Committee Members:

• Prof. David L. McDowell, Advisor, ME/MSE
• Prof. Ting Zhu, ME/MSE
• Prof. Naresh Tadhani, MSE
• Prof. Richard Neu, ME/MSE
• Edwin Antillon, Ph.D., U.S. Naval Research Laboratory

Atomistic and Coarse-Grained Atomistic Modeling of Solute Ordering Effects on Dislocation Migration

Abstract:

Dislocation-mediated plasticity in alloy systems is strongly affected by the concentration and distribution of solute atoms. These effects manifest as changes in the strength or ductility of the alloy. Such microstructural variables are especially relevant in state-of-the-art alloy systems such as high entropy alloys or alloys produced by additive manufacturing (AM). The temperature and stress gradients induced during processing lead to solute configurations that can no longer be considered random. In 316L stainless steel, the AM process leads to the development of sub-grain solute segregation which in turn leads to dislocation cellular structures that simultaneously enhance strength and ductility. At certain target composition ranges in this system, heat treatment procedures lead to chemical short-range ordering (CSRO) and solute clustering which affect strength retention at high temperature. Targeted design of alloys for such improved mechanical properties requires a fundamental understanding of these dislocation-solute interactions at the nanoscale.

Computational modeling is one such strategy that offers insight into the atomic scale mechanisms and can screen large parameter spaces more quickly than experiments. This work begins by building a bottom-up understanding of the solute composition and temperature dependent dislocation mobility in 316L from molecular dynamics (MD), equipping reduced-order modeling approaches with the ability to directly simulate dislocation cell structure formation. Then, a physics-based analytical model is developed and validated to predict the effects of varying CSRO on yield strength. While powerful as modeling approach, the computational cost of MD can become quite demanding when exploring multidimensional parameter spaces. An application of average-atom interatomic potentials to the Concurrent Atomistic-Continuum (CAC) coarse-grained atomistics method is presented that enables study of multicomponent alloy systems at reduced computational cost while preserving key nanoscale mechanisms. Finally, a coarse-grained CAC implementation of the nudged elastic band (NEB) method is validated as a length and time scale bridging approach, again extending accessible parameter ranges through coarse graining while retaining a full atomistic description of the reaction pathway.