Twinning in Single Crystal Steels

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Orientation and stress state dependence of twinning is studied with novel experiments in materials with low stacking fault energy. These materials include Hadfield and austenitic stainless steels with nitrogen additions and have enormous potential for applications where high strength, ultra-high strain hardening and high fracture strains are needed. One of the unusual attributes of these steels is that during deformation an upward curcature in stress-strain curves develops. Single crystal experiments that study this unusual strain hardening response will be coupled with mechanics modeling to explain the experimental findings. The addition of nitrogen to these steels increases the deformation resistance and imparts beneficial properties. The role of nitrogen will be clarified with experiments and in modeling efforts examining twin nucleation and twin-slip interaction. The uniaxial stress-strain behavior of single crystals specimens will be determined as a function of crystal orientation, stress direction (tension and compression), maximum strain level and governing deformation mechanism. Considerable tension-compresson asymmetry develops in these classes of materials because of directionality of twinning. These results will be augmented with stress state experiments where hydrostatic stresses are systematically changed. The sress state will decide the effective stacking fault energy hence the deformation behavior. The effective stress required for onset of twinning. These results will be augmeted with stress states experiments where dydrostatic stresses are systematically changed. These stress state will decide the effective stacking fault energy hance the deformation behavior. The effective stress required for onset of twinning will be determined as well as its variation with compressice hydrostatic stress. Local strain field measurements using digital strain mapping techniques will be utilized to study the evolution of twin formation and propagation. A specialized marker will be deposited on the specimen surface, then the motion of the markers will be tracked during the deformation experiments. This technique allows the entire strain field on the specimen surface to be determined including local strain gradients. Texture evolution measurements will establish the orientation changes with deformation will help identify the activation of slip and twinning as a function of deformation. A micro-mechancis modeling effort, incorporating the twin volume fraction and twin evolution, will be undertaken for predicting the stress-strain response as a function of orientation, stress state, and texture evolution.