体心立方金属钽、钨及其合金韧脆转变机制的定量电子显微学研究

Refractory body-centered cubic metals such as Ta and W，which have high melting temperature and strength, good irradiation and corrosion resistance, have a wide range of applications in aviation, spaceflight navigation, arms industry, nuclear power, energy sources and other fields. Moreover, these metals can maintain their great mechanical properties in a large temperature and strain range. The ductile-brittle transition of BCC metals has important influence on their processing conditions and service environment, such as temperature and strain rate, which arouses great interest among researchers. The ductile-brittle transition of BCC metals is greatly influenced by their deformation mechanisms. There is a significant toughness difference between Ta and W, which is related to their valence electron number difference. The present proposal aims to investigate the ductile-brittle transition mechanisms in BCC metals with different valence electron number, such as Ta, W and their alloys, with the combination of transmission electron microscopy techniques and quantitative electron microscopy methods. The goal is to clarify deformation mechanisms of Ta, W and their alloys, such as dislocation slipping, deformation twinning and phase transformations, by using in situ and aberration corrected transmission electron microscopy under applied stress. The effect of valence electron number on the ductile-brittle transition temperature and activation energy of Ta-W alloys is planning to be investigated by combining experimental mechanical testing and theoretical calculations. The results will provide insights to the internal relation between deformation mechanisms and the ductile-brittle transition of BCC metals, and theoretical basis for the toughness control of BCC alloys. Besides, deformation behaviors of Ta, W and their alloys under high strain rates are going to be investigated by using Hopkinson pressure bar testing and transmission electron microscopy, to clarify the effect of high strain rates on the ductile-brittle transition process and corresponding deformation mechanisms. These results can provide theoretical references for the engineering applications of BCC metals.

BCC金属Ta、W具有极高的熔点和强度、良好的耐辐照和抗腐蚀性能、并且能够在较大的温度和应变范围都保持良好的力学性能，因而在航空航天、国防工业、核电能源等领域有广泛的应用。BCC金属的韧脆转变过程对其加工条件和使用环境如温度、应变速率等具有重要的限制。本申请拟结合原子分辨率透射电镜的原位、离位表征和定量电子显微学分析对平均价电子数不同的Ta、W及其二元合金的韧脆转变机制进行研究。重点从实验上厘清Ta、W及其合金在加载过程中位错滑移、孪生、相变等变形机制，结合力学性能实验和理论计算探讨平均价电子数变化对Ta-W合金韧脆转变温度和激活能的影响，从而阐明BCC金属的变形机制与其韧脆转变过程的内在联系，并为难熔BCC金属的韧性调控提供理论依据。利用霍普金森压杆冲击实验和像差校正透射电镜表征系统研究高应变速率对BCC金属韧脆转变过程和机制的影响，以期为BCC结构金属的工程应用提供理论参考。

体心立方结构的金属或合金能够在较大的温度和应变范围都保持良好的力学性能，因此被广泛应用于日常生活当中。体心立方金属的韧脆转变过程对其加工条件和使用环境如温度、应变速率等具有重要的限制。这种特殊的塑性变形行为与体心立方金属复杂的微观变形机制有关。本项目利用透射电镜实验，对比研究了不同韧性的V族、VI族体心立方金属及其合金的变形机制，发现随着合金中VI族金属含量的增加，合金韧性逐渐降低，合金的变形机制以孪生、位错滑移和相变的顺序发生演变，这种变形机制的演化揭示了体心立方金属的变形机制与其韧脆转变过程的内在联系。在此基础上我们研究了体心立方金属在冲击变形过程中的微观机制，揭示了其在高应变速率下的变形机制包含{112}孪生、{332}孪生及马氏体相变机制。针对位错与相界面的交互作用，我们在双相合金中阐明了三种位错与α/β相界面的滑移传递机制，前两种机制中的α/β相界对位错有很强的阻碍和分散作用，而第三种机制中的位错会形成滑移带，揭示了α/β双相合金存在塑性各向异性的原因。针对位错与孪晶界的交互作用，我们研究了纳米孪晶片层对位错形核的影响，阐明了纳米孪晶金属中孪晶片层厚度越小位错形核所需的临界剪切应力越大，揭示了孪晶界对位错形核的限制增加了纳米孪晶材料的可回复性，建立了纳米孪晶金属可回复性与孪晶片层厚度的关联。项目执行期间，共发表SCI论文3篇；协助指导3名研究生，其中1名已毕业。