Mechanical behaviour of nickel symmetric CSL grain boundaries under triaxial loading: from dislocation nucleation to decohesion
Résumé
The investigation of the mechanical behaviour of grain boundaries (GB) at the atomic scale is of particular interest for the prediction of intergranular microcrack initiation in metals and alloys. At the polycrystal scale, many experimental observations report the formation of plastic slip bands impacting grain boundaries, leading to different phenomena: slip band transmission, strong deformations in the neighbouring grain or GB crack initiation. Further analytical or numerical computations based on crystal finite element (FE) analysis or discrete dislocation dynamics reveal strong multiaxial intergranular stress fields in the vicinity of the corners of the impinging slip bands [1], [2]. However, to our knowledge, few atomistic studies [3], [4] consider grain boundaries subjected to multiaxial loadings, as only uniaxial loadings are generally applied.
In this study, the mechanical behaviour of various grain boundaries is investigated through molecular dynamics (MD) computations. Loadings consisting in normal straining to the grain boundary plane coupled with transverse straining, are applied to numerous CSL tilt symmetrical [1 0 0] and [1 1 0] grain boundaries in nickel using the EAM potential proposed by Mishin [5]. For each grain boundary, several tensile loadings are carried out for different transverse/normal strain ratio values varying between -0.3 and 0.3.
Geometrical and mechanical parameters such as: thicknesses, axial elastic moduli, critical normal stresses triggering either GB fracture or dislocation nucleation and Griffith energies, are reported for the grain boundaries under study.
An increase of the strain ratio correlates with an increase of the normal critical stress as a progressive behaviour transition from the emission of Shockley partial dislocations to GB brittle decohesion is observed. For all the considered grain boundaries, it is found that as the strain ratio increases from one tensile test to another, the first occurrence of GB crack initiation occurs as the critical stress is saturated, which may correspond to a GB intrinsic parameter of fracture.
Such thorough characterizations provide necessary information at the atomic scale for a bottom-up approach considering a multi-scale analytical model for the prediction of an intergranular crack initiation with a double criterion [6]. In conjunction, a top-down approach brings a consistency to this multi-scale modeling; this reversed approach is based on MD simulations using displacement fields provided by crystal FE simulations of slip bands/GB interactions, thus allowing us to apply realistic intergranular loadings to our atomistic GB models.