亚稳beta型钛合金在循环加载下形变和相变机制的定量电子显微学研究

Metastable beta-type titanium alloys such as the Ti-24Nb-4Zr-8Sn alloy with a body-centered cubic lattice have attracted extensive research because they exhibit an attractive combination of high strength and toughness. Moreover, these alloys usually have low Young’s modulus, good workability and nonlinear pseudo-elasticity, which generate strong research interest. Under cyclic loading, these alloys could demonstrate reversible, partially reversible and irreversible deformations with different strains or orientations. These effects could bring excellent fatigue and damping properties of the alloys. Mechanical properties of metastable beta-type titanium alloys are influenced by the presence of various intermediate phases, which can be controlled by materials processing and alloy design. Besides, the addition of different alloying elements can change the stability of beta phase substantially. Therefore, extensive efforts have been carried out in these alloy systems. Depending on phase stability, deformation mechanisms such as dislocation slip, deformation twinning and stress-induced martensitic transformation are reported during plastic deformation of metastable beta-type titanium alloys. These complicated deformation mechanisms as well as the evolution of and transitions between them bring interesting even unique mechanical properties and performance of the metastable beta-type titanium alloys. As metastable titanium alloys are developed for aerospace and medical applications that require high reliability and safety, a detailed understanding of deformation and work hardening mechanisms are desired for the alloys. The present proposal aims to investigate the evolution of deformation mechanisms and microstructures in metastable beta-type titanium alloys such as Ti2448 alloy under cyclic loading with the combination of transmission electron microscopy techniques and quantitative electron microscopy methods. The focus is to determine local stress and strain in the materials at the atomic resolution with the quantitative high resolution transmission electron microscopy methods, thus to estimate driving forces of plastic deformations and phase transformations in the alloys. In situ as well as quasi-in situ electron microscopy observations are employed to resolve the evolution of deformation mechanisms and phase transformations. The results will provide insights to plastic deformation and phase transformations of metastable beta-type titanium alloys with the aids of theoretical calculation and experimental mechanical testing. It will also have implication on fatigue processes and damping properties of the alloys and assist the development of high performance titanium alloys.

亚稳beta型钛合金(如Ti2448合金等)表现出了低模量、高强度、非线弹性等特殊的力学行为，特别是在循环加载下，各种形变方式会出现可逆、部分可逆、不可逆的演化，相应地材料表现出独特的疲劳和阻尼等性能。本申请拟结合原子分辨率透射电镜的原位、准原位表征和定量电子显微学分析对Ti2448合金等的微结构及其在循环加载等使役条件下的演化进行研究，重点从实验上厘清在循环加载-卸载时，亚稳beta型钛合金形变过程中形成的局部应力、应变如何诱发相变、孪生等机制，阐明局部应力、应变的演化及其对滑移、孪生、相变等形变行为的影响，从而深入认识亚稳beta型钛合金的结构演化和力学行为与使役性能之间的相互关系。通过和已有的材料力学性能实验结果的比较，以期加深对亚稳beta型钛合金材料的力学行为本质的理解，深入认识该合金疲劳失效中的物理过程，阐明合金阻尼性能的物理本质，为新型钛合金的研究和发展提供理论参考。

亚稳beta型钛合金表现出低模量、高强度、非线弹性等特殊力学行为，特别是在循环加载下，各种形变方式会出现可逆、部分可逆、不可逆的演化，相应地材料表现出独特的疲劳和阻尼等性能。本项目利用原位扫描和透射电镜实验，以及有限元计算，确定了亚稳beta型钛合金主要形变机制随着应力的增加以马氏体相变、孪晶和位错滑移带的顺序依次进行，这一形变机制的演化解释了 Ti2448 合金在宏观变形中随着应变量的增加出现应力平台、加工硬化及颈缩的现象。发现了亚稳beta型钛合金中形变诱发的菱方结构omega相，揭示了在变形过程中产生的菱方结构omega相以及在beta与omega相之间连续过渡的共格界面保证了该可逆omega相变的发生。这个菱方omega相的可逆性为材料带来了约2.3%的可逆应变，对材料超弹性和形状记忆性能的提高具有意义。结合原子分辨透射电子显微镜、能量色散谱分析和第一性原理计算，在Ti2448合金时效过程中从原子尺度定量表征了一种连续的扩散位移型beta→alphaʺ→alpha转变。这种转变是通过伪调幅分解机制进行，是在beta基体成分涨落的辅助下，结构和成分同时向平衡态连续变化的过程。利用旋进电子衍射技术定量测量出析出相-基体界面上的应变分布，发现随着时效温度升高，共格应变场的宽度逐渐增加。析出相-基体界面上共格应变的存在产生了显著的强化效应。通过原子尺度的动态观察、定量应变分析和理论计算确定了亚稳beta型钛合金中的形变和相变机制，阐明了循环载荷下亚稳beta型钛合金中微结构演化与力学性能之间的关系，为亚稳beta型钛合金的研究发展提供了理论依据。在Acta Materialia等国际期刊上发表9篇论文，在国内期刊上发表3篇论文，参加国际国内会议并给出邀请报告8次，培养博士生1人。

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