亚微米尺度下Beta钛合金单晶力学行为及变形机理研究

The dependence of mechanical behavior and microscopic deformation mechanism on geometric size will be studied in beta-Ti alloys single crystals of Ti-36Nb-2Ta-3Zr-0.3O gum metal that demonstrates an ultrahigh strength, and Ti-10V-2Fe-3Al (Ti1023) subjected to tensile and compressive deformation in submicron scale. The variety curves of stress-strain as a function of sample size will be measured in Ti alloys single crystals under different orientations. The plastic deformation mechanism and size effect will be systematically studied in gum metal, which appears to deformation by a dislocation-free mechanism triggered by elastic instability at the limit of strength. The critical stresses and their size effect of giant faults, nanodisturbances and deformation-induced martensitic phase transformation will be determined in beta-Ti alloys during plastic deformation. The constitutional relationship between the size effect of martensitic phase transformation and the size effect of mechanical behavior of pillars will be established in gum metal deformed by dislocation-free mechanism and Ti1023 deformed by conventional dislocation slip and twinning. The influence of plastic deformation behavior of second-phase precipitates and nanodisturbances with the fine-scale with respect to the sample pillars on the size effect of mechanical behavior of pillars will be systematically studied. A theoretic model will be suggested describing the plastic deformation mechanism and the size effect in beta-Ti alloys single crystals in submicron scale.

通过对不同尺寸Ti-36Nb-2Ta-3Zr-0.3O橡胶金属和Ti-10V-2Fe-3Al (Ti1023)两种Beta钛合金单晶拉伸和压缩加载下宏观变形行为和微观变形机理的研究，建立不同位向下微尺度单晶流变应力、塑性变形随单晶尺寸的变化规律。弄清非传统位错滑移和孪生等塑性变形方式下，亚微米尺度下橡胶金属本征塑性变形机理及其尺寸依赖关系。确定扩展剪切带、严重扭曲的纳米畴和应力诱发马氏体相变的临界分切应力对材料尺寸的依赖关系和应力应变突变的物理极限尺寸范围。分析亚微米尺度下橡胶金属内部非位错塑性变形与传统Ti1023钛合金滑移、孪生塑性变形时应力诱发马氏体相变对材料尺寸的依赖关系和亚微米尺度下材料力学行为的尺寸效应之间的本征关系。探讨橡胶金属内部不同尺度第二相或点缺陷塑性变形行为对微尺度材料整体力学性能尺寸效应的影响。为亚微米尺度下Beta钛合金塑性变形机理及尺度效应提出一个合理的解释。

研究了亚微米尺度下Ti-24Nb-4Zr-8Sn橡胶金属和Ti-10V-2Fe-3Al两种Beta钛合金宏观应力响应及微观变形机理。发现了橡胶金属内部在位错大规模形核与扩展之前出现一种新的形变诱发纳米马氏体相变机制。当试样尺寸减小到0.3微米时，试样强度可高达理论强度的78%。试样尺寸减小，位错形核所需临界应力上升，材料通过形成更多的纳米马氏体相变来释放更高的应变能，引入更多的弥散相颗粒，位错尺寸效应和弥散强化共同作用大大提高了材料的强化效应。.微尺度下淬火亚稳Ti-10V-2Fe-3Al合金单晶在理论屈服强度区域应力应变曲线表现出平滑连续无应变突跳，并有持续的加工硬化现象。分析发现在β基体中形成高密度有四种变体的纳米ω相阻碍位错运动，减弱位错瞬时大量滑出试样的机会。应变突跳现象被抑制，使得曲线连续平滑及加工硬化。ω相通过从一种不利于滑移的变体到另一种利于滑移变体的转变来促进滑移变形。该种位错滑移和应变诱发相变机理能非常有效地提高材料服役过程在理论强度区域塑性变形稳定性。与传统的位错切过或绕过沉淀物粒子引起钛合金强化不同，我们提出了一种新的相晶格畸变强化机理。位错塞积应力可高达相的理论强度，导致该新型晶格畸变强化机理介于位错切过与绕过机理之间。晶体学分析及第一原理能谱分析表明：通过位错切过一个变体和第二、三和四变体联合作用导致无沉淀物变形通道的形成。亚微米尺度下亚稳钛合金在理论强度区域的稳态塑性变形现象对于微小元器件强度设计及应用具有潜在的重大应用价值。.用分子动力学模拟研究了纳米尺度下α-Ti单晶变形晶体学及尺寸效应。发现材料尺寸小于某一临界尺寸时，尺寸效应由“越小越强”转变为“越小越弱”。 即反尺寸效应。不同位向纳米柱的表面能差异导致该临界尺寸不同。发现α-Ti单晶纳米柱在沿[0001]位向拉伸时，{101-2}孪晶是主要的变形机制；然而随着应变的增加，在孪晶区域出现比较罕见的HCP到FCC相的转变。这种FCC相的转变主要由肖克莱分位错滑移和堆垛层错导致。