Structural instability of metallic glasses under radio-frequency-ultrasonic perturbation and its correlation with glass-to-crystal transition of less-stable metallic glasses

Abstract

It has been reported that the structural stability is significantly deteriorated under radio-frequency-ultrasonic perturbation at relatively low temperatures, e.g., near/below the glass transition temperature T(g), even for thermally stable metallic glasses. Here, we consider an underlying mechanism of the ultrasound-induced instability, i.e., crystallization, of a glass structure to grasp the nature of the glass-to-liquid transition of metallic glasses. Mechanical spectroscopy analysis indicates that the instability is caused by atomic motions resonant with the dynamic ultrasonic-strain field, i.e., atomic jumps associated with the beta relaxation that is usually observed for low frequencies of the order of 1 Hz at temperatures far below T(g). Such atomic motions at temperatures lower than the so-called kinetic freezing temperature T(g) originate from relatively weakly bonded (and/or low-density) regions in a nanoscale inhomogeneous microstructure of glass, which can be straightforwardly inferred from a partially crystallized microstructure obtained by annealing of a Pd-based metallic glass just below T(g) under ultrasonic perturbation. According to this nanoscale inhomogeneity concept, we can reasonably understand an intriguing characteristic feature of less-stable metallic glasses (fabricated only by rapid melt quenching) that the crystallization precedes the glass transition upon standard heating but the glass transition is observable at extremely high rates. Namely, in such less-stable metallic glasses, atomic motions are considerably active at some local regions even below the kinetic freezing temperature. Thus, the glass-to-crystal transition of less-stable metallic glasses is, in part, explained with the present nanoscale inhomogeneity concept.

Relaxation of Bulk Metallic Glasses Studied by Mechanical Spectroscopy

“…A single semicircle would correspond to a Debye-like process, an asymmetric arc to a KWW-like process, and multiple arcs to multiple processes. The Cole−Cole plot for the La 60 Ni 15 Al 25 bulk metallic glasses shows a pronounced inflection corresponding to a well separated β relaxation, while the Pd 40 Ni 10 Cu 30 P 20 metallic glass exhibits a pronounced shoulder in the range 0.6 < G′/G u < 1, which is associated with an "excess wing". In Zn 38 Mg 12 Ca 32 Yb 18 bulk metallic glass, the Cole−Cole diagram displays a nearly symmetrical semicircle and the β relaxation is not detected in the current study.…”

“…Heating to a temperature much higher than T g leads to a crystallization of the specimen curve 4. In a crystalline 40 Ni 10 Cu 30 P 20 log(τ α ) = 0.67, β KWW = 0.54, ΔG α /G″ max = 0.7, log(τ β ) = −3.9, ΔG β /G″ max = 0.13; a = 0.56, b = 1. The arrow is the τ 0 of the CM calculated using these parameters.…”

“…With decreasing temperature the peak frequency of the α-peak in the loss modulus G″ decreases. With the assumption of the time− temperature superposition principle, a master curve can be plotted for the loss modulus G″/G u in Pd 40 Ni 10 Cu 30 P 20 and La 60 Ni 15 Al 25 bulk metallic glasses in a large frequency domain (Figure 8(b)). …”

“…Loss factor tan δ = G″/G′ was also determined. Figure 2 displays the dynamic mechanical curves in Pd 40 Ni 10 Cu 30 P 20 bulk metallic glass, obtained at a fixed driving frequency (0.3 Hz) and changing the temperature at a constant heating rate (3 K/min). G u corresponds to the value of the unrelaxed modulus, i.e., the value of the shear modulus at low temperature.…”

“…The β relaxation has been attributed to structural heterogeneity in metallic glasses and is linked to local movements of "weak spots" or "soft zones" (i.e., free volume, flow defects, or quasi-point defects, 35−37 liquid-like sites, 38,39 weakly bonded zones, 40,41 loose packing regions 42 ). The presence of structural heterogeneity in nano-or microscale domain has been verified in metallic glasses by transmission electron microscopy (TEM) and simulation method.…”