Microscopic Characterization of Relaxation Behavior in Metallic Glasses
Lei, Tianjiao
2019
Abstract
Metallic glasses (MGs) show high strength and elastic limit, but they also exhibit little macroscopic plasticity, which limits their structural applications. The main reason is strain localization within dominant shear bands. Because of the amorphous structure of MGs, it is challenging to define defects that accommodate deformation. Observations in physical analogs have shown that macroscopic deformation of MGs is accommodated by cooperative shearing of atomic clusters, termed shear transformation zones (STZs). At small strains, STZs are isolated. They can be reversed by back stress in the elastic matrix upon removal of the applied stress, which results in anelastic behavior. The common observation of a main (α) relaxation, and high-frequency (β) relaxation in mechanical spectroscopy is readily explained in molecular glasses, but their microscopic mechanisms in MGs are still not clear. Recently, the intensity of the β relaxation has been correlated with macroscopic plasticity of MGs. One goal of the present project is to use anelasticity to characterize the STZ spectra of La-based MGs with and without a pronounced β relaxation, and to study the α vs. β relaxation and structural relaxation vs. cryogenic cycling from a microscopic view. La55Ni20Al25 and La70(NixCu1-x)15Al15, x=0,1 MGs have been investigated. The results suggest that the chemical composition of STZs corresponding to α vs. β relaxation is different, indicated by two regimes of STZ activation volume. Room-temperature structural relaxation only affects the larger/slower STZs (corresponding to α relaxation) by decreasing the number of the corresponding potential STZs (i.e., atomic clusters capable of shear transformation) while increasing the relaxation time constants. A detailed description of structural relaxation emerges: its dominant effect is on the largest, and therefore slowest, STZs observed in the present kinetics window. Cycling between liquid-nitrogen temperature and room temperature reverses the increasing time constants due to structural relaxation in La70(NixCu1-x)15Al15, x=0,1, pointing to a rejuvenation effect. However, cycling does not significantly affect the anelastic behavior. The pronounced β peak observed in normalized loss modulus of La70Ni15Al15 but not La70Cu15Al15 is a result of both larger volume fraction of fast and small potential STZs and smaller volume fraction of slow and larger potential STZs in La70Ni15Al15 vs. La70Cu15Al15. Another focus of the present project is the nonlinear anelastic relaxation in MGs under high stress, for which the viscosity is non-Newtonian, and therefore the rate of anelastic relaxation is not linear in the applied stress. In this regime, the details of the activation volume, not available in the linear regime, can be obtained. Despite the complicated stress state due to nonlinearity, bending allows for stable measurements for a long period. A method of controlled sample bending to a strain of up to ~ 0.0155 has been developed, and applied to Al86.8Ni3.7Y9.5 MG. Significant nonlinearity of the anelastic strain in the stress was observed, which is mainly associated with the largest and slowest active STZs not reaching mechanical equilibrium at the end of the constraining period. Combining nonlinear kinetics under constraint and zero bending moment after constraint removal, the volume of the largest active STZs and the transformation shear strain were obtained independently for the inherent state – their most likely values are 4.8×10-28 m3 and 0.18, respectively.Subjects
metallic glasses mechanical relaxation structural relaxation
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