Roman Gröger | Searcharxiv

arXiv2021

Symmetry-adapted single crystal yield criterion for non-Schmid materials

All yield criteria that determine the onset of plastic deformation in crystalline materials must be invariant under the inversion symmetry associated with a simultaneous change of sign of the slip direction and the slip plane normal. We demonstrate the consequences of this symmetry on the functional form of the effective stress, where only the lowest order terms that obey this symmetry are retained. A particular form of yield criterion is obtained for materials that do not obey the Schmid law, hereafter called non-Schmid materials. Application of this model to body-centered cubic and hexagonal close-packed metals shows under which conditions the non-Schmid stress terms become significant in predicting the onset of yielding. In the special case, where the contributions of all non-Schmid stresses vanish, this model reduces to the maximum shear stress theory of Tresca.

cond-mat.mtrl-sci↗

arXiv2007

Development of physically based plastic flow rules for body-centered cubic metals with temperature and strain rate dependencies

In this Thesis we first show how the shape of the Peierls barrier and its dependence on the applied loading can be extracted from the data obtained in atomistic studies at 0 K. We consider the Peierls barrier as a two-dimensional periodic function of the position of the intersection of the dislocation line with the perpendicular {111} plane, with adjustable terms dependent on the shear stresses parallel and perpendicular to the slip direction. The functional forms of these terms are based on the effective yield criterion recently developed on the basis of atomistic modeling of the glide of screw dislocations at 0 K. The minimum energy path between two potential minima, and thus the corresponding activation barrier, is obtained using the Nudged Elastic Band method. The constructed Peierls barrier reproduces correctly both the well-known twinning-antitwinning asymmetry observed for pure shear parallel to the slip direction and the effect of shear stresses perpendicular to the slip direction. This advancement introduces for the first time the effect of both shear stresses parallel and perpendicular to the slip direction into the model of thermally activated dislocation motion. Based on this model we formulate a general yield criterion that includes not only the full stress tensor but also effects of temperature and strain rate. This approach forms a basis for multislip yield criteria and flow relations for continuum analyses in both single and polycrystals the results of which can be compared with experimental observations.

cond-mat.mtrl-sci↗