The role of structural relaxation in the plastic flow of metallic glasses

Abstract

The role of structural relaxation in the plastic flow behavior of metallic glasses is analyzed both theoretically and experimentally. The characteristic time of structural relaxation is calculated as a function of glass thermal prehistory. It is revealed that heating above the room temperature by several tens of Kelvins results in a sharp, by several orders of magnitude, decrease of this time. It is argued that localized “inhomogeneous” dislocation-like flow occurs on loading if the characteristic time of structural relaxation is much greater than the characteristic loading time, while “homogeneous” viscous deformation is observed in the opposite case. Precise measurements of acoustic emission in a Co-based metallic glass being loaded at different temperatures and strain rates are employed for verification of this statement. It is shown that the inhomogeneous → homogeneous flow transition occurs at temperatures somewhat higher than T=400 K, and the transition temperature increases by ≈ 40 K as the strain rate increases by two orders of magnitude. Theoretical estimations show that for the inhomogeneous flow the characteristic time of structural relaxation in the loaded state is indeed much greater than the characteristic loading time. It is concluded that the kinetics of structural relaxation determines the flow mode of metallic glasses in a unique manner. The kinetically “frozen” structural relaxation gives rise to a crystalline-like localized flow under load while intensive structural relaxation facilitates a viscous glass-like behavior.

The Kinetics of Irreversible Structural Relaxation and Homogeneous Plastic Flow of Metallic Glasses

“…In fact, this statement has been successfully proved using measurements of non-isothermal strain recovery [17] and acoustic emission [11 to 13] as fine indicators of the flow mode. It has been shown that under the strain rates of 10 À4 to 10 À6 s À1 this transition in samples preannealed at room temperature occurs slightly higher T 400 K [13] and, most likely, is independent of the glass chemical composition [17]. This result is in excellent agreement with the calculations of the relative SR rate for samples preannealed at room temperature (curve 1 in Fig.…”

“…If a glass is preannealed at a temperature T a during time t, cooled to a temperature T 0 and linear heating at a rate of T is started after that, it is easy to show that after heating during time t the RC volume spectral density will decrease to [13] …”

“…the number of elementary relaxation events per unit time and unit volume) that determines the plastic flow mode. If the SR rate is small enough to ensure the condition t sr ) t d (t sr is the characteristic SR time, t d the characteristic loading time) to be fulfilled, the plastic flow is localized and occurs via a dislocation-like mechanism [13]. In the case of intensive SR t sr ( t d and homogeneous flow is observed [13].…”

Structural relaxation and related viscous flow of Zr–Cu–Al-based bulk glasses produced from the melts with different glass-forming ability

“…There is also another argument leading to the same conclusion. In situ measurements of the acoustic emission in a metallic glass being deformed at different temperatures showed that the transition from homogeneous to inhomogeneous (localized) plastic flow is also controlled by the rate of structural relaxation but not by the homologous testing temperature [34].…”

Non-isothermal strain recovery as a result of irreversible structural relaxation of metallic glasses

“…The temperature dependence of r,, thus defined was calculated by Khonik et al (1998b). It was determined that this quantity is sharply dependent on the thermal pre-history, decreasing rapidly above the pre-annealing temperature.…”