Electron channeling contrast imaging characterization and crystal plasticity modelling of dislocation activity in Ti21S BCC material

Abstract

In this paper, an original approach is proposed to compare modeling and relevant statistical experiments using -Ti21S Body Centered Cubic (BCC) metal as a challenging benchmark. Our procedure allows the evolution of microstructural defects to be tracked in situ with excellent spatial resolution, while observing a bulk sample region sufficiently large to be statistically representative of the material. We identify multiple mechanisms such as slip transfer, slip traces, pencil glide, etc. We demonstrate that for small plastic strains ( < 0.25 %) under uniaxial tensile loading, the Schmid law is satisfied statistically. Under these circumstances, changes in Critical Resolved Shear Stress (CRSS) are minimal and accommodation of incompatible deformation between grains has not yet become important. The majority of the observed slip plane traces at the mesoscale corresponds to the {123} family. Fully automated while precise, the reported approach compares this data with four crystal plasticity models, and provides a methodology for similar analyses in other materials. * Corresponding author. X-Ray Diffraction HR-XRD, Electron BackScattered Diffraction EBSD…) that provide valuable atomic scale information, such as activation energies and critical shear stresses. However, one may argue that most multiscale strategies usually address only very specific cases, and that they are not necessarily statistically representative of the myriad of mechanisms that can occur in real polycrystals. For instance, one can cite HR-TEM [4] studies of special grain boundaries (structure, energy, migration under stress and temperature…) and their interactions with dislocations (emission, absorption, transmission…). Indeed, such available studies are far from covering the full range of possible grain boundary configurations in a real polycrystal. In addition, preparing and testing of thin foils for electron transmission-based techniques [5-7] (TEM, Transmission Kikuchi Diffraction TKD) can unfortunately affect the defect microstructure and the mechanisms investigated because of inherent size, strain rate, and external surface effects. Besides, these techniques do not provide enough data for statistical analysis of physical mechanisms.Experimental findings at an intermediate, mesoscopic scale, can be used to improve our knowledge of polycrystal plasticity and to improve models at the grain scale. They provide validation data for advanced numerical simulations that predict the mechanical behavior of polycrystals (strain hardening, ductility etc.

Experimental investigation of early strain heterogeneities and localizations in polycrystalline α-Fe during monotonic loading

“…This might explain why the maximum Schmid factor is sometimes reported to show no influence on strain heterogeneities, when the strain accumulation stage is not differentiated from the build-up of the plastic deformation pattern. On the opposite, the Schmid law was reported to be statistically satisfied at very low plastic strains [55]. This scale of investigation allows for a hint at how the plastic deformation pattern is built-up around the macroscopic yielding.…”

“…For a given slip mark, the angle between the mark and the loading direction (horizontal axis) is measured. With knowledge of the crystallographic orientation of the grain where the mark lies, the measured angle can be compared to the theoretical angles associated to each of the 48 slip systems among <111>{110}, <111>{112} and <111>{123} [55]. The computation of the theoretical angles is detailed in appendix A.…”

Tuning critical resolved shear stress ratios for bcc-titanium Ti21S via an automated data analysis approach

“…An optimized procedure referred as Accurate Electron Channeling Contrast Imaging (A-ECCI), and based on the precise control of the channeling conditions taking advantage of the 0.1° angular resolution of High Resolution Selected Area Channeling Patterns with an improved spatial resolution of 500nm was developed some years ago [27], [28], [30], [31]. A-ECCI thus enables the comprehensive characterization of defects such as stacking faults or dislocations, and can be used in conjunction with in situ tensile test inside the SEM [32], [33]. This technique provides access to multiple regions of interest; from sample scale to dislocation scale; with one unique tool while loading a bulk sample, thus providing access to large datasets amenable to the application of data mining procedures.…”