Modeling of twinning as a deformation mechanism in iron (bcc) crystals and iron-based alloys

body-centered cubic (bcc);twinning;molecular dynamics;energy landscape

Ojha, Avinesh ; Sehitoglu ; Huseyin

关键词: body-centered cubic (bcc);    twinning;    molecular dynamics;    energy landscape;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/44761/Avinesh_Ojha.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

We develop an analytical expression for twin nucleation stress in bcc metals considering GPFE(generalized planar fault energy) and the dislocations bounding the twin nucleus. We minimizethe total energy to predict the twinning stress relying only on parameters that are obtainedthrough atomistic calculations, thus excluding the need for any empirical constants. We validatethe present approach by means of precise measurements of onset of twinning in bcc Fe50Crsingle crystals showing excellent agreement. The experimental observations of the three slipsystems resulting in the twin formation was demonstrated via EBSD and TEM techniques alongwith DIC (Digital Image Correlation).In addition, the present work is geared towards understanding the twin-slip and twin-twininteractions in alpha -Fe crystals using Molecular Dynamics (MD) technique. Five types of twintwinand twin-slip intersections have been analyzed, namely <111>, <113>, <210>, <513>,<110>, and the magnitude of the residual dislocation left at the twin boundary for each type ofintersection was identified. Further, the role of the residual dislocations in affecting themagnitude of the critical stress required for twin migration has been established for eachintersection type. We are able to investigate the Schmid factor criteria for slip and twinnucleation under tensile and compressive loading orientations, and the results obtained are inclose agreement with the theoretical critical resolved shear stress (CRSS) of the activatedsystems.Furthermore, the results obtained through MD simulations and experiments utilizing EBSD andDIC are used to investigate the effect of the residual dislocations on the energetics of twin-slipand twin-twin interactions. An analytical expression is developed based on the geometricalparameters such as twin width and length, and the magnitude of the residual dislocation thatquantifies the critical stress required for twin migration. Energy analysis based on the totalelastic energies of the interacting dislocations and the associated fault energies reveal a strongdependence of the twin migration stress as a function of the magnitude of the residualdislocation. A higher magnitude of the residual dislocation causes an increase in the twin-migration stress and makes the transmission of slip/twin more difficult through the coherent twin boundary (CTB).