some authors for better understanding of the role of free volume in the phenomenological shear localization in metallic glasses [12–۱۶]. Yoo et al. [13] found that the hardness of the shear bands, whose thickness was reported in the range of hundreds of nanometers, is much lower than the respective hardness of undeformed region. From the above mentioned, we can see as the interests in exploring mechanical properties of small samples have increased, indentation techniques should be employed on an even finer scale in order to match the imprint size to the object dimensions. To fulfill this aim, atomic force microscopy (AFM) nanoindentation [17] which has been claimed to be able to extract quantitative mechanical information coupled with its inherently high spatial resolution of imaging, may be a powerful tool to characterize nanoscale properties of BMGs. The prior scanning of the surface with AFM allows one to select areas of interest [18]. Nowadays, this technique has been extensively employed to evaluate hardness on the nanometer scale in a variety of materials [18–۲۱], but its application on BMGs, to the best of our knowledge, is still an open issue.

During the indentation, the most interesting phenomenon is the indentation size effect (ISE) [22,23], which is manifested as an increase in hardness H with decreasing indentation peak load Pmax. Normally, ISE is not expected to occur in BMGs due to the absence of dislocations and strain hardening. However, in some investigations ISE was reported in Zr [24–۲۶], Pd [27], Au-based BMGs [28] and amorphous solids [22]. The strain-induced softening due to the excessive free volume creation during plastic deformation was regarded as the main cause for ISE [24]. To date, the reported ISE in BMGs are mainly studied by instrumented indentation on the micrometer scale (with the peak load of
mN). Therefore, it is intriguing to know whether and how ISE would exist on a nanometer scale under AFM nanoindentation, as the possible mechanisms causing the ISE may play a key role on better understanding of the plastic deformation in BMGs.

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