Deformation map of metallic glass: Normal stress effect
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Published online 24 September 2020 | https://doi.org/10.1007/s40843-020-1454-x
Deformation map of metallic glass: Normal stress effect 1,2
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Shaojie Wu , Ruitao Qu
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Up to now, it remains unclear how normal stress affects the deformation mode transition of metallic glasses (MGs) at high temperatures. Here by constructing the deformation maps under both compression and tension, we show that the tensile normal stress can largely promote the transition from shear localization to homogeneous deformation while the compressive one may slightly suppress it, demonstrating a significant normal stress effect. The present results emphasize the critical role of stress state in high-temperature deformation of MGs, implying that the thermoplastic forming of MGs could be better controlled via adjusting the local stress state. MGs prefer localized shear-banding plastic deformation at ambient temperature but homogenous deformation at elevated temperatures [1–3]. However, these unique deformation behaviors are susceptible to testing conditions such as the temperature and strain rate [1,4–6]. In order to understand how such external conditions affect the deformation behavior of MGs, one way is to experimentally construct the deformation map, which could also act as an important tool for predicting the mechanism by which the MG deforms and hence guiding the optimization of processing parameters such as the best thermoplastic forming conditions. Spaepen [7] developed the first deformation map for MG. In the stress-temperature space, he distinguished the plastic deformation into two modes: inhomogeneous deformation and homogeneous deformation. The former mode occurs at low temperatures with high applied stress while the latter happens at high temperatures with low applied stress. Subsequently, Megusar et al. [8] examined the strain rate effect on the deformation mode transition and then established another deformation map, which was plotted in the strain rate-temperature space. Since then, efforts on understanding the deformation map have 1 2
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, Zhengwang Zhu , Haifeng Zhang
and Zhefeng Zhang
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not been suspended [4,9–15]. For instance, in homogeneous deformation regime, a characteristic transition from non-Newtonian flow to Newtonian flow was observed with decreasing strain rate or increasing temperature [4,9,13,14]. In addition, by deforming MGs near the glass transition temperature (Tg), Schuh et al. [11] discovered a new region of homogeneous flow, which occurs when the high deformation rate exceeds the rate of shear band nucleation and is different from the traditional viscous-flow-induced homogeneous deformation. Given the new discovery, more detailed deformation maps of MGs have been developed individually by Lu et al. [4], Schuh et al. [1] and Sun et al. [15]. Nevertheless, the previous researchers usually investigated the deformation maps using one testing method (e.g., compression, tension or nanoindentation) [1,4,11–15], and few studies [16–18] focused on the comparison of deformation mode features under
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