深冷轧制对锆合金薄板退火织构与成形性能的影响规律及机制

Due to the poor room temperature stretch formability of Zirconium alloys (Zr-alloys) sheets, cracks often occur during the forming process of spacer grids, which are crucial components of nuclear reactor. In this project, a method based on deformation twinning is proposed to improve the stretch formability of Zr-alloys sheets. Cryogenic rolling, which is suitable for mass production and without risk of oxidation, is proposed to activate the twinning system of Zr-alloys. The bimodel basal texture is supposed to be weaken after the activation of twinning, thus leading to good strain accommodation in the thickness direction of Zr-alloys sheets. The effects of cryogenic rolling on the texture and microstructure (including the twins category, twins fraction and grain size) of rolled Zr-4 alloy (the model material in this project) sheet will be firstly investigated by the method of EBSD, TEM and so on in this project. Then, the recrystallization mechanism of Zr-4 alloy fabricated by cryogenic rolling will be clarified. Additionally, the plastic deformation mechanism of Zr-4 alloy sheet prepared by cryogenic rolling during sheet forming will also be explored by the method of combined multi-scale numerical simulation and experiments. Based on these basic studies, a method to improve stretch formability of Zr-alloys sheet through texture tailoring can be developed. The results of this project will help to improve the forming quality and the resistance of stress relaxation of Zr-alloys spacer grids and to ensure safety and reliability of the nuclear reactor, thus contributing to the development of nuclear power energy in China.

针对锆合金板材成形性能较低，在制备核电站反应堆用定位格架过程中常出现开裂的问题，本项目提出激活孪生调控织构进而改善成形性能的方法。采用易量产且无氧化的深冷轧制方法激活锆合金孪生，减弱锆合金板材基面织构，提高板材厚度方向的应变协调能力，从而改善板材成形性能。利用EBSD、TEM等先进的材料表征手段，实现深冷轧制诱导Zr-4合金（模型材料）的微观组织与织构的定量表征，阐明深冷轧制诱导孪晶条件下Zr-4合金退火织构的形成机制。开展单向拉伸、杯突、筒形件拉深等性能测试，并采用多尺度数值模拟与实验相结合的方法，探讨深冷轧制对Zr-4合金板材成形性能的影响规律及机制，最终发展一种基于深冷轧制的锆合金板材成形性能调控方法。项目研究成果对于提高锆合金定位格架成形质量（理论上也能提高抗应力松弛能力），确保反应堆的安全与可靠运行，进而促进我国核电事业不断发展及核能自主化的不断推进等具有重要的理论与工程意义。

织构不仅对锆合金的力学性能、吸氢性能、耐腐蚀性能有显著影响，而且还会影响锆合金板材成形性能以及服役过程中的辐照性能等。因此，研究锆合金的织构调控方法与机制对于提高锆合金的力学性能、成形性能、抗辐照性能等具有重要的理论与实际意义。. 针对锆合金板材室温成形性能较低，冲压成形过程中常出现开裂的具体问题，本项目提出激活孪生调控织构进而改善成形性能的方法。本项目主要围绕锆合金的孪晶激活、孪晶对织构的影响以及后续退火过程中孪晶组织与织构的演变开展研究。研究发现：（1）对于Zr-4合金，液氮温度条件下的常规轧制变形并不能有效促使孪晶的激活，可能与变形速率较低、初始晶粒尺寸较小有关；（2）相同变形量条件下，液氮温度轧制组织在后续退火过程中可能更易于晶粒的粗化，可能与低温促使的变形不均匀有关；（3）在板材胀形过程中，多尺度数值模拟表明趋于平面应变状态的变形将引起Zr-4合金晶粒C轴逐渐转向平行于板材平面以内；（4）对于轧制成形的Zr-4合金、Zr702合金（具有较强的基面双峰织构），沿着轧制方向（RD）的室温压缩变形就能有效激活{10-12}拉伸孪生，孪晶面积分数随变形量的增加而增加，拉伸孪晶的激活将引入{0002}∥RD新织构组份；（5）在后续退火过程中，拉伸孪晶片层组织会逐渐长大（而不是被基体所吞并），可能与孪生区域积累的形变能较基体形变能低所致，孪晶片层组织的长大导致{0002}∥RD织构组份的强化；（6）在相同变形量条件下，相对于RD方向，沿Zr-4板材法向（ND）的压缩变形更容易促使退火过程中晶粒的长大，可能与ND方向变形时有更多的锥面<c+a>滑移系启动有关，锥面<c+a>位错的伯氏矢量的模更大，存储有更高的畸变能；（7）RD方向预压缩变形引入拉伸孪晶后，ND方向的再次压缩变形会激活退孪生，而不是二次孪晶。. 上述研究结果进一步加深了对锆合金变形以及退火再结晶行为的理解，提出了一种基于孪生调控锆合金织构的方法，可为锆合金性能的提升提供一定的理论指导。