Twin Nucleation and Migration - Modeling and Experiments

该项目旨在利用最先进的第一性原理/细观力学工具结合实验，开发一种先进材料设计的分层方法。我们计划专注于bcc合金，如Fe-Cr， Fe-Cr- co和奥氏体B2形状记忆合金，以发展我们的顺序多尺度设计方法。这些合金引起了人们的极大兴趣，但没有人试图建立模型来预测它们的变形响应。这些合金的变形行为具有显著的孪生活性。一个基于第一性原理计算的连续双晶核模型将被开发。该研究将通过仔细定义成核和迁移现象，通过考虑整个能量势垒瞬态，确定能量势垒如何演变并建立孪核和孪核迁移应力水平。设想通过精确测量与孪生相关的局部变形，包括孪生剪切应变，在相应的长度尺度上确认模型的有效性。变形的观察是在宏观和微观两个层面上使用数字成像技术，在倍数放大。这项工作的广泛影响是，我们的方法将通过避免大型测试矩阵方法和优化试验来加速先进结构材料的设计。基于晶体塑性模型预测了bcc合金的力学响应，该模型依赖于大量实验确定的常数。总的来说，强调第一性原理/中尺度力学与实验相结合是独一无二的。此外，还将举办一些外展活动，从针对行业和高级研究人员的短期课程，到为K-12学生及其教师提供活动工具包的学习模块，这将扩大该方法的教育影响。

This project is aimed at developing a hierarchical methodology for advanced materials design utilizing the most advanced first principles/mesomechanics tools combined with experiments. We plan to focus on bcc alloys such as Fe-Cr, Fe-Cr-Co and austenitic B2 shape memory alloys to develop our sequential multi-scale design approach. These alloys are of significant interest, but there has been no attempt to develop models to predict their deformation response from first principles. The deformation behavior of these alloys is characterized by significant twinning activity. A continuum twin nucleation model based on first-principle calculations will be developed. The research will determine how the energy barriers evolve and establish twin nucleation and twin migration stress levels, through consideration of the entire energy barrier transients, by carefully defining nucleation and migration phenomena. It is envisaged that confirmation of the model validity at the corresponding length scale by precisely measuring the local deformations associated with twinning including the twin shear strain. Observations of deformation are made at both the macro- and micro- levels using digital imaging techniques at multiple magnifications.The broad impact of the work is that our methodology will accelerate the design of advanced structural materials by avoiding the large test matrix approach and optimization trials. The mechanical response of bcc alloys have been predicted based on crystal plasticity models, which rely on a large number of experimentally determined constants. Overall, the emphasis on first principles/mesoscale mechanics combined with experiments is unique. In addition, there will be several outreach activities ranging from short courses to industry and advanced researchers, to learning modules with activity kits for K-12 students and their teachers, which will broaden the educational impact of the approach.