Effects of non-glide stresses on the plastic flow of single and polycrystals of molybdenum

Abstract

The Schmid law, which has been ubiquitously utilized in large-scale continuum computations, asserts that only the shear stress acting in the slip plane in the slip direction controls the plastic deformation. This rule is accurate for fcc metals but it does not apply in bcc metals owing to the spacial spreading of the core of screw dislocations that control their plastic deformation. In this paper we present resulsts of atomic-level simulations of the effect of externally applied stresses on 1/2[1 1 1] screw dislocations in molybdenum. We concentrate on the effect of other components of the applied stress tensor than the Schmid stress in the slip plane (1 0 1). These are shear stresses parallel to the Burgers vector in other {1 1 0} planes of the [1 1 1] zone as well as shear stresses perpendicular to the Burgers vector acting in {1 1 0} planes. We thus identify three distinct non-glide shear stresses that affect the glide of 1/2<1 1 1> screw dislocations and formulate single crystal yield criteria that include the effects of these stress components. This forms a basis for multislip yield criteria and flow relations for continuum analyses. Using this approach we demonstrate that the effects of non-glide stresses that originate at the level of individual dislocations have significant influence on plastic yielding of polycrystals.

Overview of constitutive laws, kinematics, homogenization and multiscale methods in crystal plasticity finite-element modeling: Theory, experiments, applications

“…In order to take these effects into account, the approach of Bassani et al [266] and Vitek et al [267] is adopted, where the effect of non-glide stress is incorporated in the model by modifying the critical resolved shear stress according to:…”

“…The deformation rates in such cases are typically orders of magnitude above realistic scales so that the transfer of such information must be done with great care since the results cannot in all cases be used for low-rate deformation constitutive models that typically prevail in CPFE simulations. Therefore, it may be more pertinent for molecular dynamics simulations of plasticity to provide certain constants or mechanisms for improved formulations of dislocation core structures, mobility, non-Schmid behavior, dislocation reactions, damage initiation and hardening mechanisms [265][266][267]. Other combinations are conceivable where a continuum-scale FE method is used for a proper boundary condition treatment around a simulation regime that is treated by an atomistic method.…”

Analysis of grain size effects on transformation-induced plasticity based on a discrete dislocation–transformation model

“…Due to the lack of close-packed planes in BCC crystals, there is no clear consensus among researchers on which planes crystallographic slip occurs, though, experimentally, slip appears to occur mainly on the f1 1 0g f and f1 1 2g f planes. Simulations have suggested that glide on the f1 1 2g f planes is in fact composed of alternating short slip steps on the f110g f planes (see Vitek et al, 2004), although this is still a somewhat open issue. As mentioned in Section 2.2, the amount of plastic slip in high-carbon martensite is very limited and is not accounted for in the model.…”

Bond-order potential for simulations of extended defects in tungsten

“…[64][65][66][67] Furthermore, studies of the motion of screw dislocations in molybdenum revealed the significance of stresses perpendicular to the slip direction on the onset of plastic deformation. 68,69 In iridium, unlike in any other fcc metal, two core structures for the screw dislocation have been found. A transformation between these two structures may lead to an exceptionally high rate of cross slip during plastic deformation and associated rapid increase in the dislocation density and strong local hardening.…”