基于HCP马氏体转变实现优良性能的Co基形状记忆合金研究

It is not clear why excellent shape memory effect (SME) cannot be realized in Co-based alloys by HCP martensitic transformation, although it can be realized in FeMnSi-based alloys. We put forward that the low yield strength and the pre-existence of many thermal-induced martensite are responsible for poor SME in the Co-based alloys. To directly prove the above idea experimentally, we will investigate:(1) Effects of different kinds of solute atoms on the yield strength and stacking fault energy in the Co-based alloys and their relationship with the SME; (2) effects of methods improving the SME in the FeMnSi-based alloys on the amount of annealing twin boundaries and HCP martensitic transformation as well as their relationship with the SME; (3) precipitations of different ordered phases and their effects on the process and characteristics of HCP martensitic transformation (thermoelastic or nonthermoelastic) as well as their relationship with the SME and superelacticity; (4) effect of external filed on the process of HCP martensitic transformation and its relationship with SME. This research will also lay foundation for composition design and preparation techniques for Co-based alloys with excellent performance realized by the HCP martensitic transformation. In addition, this research will help us clarify the key role of Si playing in improving the SME of FeMn-based alloy and confirm the key factor controlling the recovery strain in the FeMnSi-based alloys.

针对HCP马氏体转变未能在Co基合金中实现与FeMnSi基合金相当的优良形状记忆效应（SME）及原因不清楚的科学问题，申请人提出低的母相屈服强度和大量预先存在的热马氏体是其未能实现的原因；拟研究不同种类溶质原子的加入对Co基合金母相屈服强度和层错能的影响及其与SME的关系；提高FeMnSi基合金SME的方法对Co基合金孪晶界面分数和HCP马氏体转变的影响及其与SME的关系；不同结构有序相的析出对HCP马氏体转变性质的影响（热弹性或非热弹性）及其与SME和超弹性关系；外场对HCP马氏体转变过程的影响及其与SME的关系。上述研究不仅能直接验证明确HCP马氏体转变未能在Co基合金中实现优良SME的原因，而且将为基于HCP马氏体转变在Co基合金中实现优良的性能奠定成分设计和制备技术的基础；同时有助于清楚辨析Si显著提高FeMn基合金SME的关键角色，验证控制FeMnSi基合金SME的关键因素。

Fe-Mn-Si基合金和Co基合金均拥有面心立方（FCC）⇌密排六方（HCP）马氏体相变。但是，Co基合金未能实现与Fe-Mn-Si基合金相当的优良形状记忆效应。针对上述问题，本项目研究了Co-xAl（x=2,4,7,8 wt%）二元合金中热诱发HCP马氏体与形状记忆效应的量效关系；Co-7Al-xW（x=0,4,6,9 wt%）合金以及Co-xNi-6Si（x=15,20,25 wt%）合金的马氏体相变和形状记忆效应演化规律；热处理工艺和时效方式对锻造和铸造Co-7Al-6W合金中β-CoAl有序相析出行为以及马氏体相变和形状记忆效应的影响；L12结构共格纳米有序相对Co-20Ni-6Si合金马氏体相变和形状记忆效应的影响；脉冲磁场对Co基合金形状记忆效应的影响。上述研究获得了如下结果：Co基合金形状记忆效应的来源是应力诱发HCP马氏体相变及其逆相变，而不是热诱发HCP马氏体的合并；在Co-Al-W和Co-Ni-Si合金中均实现了与Fe-Mn-Si基合金相当的形状记忆效应；通过高温淬火后直接时效可在Co-Al-W合金中方向性析出β-CoAl有序相，进而显著提高其形状记忆效应；通过“铸造+高温固溶后水淬”直接制备出具有优良形状记忆效应的Co-Al-W合金；析出L12结构共格纳米有序相会显著恶化Co-Ni-Si合金的形状记忆效应，从而揭示了“共格纳米相诱发非热弹向热弹性马氏体相变转变”不具备普适性；初步研究表明脉冲磁场处理对提高Co基合金形状记忆效应的规律不明显。综上，本项目的研究结果为基于FCC⇌HCP马氏体转变在Co基合金中实现优良形状记忆效应提供了成分设计原则和制备技术，并为开发新型形状记忆合金奠定了理论基础。