微尺度亚稳β钛合金单晶相变诱发塑性行为及其尺寸效应

The geometric dimension of applied materials has been reduced to micron and nano scales due to the development of microelectronic devices and microelectromechanical systems (MEMS), but there is little known about the design rules and constitutive descriptions of these micron-scale materials. Thus, understanding their mechanical response and microstructural mechanism is necessary in order to realize the full potential applications. In this project, transformation-induced plasticity (TRIP) and strain hardening behaviors in the micron-scale materials will be systematically studied. A type of metastable β titanium alloy single crystal, i.e. Ti-10V-2Fe-3Al, will be chosen as a model material. A group of square micropillars with specific crystal orientation will be fabricated using a combined lithography and focused ion beam (FIB) micromachining technique. The microcompression testing will be carried out in an MTS Nanoindenter XP, outfitted with a flat-tip diamond punch. With these experiments, the variety curves of stress-strain as a function of sample size will be measured. The dependence of hardening parameters such as work hardening rate and strain hardening exponent on strain will be revealed. The critical sample size, below which the strain-dependent hardening trend would be broken down, will be determined. Secondly, the deformed samples will be observed by scanning electron microscopy (SEM), and their microstructures will be analyzed by transmission electron microscopy (TEM). The plastic deformation mechanism and intrinsic reason of size dependence will be illuminated. Thirdly, the quantitative relationship between the transformed martensites and the applied strain will be formulated from experimental data. Furthermore, a martensitic transformation model defining TRIP effect, in which the volume fraction of transformed martensite, strain and sample size are taken as parameters, will be established. Lastly, the inherent relationship of hardening behavior-deformed microstructure-dimensional constraint will be exploited, which permits the disclosure of the physics of strain hardening behavior in the micron-scale samples. The results would provide with the theoretical support to evaluate mechanical properties of micron-scale marterials and design their devices with better reliability and higher performance.

微电子元器件与微机电系统等制造技术的发展使得所用材料的外观尺度逐渐减至微纳量级。因而，有必要就该尺度材料的力学性能和微观机理展开深入研究。本项目选用Ti-10V-2Fe-3Al亚稳β钛合金单晶为模型材料，设计加工特定晶体位向的微尺度方柱试样并进行微柱体压缩实验，着重研究微尺度材料的相变诱发塑性行为及应变硬化特性。测试试样不同外观尺寸下的力学响应，确定加工硬化率、应变硬化指数等材料硬化特征参数随应变的变化规律，以及这种规律突变时对应的临界外观尺寸；观察试样变形后的微观结构特征及其随外观尺寸的演化过程，阐明应变硬化特性的微观机制和性能突变的内禀原因；探寻相变马氏体与试样外观尺寸、应变的定量关系，尝试建立微尺度材料相变诱发塑性效应的计算模型；探索试样硬化行为－微观结构特征－外观尺寸约束三者的内在关联，揭示微尺度材料应变硬化特性的物理本质。为微尺度材料力学性能评价及微器构件安全设计提供理论依据。

微电子机械系统(MEMS)及微传感器中广泛使用的元器件的外观尺度已从传统的毫米量级降至微纳米量级，在复杂的微加工制备和随后的服役过程中其变形损伤断裂是导致这些微纳系统失效的关键原因。本项目将材料科学中传统的微观组织-性能关系二维研究空间拓宽至微观组织-外观尺寸-性能关系的三维研究空间，以β钛合金Ti-10V-2Fe-3Al(Ti1023)为模型材料，系统研究了微尺度材料的力学性能及变形行为的尺寸依赖关系，取得了以下三方面重要研究结果：. (1)研究了微尺度Ti1023合金单晶的压缩变形行为及其微观机制。我们发现，Ti1023合金微柱沿[011]位向压缩的塑性变形阶段应力-应变曲线光滑，表现出持续的加工硬化。微柱屈服强度随试样尺寸的减小而增加。TEM观察表明，Ti1023微柱压缩时表现出来的持续应变硬化归因于晶体中纳米尺度的ω相和应力诱发的α”相对位错滑移的持续阻碍作用。. (2)研究了Ti023合金微柱中第二相的变形特性及其强化作用。结果发现，位错与第二相颗粒除了传统的位错切过和位错绕过两种交互作用机制外，还包括另一种作用机制——颗粒无序化(precipitate disordering)。微柱试样压缩变形时，ω第二相表现出强烈的变形各向异性。当其位向有利于β基体中位错通过时，第二相被位错切过。与之相反，当ω颗粒的位向特别不利于变形时，位错在ω/β相界面处塞积，最终第二相颗粒发生晶格紊乱以协调变形。同时，我们通过晶体学分析和第一性原理计算模拟进一步证实了以上现象的存在。这些研究结果丰富了对沉淀硬化合金变形机制的理解，同时为微尺度材料的强度设计提供了一定的指导。. (3)研究了Ti023合金微柱力学性能的应变速率效应。我们发现，微柱表现出“高速增塑”现象，即随着应变速率的增加，微柱的塑性性能随之增加。同时，与经典的“越小越强”强度关系耦合，微尺度材料显示出“自韧化”特性。微观观察表明这与试样组织内愈发活跃的位错运动有关。这一研究结果对于金属材料，特别是脆性材料的强韧化设计具有重要的指导意义。