钛合金微观力学行为的中子原位衍射分析与弹塑性自洽模拟

Neutron diffraction technique is the key technology which is widely used for the measurement of three dimensional residual stresses inside the material and the averaged stress in a block material. Elastic-plastic self-consistent (EPSC) model fits the neutron diffraction data well, which is used to quantitatively analyz the deformation systems, i.e. slip and twinning, that contribute to the measured strain. Titanium (Ti) alloys are the essential materials used in a large variety of aeronautic, energy and biomedical applications. It’s necessary to investigate the internal stresses and the deformation mechanism of the structural Ti alloys under the service environment, such as temperature and external load. There is little work focuses on the deformation mechanism of Ti alloys at present. In this project, in situ neutron diffraction combined with EPSC modeling are used to study the tensile deformation behaviors of the α-type Ti−5Al−2.5Sn alloy, the β-type Ti-3Al-8Mo-11Cr alloy and the (α+β)-type Ti−6Al−4V alloy at the temperatures range from room temperature (RT) to 400 ℃. The focused issue of this work is to set up the EPSC models for the three types of Ti alloy accurately. Crystal structure, orientation relationship between the two phases, texture and its evolution, deformation systems (slip and twinning) and the choices of hardening models for different deformation systems must be included appropriately in the models. Finally, the deformation mechanism for the three types of Ti alloy at the temperatures of RT~400 ℃ will be quantitatively provided based on the in situ neutron diffraction measurements.

中子衍射技术是当前准确测定材料内部三维残余应力及大块材料平均应力的关键技术。弹塑性自洽（EPSC）模型可以很好地拟合中子衍射数据，定量地给出各种形变系统（滑移系、孪生等）对形变量的贡献。钛合金是航空航天、能源、生物医学等领域必不可少的材料，研究作为结构材料使用的钛合金在其服役环境（温度、力）下的内部应力状态演化及变形机制非常必要。目前关于钛合金形变机理的研究工作相对较少。本项目拟利用原位中子衍射技术并结合EPSC模拟，研究α型Ti−5Al−2.5Sn合金、β型Ti-3Al-8Mo-11Cr合金及α+β型Ti−6Al−4V合金在其使用温度范围（RT~400℃）内的拉伸形变行为。准确地建立三类钛合金的EPSC模型，模型考虑晶格结构、相间晶格取向关系、织构及其演化、形变系统（滑移、孪生）及各个形变系统硬化模型的选择。基于中子原位拉伸衍射数据，定量地分析三类钛合金在RT~400℃下的力学形变机制。

钛合金具有强度高、韧性好、疲劳性能和抗裂纹扩展性能优异等特点，是航空航天及汽车制造领域必不可少的材料。研究作为结构材料使用的钛合金在其服役环境下的内部应力状态演化及变形机制非常必要。目前关于钛合金变形行为的研究主要集中于宏观性能与离线（断裂或失效时）微结构的对应分析，关于材料在变形过程中微结构演化、变形机制及二者关联性的分析工作较少。本课题主要结合原位衍射技术及弹塑性自洽模拟，尝试分析不同微结构的材料在外载作用下的微观形变行为，形成可扩展应用的研究方法。本课题以钛合金为主要研究对象，辅以锆、铍用于模型开发和校验。利用原位XRD技术对比研究了CPTi和TB2的原位准静态拉伸形变行为，通过分析TB2中两相之间的相互作用，解释了(211)β 和 (101 ̅0)α 晶面弹性应变同时异常增大及其宏观应力-应变曲线上出现屈服峰的原因；利用原位中子衍射技术研究了Zr4合金加载过程中的微观形变机制，获得了形变织构、位错演化和孪晶引起的应力再分布等信息；发展了大变形双相弹塑性自洽模型，能够很好地拟合（取向不相关及取向相关）双相材料宏观及微观（文献中不锈钢和高温合金的）实验数据，定量地分析材料的微观力学及形变行为，该模型可拓展应用于其它单、双相材料在复杂加载条件下宏观性能及微结构演化的模拟。