Anelastic to Plastic Transition in Metallic Glass-Forming Liquids

Abstract

The configurational properties associated with the transition from anelasticity to plasticity in a transiently deforming metallic glass-forming liquid are studied. The data reveal that the underlying transition kinetics for flow can be separated into reversible and irreversible configurational hopping across the liquid energy landscape, identified with and relaxation processes, respectively. A critical stress characterizing the transition is recognized as an effective Eshelby ''backstress,'' revealing a link between the apparent anelasticity and the ''confinement stress'' of the elastic matrix surrounding the plastic core of a shear transformation zone. DOI: 10.1103/PhysRevLett.99.135502 PACS numbers: 61.43.Dq, 62.10.+s, 62.20.Dc, 62.40.+i Some of the earliest efforts to describe the mechanics of deformation and flow of metallic glasses and liquids were carried out by Argon [1]. Inspired by the deformation of soap bubble rafts [2], Argon proposed that these materials deform by plastic rearrangements of atomic regions involving tens of atoms, termed shear transformation zones (STZ's). Argon further recognized that these plastically rearranging regions were not free but confined within an elastic medium [3]. Using Eshelby's insightful analysis [4], he estimated the effect of elastic matrix confinement on the energy barrier separating the initial and transformed state of the STZ. He further argued that the elastic matrix confinement of an isolated transformed STZ would lead to reversible elastic energy storage in the STZ-matrix system, implying that transformed STZ's have a memory of their original untransformed state. Interestingly, Argon's concept was recently studied in a deforming metallic glass foam, where buckled membranes and their accommodating stress fields were observed to behave like elastically confined STZ's [5].As recognized by Johari and Goldstein [6], the underlying relaxation mechanisms of liquids and glasses are governed by two kinetic processes: a fast process, termed the process, viewed as a locally initiated and reversible process, and a slow process, termed the process, viewed as a large scale irreversible rearrangement of the material. From a potential energy landscape perspective, Debenedetti and Stillinger [7] have identified the transitions as stochastically activated hopping events across ''subbasins'' confined within the inherent ''megabasin'' (intrabasin hopping) and the transitions as irreversible hopping events extending across different landscape megabasins (interbasin hopping). A 1D section of a potential energy landscape illustrating this concept is presented schematically in Fig. 1. Johnson and Samwer [8] have recently shown that, by employing a sinusoidal function to describe the megabasin potential energy density, a scaling law for the yield strength of a frozen-in configuration arises in terms of the curvature of the potential energy density function (i.e., the isoconfigurational shear modulus), revealing a universal shear strain limit of c 0:036. More recently, Demetriou ...

Effects of cooling rate on particle rearrangement statistics: Rapidly cooled glasses are more ductile and less reversible

“…For example, materials are typically deemed reversible in the regime where the stress-strain curve is linear, and irreversible in the regime where plastic flow occurs [48]. Reversibility has been studied experimentally using enthalpy [18] and strain recovery [19], elastostatic compression [16], nanoindentation [10], and quality factor measurements [49]. In simulations, reversibility has been studied using cyclic shear of model glasses [17,[50][51][52][53][54].…”

“…near γ ∼ 0.05), the shear stress reaches a peak (whose height depends on the thermal history, as shown in Fig. 1 (c)) and then begins to decrease until it plateaus at a steady state value in the plastic flow regime [2,18]. (For this system, we employed periodic boundary conditions that prevent fracture.…”

“…In crystalline materials, topological defects reflecting the symmetry of the crystalline phase govern response to deformation. In amorphous solids without long-range positional order, it is more difficult to detect and predict changes from elastic response to irreversible behavior [8,15], such as yielding [16,17] and flow [4,18]. The typical response of the deviatoric stress to an applied (pure) shear strain for amorphous solids is depicted in Fig.…”

Evolution of hidden localized flow during glass-to-liquid transition in metallic glass

“…The fraction of the flow units is B8% for this specific MG system. Such isolated reversible motions within matrix can be regarded as hopping events across inherent structures and contribute to the brelaxation 26,56 . These few easy flowing spots do not change the brittle nature of MGs.…”

“…Recent studies on the atomic-scale glassy structure have revealed the existence of liquid-like sites in glassy state 4,18,19 , which are presumed to be responsible for the viscoelastic flow behaviour in glasses [20][21][22] . Meanwhile, the studies on glasses have demonstrated that the b-relaxation is identified to play an essential role in the GLT process 2,8,9,11,23 , and the b-relaxation has comparable activation energy with that of the deformation unit and strongly correlated with mechanical brittle-to-ductile transition in MGs 20,21,[24][25][26] , indicating that the b-relaxation is closely related to the initiation and evolution of the localized liquid-like deformation units or flow units in MGs [27][28][29][30] . Yet, the fraction and evolution of these liquid-like zones, leading to the flow phenomena such as elastic and plastic deformations and GLT, are still speculative due to the lack of abundant experimental evidence, and the intrinsic correlations between such deformation transition, relaxation mode and structural characteristics changes during GLT are still poorly understood.…”