高应变率下双相钛合金变形孪晶诱导界面相交及局部协调机理研究

Twin-twin interactions are widely investigated in magnesium alloys, indicating that the strain accommodation at the intersection interfaces cannot be provided by the same type of twins and basal slip is required. The limited accommodation at the intersection interface results in the nucleation of cracks. However, our recent research on titanium reveals that the interaction between {11-22} and {11-24} twins stimulates the formation of secondary {10-12} twins. This result gives us a new perspective that multiple tension and compression twins in titanium provide more possibilities of accommodating the strain at intersection interfaces. In order to enrich the understanding of accommodation mechanism at intersection interfaces, EBSD and TEM characterizations will be performed in this project to statistically investigate the accommodation mechanism at the intersection interfaces in dual-phase titanium alloy under high strain rate, which is produced because the growth of deformation twins is blocked by α/β interfaces and other twin boundaries. The evolution of the intersection interfaces during plastic deformation will be clarified. The relation between the accommodation systems and the types of the intersection interfaces will be revealed. The crystallographic model for accommodation mechanism at intersection interfaces will be built. The results will provide theoretic basis for designing dual-phase titanium alloys which exhibit excellent accommodation ability at intersection interfaces. It will help us to realize the goal of improving the strength and avoiding the reduction of plasticity via the introduction of twin boundaries and phase interfaces.

孪晶与孪晶界面相交在镁合金中已广泛关注，研究表明在界面相交处不能由同种孪晶提供变形协调而只能通过基面滑移，有限的协调导致在界面相交处易形成微裂纹。但我们最近在钛中发现，{11-22}孪晶与{11-24}孪晶相交，在界面相交处激发了{10-12}二次孪晶，给我们提供了一种新思路，钛中孪晶的多样性给予更多释放界面相交处应变集中的途径。为丰富对孪晶诱导界面相交及局部协调机理的认知，本项目拟通过背散射电子衍射技术和透射电子显微镜等实验方法，系统研究双相钛合金在高应变率下，孪晶扩展遇到α/β相界面或孪晶界形成的界面相交，明晰相交界面在塑性变形中的演化规律，揭示不同界面相交处需要的协调方式与相交界面类型的相关性，建立界面相交处变形协调机理的晶体学模型。这些结果将为设计界面相交处协调能力强的双相钛合金材料奠定理论依据，从而实现通过引入孪晶界和相界面提高材料强度的同时又不损失塑性的目标。

孪生作为一种重要的变形模式，在钛合金和镁合金等金属材料中普遍发生，对于改善材料的强塑性有重要的作用。但孪晶的扩展被界面阻碍后，要求周围晶体启动变形机制以释放局部堆积的应力集中，从而避免微裂纹形核和结构失稳，为实现通过界面实现强化又避免塑性损失，需揭示易发生孪生行为的材料中，界面对于孪生的阻碍作用及激发的变形协调机制与界面特征的关系。.本项目基于实验数据，统计了变形孪晶遇不同界面后，在相邻晶体激发的变形协调机理，变形孪晶在小角度界面的相邻晶体内可激活两种同类型孪晶，在大角度界面处启动两种相反类型孪晶，也可能终止于大角度界面。本项目讨论了界面取向差对局部变形协调模式及变体选择的影响，开发了评价相邻晶体对局部剪切协调能力的计算方法，建立了变形孪晶与界面相交诱发局部协调机理的预测模型。针对变形孪晶与非同族型孪晶界相交问题，本项目发现非同族型孪晶相交激发的二次孪晶不符合当前广泛适用的孪晶择优准则，本项目中理论计算表明实验中观察到的二次孪晶变体能最大程度协调被阻碍的孪晶引起的局部位移梯度张量，提出了非同族孪晶相交激发二次孪晶类型和变体的预测机制。.本项目将孪晶与界面相交问题及界面特征影响局部协调机理的预测模型，扩展到位错与界面的相交问题，实验表明随着取向差角度增大，位错穿过晶界减少，但在较大取向差晶界处，依然发生了较多位错穿过晶界，且在较小取向差晶界处，也有较多位错未穿过晶界，本项目建立了相邻晶粒取向差图谱与位错能否穿过晶界的关系。.利用孪晶遇界面后的相互作用机理，本项目在钛合金中引入相界面，通过孪晶与相界面的相互作用，成功进一步提升了钛合金的屈服强度，本项目结果有助于在多尺度建模中考虑界面处变形协调机制，模拟TRIP和TWIP金属材料的变形组织演化与服役行为，为优化和设计具有良好界面协调能力的高强高塑金属材料奠定理论基础，以满足日益严峻的服役环境对钛合金强度与塑性的极高要求。