Hardening and crystallization in monatomic metallic glass during elastic cycling
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Yun Deng Key Laboratory of Magnetic Resonance in Biological Systems, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
Chuang Denga) Department of Mechanical Engineering, The University of Manitoba, Winnipeg, Manitoba R3T 5V6, Canada (Received 16 January 2015; accepted 22 April 2015)
While conventional metallic glass (MG) is usually an alloy that contains at least two types of different elements, monatomic metallic glass (MMG) in body-centered cubic metals has recently been vitrified experimentally through ultrafast quenching. In this research, MMG in Ta was vitrified by molecular dynamics simulations and used as a model system to explore the atomistic mechanism of hardening in MG under cyclic loading well below the yield point. It was found that significant structural ordering was caused during the elastic cycling without accumulating apparent plastic strain, which ultimately led to the crystallization of MG that has been long conjectured but rarely directly proved before. It was also revealed that tensile stresses were more likely to induce structural ordering and crystallization in MG than compressive stresses.
I. INTRODUCTION
The mechanical properties of materials are strongly linked with their microstructural evolution under external loading. Whereas structural defects such as dislocations have been found to govern the microstructural evolution and mechanical properties in crystalline materials, no such defects have been unambiguously identified in amorphous materials such as metallic glass (MG). Therefore, despite the significant efforts that have been made in the past,1–13 the microstructural evolution that leads to various mechanical responses in MGs is still under debate, among which the cyclic hardening within the elastic regime8–13 has drawn considerable attention in recent years. It has been widely reported that the yield strength of MGs can be significantly increased by cycling the sample with small amplitude, which can significantly impact the fracture and fatigue behavior of the MGs.8–13 Although no apparent plastic strain or macroscopic shear banding can be detected during the elastic cycling in MG, it has been conjectured that substantial microstructural ordering or even nanocrystallization has occurred.8–10 Recently, Wang et al.11 found that nanocrystals can be formed at a semicircle notch-tip region in cyclically strained MG
Contributing Editor: Franz Faupel a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.130 1820
J. Mater. Res., Vol. 30, No. 11, Jun 14, 2015
http://journals.cambridge.org
Downloaded: 06 May 2016
based on in situ TEM experiments. However, the molecular dynamics (MD) simulations performed by Wang et al.11 also suggested that before the formation of the nanocrystal, the region around the notch-tip has experienced severe plastic deformation due to the strong stress concentration at the notch-tip even though the overall cycling strain was below the yield point. To date, no direct experimenta
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