Twinning Studies via Experiments and DFT-Mesoscale Formulation

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

TECHNICAL: This project is aimed at developing a hierarchical methodology for advanced materials design utilizing the most advanced tools in a joint experimental and theoretical approach. It brings new and clear insight into the role of solute on the fault energies and twin nucleation stress calculations that cannot be gleaned from solely mesoscopic or atomistic perspectives. PIs have established that in order to determine the nucleation stress for twinning, the energy required for the actual atom displacements needs to be evaluated. PIs plan to focus on low stacking fault energy alloys, Fe-X and Fe-X-N (X=Mn,Cr,Ni) steels, Cu-Al systems, to develop sequential multiscale design approach. The deformation behavior of these alloys is characterized by significant twinning activity, and the changes in nucleation stress with alloying can be rather complex and require further interrogation. PI will develop a continuum twin (heterogeneous) nucleation model for multicomponent fcc alloys based on first-principle calculations. PIs will address the important issues of positional symmetries associated with twin boundaries, and obtain generalized expressions as a function of stable and unstable fault energies. PIs will determine how alloying influences the resultant twin nucleation stress levels, through intrinsic and/or unstable energies. By conducting experiments on single crystals with selected orientations, and in conjunction with local strain measurements, PIs will establish the stress at the onset of twinning with a high level of precision. The intellectual merit of the work is that PIs are the first to establish a quantitative correlation between the twinning stress and energy barriers involved in case of deformation twinning from a theory that is rooted in quantum mechanics and mesoscale dislocation theory. Unlike previous studies, PIs will focus on single crystals and develop novel digital imaging techniques with multiscale measurements to unravel the details of twinning via local strain measurements. Incorporating the nitrogen effects in complex alloy systems, such as Fe-X, have not been addressed in past studies, and with confirmation of theory with experiment PIs will develop the experimental/theoretical tools for significant advancement in the field, offering predictive design abilities. NON-TECHNICAL: PIs general methodology is unique and applicable to a wide variety of materials of research and technological interest, while not suffering from usual limitations in experiment and theory. The project will accelerate the design of advanced materials by avoiding the large test matrix approach and optimization trials. Overall, the strategy is to advance a new modeling/experiment approach for design of materials by connecting the underlying physics and continuum scales without the semi-empirical (fitting) constants. The approach has far outreaching implications in design, education, and teaching of materials and mechanical scientists.