Twinning Studies via Experiments and DFT-Mesoscale Formulation

技术：这个项目的目的是开发一种先进材料设计的分层方法，采用联合实验和理论方法，利用最先进的工具。它为溶质在断层能和孪晶成核应力计算中的作用带来了新的和清晰的见解，这些计算不能仅从介观或原子角度收集。PIS已经确定，为了确定孪晶的形核应力，需要评估实际原子位移所需的能量。PIS计划将重点放在低层错能合金，Fe-X和Fe-X-N(X=Mn，Cr，Ni)钢，Cu-Al系上，以开发顺序多尺度设计方法。这些合金的变形行为以显著的孪生活动为特征，形核应力随合金化的变化可能相当复杂，需要进一步研究。PI将在第一性原理计算的基础上建立一个多组元fcc合金的连续孪晶(异相)形核模型。PIS将解决与孪生边界相关的位置对称性的重要问题，并获得作为稳定和不稳定断层能量的函数的普遍表达式。PIS将决定合金化如何通过本征能量和/或不稳定能量影响孪晶形核应力水平。通过对具有选定取向的单晶进行实验，并结合局部应变测量，PI将以高精度确定孪生开始时的应力。这项工作的智力价值在于，PI首次从一个植根于量子力学和介观位错理论的理论出发，在形变孪生情况下建立了孪生应力和能垒之间的定量关联。与以前的研究不同，PI将专注于单晶，并开发具有多尺度测量的新型数字成像技术，通过局部应变测量来揭示孪生的细节。在复杂的合金体系中考虑氮的影响，如Fe-X，在过去的研究中没有涉及到，随着理论与实验的确认，PI将开发实验/理论工具，在该领域取得显著进展，提供预测性设计能力。非技术性：PIS的一般方法论是独特的，适用于各种具有研究和技术兴趣的材料，而不受实验和理论上的通常限制。该项目将通过避免大型测试矩阵方法和优化试验来加速先进材料的设计。总体而言，该战略是通过将基础物理和连续介质尺度联系起来，而不是半经验(拟合)常数，来推进材料设计的新的建模/实验方法。这种方法在材料和机械科学家的设计、教育和教学中具有深远的影响。

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.