Shear band relaxation in a deformed bulk metallic glass

Abstract

Relaxation of shear bands in a Pd 40 Ni 40 P 20 bulk metallic glass was investigated by a combination of radiotracer diffusion and molecular dynamics (MD) simulations, allowing to determine for the first time the effective activation enthalpy of diffusion along shear bands in a deformed glass. The shear bands relax during annealing below the glass transition temperature and the diffusion enhancement reveals unexpectedly a nonmonotonous behavior. The development of shear bands and the subsequent relaxation of stresses after switching off the shearing are characterized on microscopic to mesoscopic length scales by MD simulation subjecting the model glass to a constant strain rate. Mean-squared displacements as well as strain maps indicate that the heterogeneity, as manifested by shear bands in the systems under shear, persist after the shear is switched off. We observe a further relaxation of residual stresses that remain localized in regions where the shear band has been present before, although the system is -different from the macroscopic experiment -homogeneous with respect to the local density. These results indicate that even on a local scale one may expect strong dynamic heterogeneity in deformed glassy solids due to shear banding. The results thus suggest that plastically deformed metallic glasses present poly-amorphous systems that necessitate descriptions that are analogous to multiphase materials including the presence of heterophase interfaces.Although non-homogenous plastic deformation of bulk metallic glasses (BMGs) via the formation of shear bands attracted increased attention in the past [1,2], it is still far from being resolved, see e.g. the reviews in [3,4]. As a generally accepted concept, so-called "shear transformation zones" (STZ), i.e. areas in which groups of atoms collectively undergo a local shear transformation, have been introduced [5] as "unit carriers" of plastic deformation in metallic glasses. A cross-over from random 3-dimensional shear events (STZ formation) to correlated 2-dimensional dynamics has been brought forward to explain the observed shear banding in metallic glasses [6]. Although the activation of a single STZ event is not inevitably related to a change of the excess volume, shear bands are often described in terms of excess volume accumulation [3,7]. Recently, we investigated diffusion in deformed Pd 40 Ni 40 P 20 (at. %; in what follows we will use the abbreviation PdNiP) glass, in which almost a single family of shear bands was introduced, and were able to unambiguously prescribe the observed enormous enhancement of the diffusion rate to an ultra-fast atomic transport along these quasi-2-dimensional pathways [8]. The tracer concentrations in the corresponding concentration profiles, which were related to shear band diffusion, were shown to scale with the number density of the introduced shear bands (that in turn scales with the imposed strain),

Sub-micron strain analysis of local stick-slip motion of individual shear bands in a bulk metallic glass

“…Similar contrast changes in single shear bands have already been reported in literature, yet without analyzing the reason for the contrast alterations [2] and in fact with prime focus on the dilated areas, in which even nanometer void formation has been reported [14]. The complex structure of individual shear bands has been discussed in agreement with the relaxation behavior of deformed glasses as analyzed by measuring the Boson peak contribution to the specific heat at low temperatures [15,16] and by annealing-time dependent measurements of the shear band diffusivity that have been carried out by radiotracer diffusion analyses [17]. The results of these analyses, and specifically the observation of alternating regions along a shear band that are densified or dilated with respect to the glass matrix, suggest that slip proceeded inhomogeneously, even along this single shear band and also under rolling conditions as the deformation pathway, i.e.…”

“…Plastic deformation to a relatively low strain of about 8 % was applied to avoid massive corrugation of the surfaces and the resulting shear band density, ρ SB , was estimated to be about ρ SB = 0.06 μm -1 . Note that this is the strain used in experiments on diffusion enhancement of atomic transport along the shear bands in [17]. After the deformation, the samples were again examined by SEM.…”

Deformation-driven catalysis of nanocrystallization in amorphous Al alloys

“…More recent work by Stolpe and coworkers demonstrated that based on measured shear band densities and exothermic heat releases during heating, free volume changes must have occurred throughout the metallic glass during cold rolling and not only within shear bands [50]. Recent measurements of time-dependent tracer diffusion along shear bands that showed a non-monotonic behavior with an initial increase of the diffusivity upon relaxation moreover suggest that regions in the matrix adjacent to the shear bands are affected by the deformation [51]. It is also in line with measurements of internal friction in a bulk metallic glass, where early irreversible events upon cyclic straining have been observed at strain levels lower than 0.001.…”

Formation of adiabatic shear band and deformation mechanisms during warm compression of Ti–6Al–4V alloy

“…However, shear band formation was considered to be highly undesirable because it might give rise to subsequent fracture of the specimens. Thus, many researchers were devoted to investigating the mechanisms of shear band formation, discussing the microstructure within shear band and establishing theory models to control the occurrence of shear/ductile fractures [2][3][4][5][6][7]. For instance, Polyzois and Bassim [8] proposed that the formation of ASB in heattreated AISI 4340 steel depended highly on the grain size, the shape and the orientation of the microstructure prior to deformation.…”