Twinning in Single Crystal Steels

用新的实验方法研究了低层错能材料中孪晶的取向和应力状态相关性。这些材料包括添加了氮的哈德菲尔德不锈钢和奥氏体不锈钢，在需要高强度、超高应变硬化和高断裂应变的应用中具有巨大的潜力。这些钢的一个不同寻常的属性是，在变形过程中，应力-应变曲线会出现向上弯曲。研究这种不寻常的应变硬化响应的单晶实验将与力学模型相结合来解释实验结果。在这些钢中添加氮提高了变形抗力，并赋予了有益的性能。氮的作用将通过实验和检验孪晶成核和孪生滑移相互作用的建模工作来阐明。单晶试件的单轴应力-应变行为将取决于晶体取向、应力方向(拉伸和压缩)、最大应变水平和主导变形机制。由于孪生的方向性，在这类材料中形成了相当大的拉伸-压缩不对称性。这些结果将通过系统地改变静水应力的应力状态实验而得到加强。应力状态将决定有效的层错能，从而决定变形行为。孪生开始所需的有效应力。这些结果将被系统地改变动静应力的应力状态实验所增强。这些应力状态将决定变形行为中的有效层错能。将确定孪生开始所需的有效应力及其随压缩冰静水压力的变化。使用数字应变映射技术的局部应变场测量将被用来研究孪晶的形成和传播的演化。在试件表面放置一个专门的标记，然后在变形实验过程中跟踪标记的运动。这项技术允许确定样品表面的整个应变场，包括局部应变梯度。织构演化测量将确定取向随变形的变化，有助于识别作为变形函数的滑移和孪晶的激活。为了预测作为取向、应力状态和织构演化的函数的应力-应变响应，将进行包含孪晶体积分数和孪晶演化的微观机制模拟工作。

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.