Tianjiao Lei1 Luis Rangel DaCosta1 Yonghao Sun3 2 A. Lindsay Greer3 Ming Liu2 Zhen Lu2 Wei Hua Wang2 Michael Atzmon1 4

1, Materials Science and Engineering, University of Michigan-Ann Arbor, Ann Arbor, Michigan, United States
3, Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, , United Kingdom
2, Institute of Physics, Chinese Academy of Sciences, Beijing, , China
4, Nuclear Engineering and Radiological Sciences, University of Michigan–Ann Arbor, Ann Arbor, Michigan, United States

Metallic glasses (MGs) possess high strength and elastic limit, but they also exhibit little macroscopic ductility due to shear band formation. Because of the disordered atomic structure, it is challenging to define defects that accommodate deformation in MGs. From observations in physical analogs, these defects have been identified as shear transformation zones (STZs), which are dissipative and thermally activated [1]. At small strains, STZs are isolated and can be reversed due to back-stress in the elastic matrix upon removal of the applied stress, which results in anelastic behavior. Recently, Ju et al. obtained a quantized hierarchy of STZs for an Al-based MG from quasi-static anelastic relaxation measurements [2]: the volume difference between STZs corresponding to adjacent peaks in the time-constant spectrum equals that of one Al atom. The volume fraction of potential STZs, i.e. atomic clusters capable of shear transformation, increases with size. After annealing at 110 oC for 1 hour, the number of potential STZs decreases, but the corresponding time constants remain unchanged [3].
The common observation of a main (α) relaxation, and high-frequency (β) relaxation in mechanical spectroscopy is readily explained in molecular glasses, but its microscopic nature in MGs is still under debate. It has been shown that the relative intensity of the β relaxation in MGs correlates with ductility [4]. The purpose of the present work is to use anelasticity to characterize the STZ spectra of La-based MGs with and without pronounced β relaxation, and to study the effect of structural relaxation from a microscopic view. La55Ni20Al25 and La70(NixCu1-x)15Al15, x=0,1 were investigated. Two regimes of volume increment are observed, suggesting a size dependence of STZ composition. Room-temperature structural relaxation only affects the larger/slower STZs -- the number of corresponding potential STZs decreases, while their time constants increase. Notably, a detailed description of structural relaxation emerges: its dominant effect is on the largest, and therefore slowest, STZs. Cycling to liquid-nitrogen temperature, observed to induce rejuvenation [5], only leads to subtle changes in the anelastic behavior. The effect of the composition x above will be discussed in the context of observations on its effect on ductility [4].
[1] Argon, Acta Metall. 27, 4758 (1979).
[2] Ju et al. J. Appl. Phys. 109, 053522 (2011).
[3] Atzmon and Ju, Phys. Rev. E 90, 042313 (2014).
[4] Yu et al. Phys. Rev. Lett. 108, 015504 (2012).
[5] Ketov et al., Nature 524, 200 (2015).