Development of polycrystalline two-phase CoNiAl shape memory alloys with high functional stability

CoNiAl型形状记忆合金在单晶态具有优异的形状记忆效应，包括单向形状记忆效应、双向形状记忆效应和超弹性形状记忆效应。它们也可以在高温下使用，是其他SMA的一种具有成本效益和前景的替代方案。由于单晶的工业应用和生产涉及到各种困难和限制，因此需要功能性多晶基SMA。然而，根据目前的研究现状，由于材料的脆性、晶体的各向异性和相变应变的取向相关性，CoNiAl多晶中的可逆马氏体相变导致沿晶界断裂。本研究项目旨在克服晶界失效的问题，从而制备出具有高功能稳定性的两相CoNiAl合金多晶形状记忆合金。这方面的策略包括在晶界控制生长相对韧性较好的第二相(γ相)，并支持该相向低温变质(ε相)的可逆转变。因此，通过在延性相中吸收相变应变来防止晶界破坏。由于该相的调整方式也是通过马氏体相变(γ/ε)来实现塑性，因此防止了晶界相的机械疲劳。晶界延性相的生长是通过晶界工程(GBE)方法实现的，包括改变CoNiAl基合金的化学成分和特殊的热处理。随后的应力诱发马氏体时效导致形成沿马氏体针取向的细小析出物，并通过稳定γ-ε相来支持完整的ε相变。SIM时效的主要任务还包括稳定材料基质中的L10马氏体，类似于已知的单晶SMA的机制。这使得可以调整相变温度，理想情况下，可以实现TWSME。在计划的工作框架内，将生产合金并接受上述措施。将详细描述其官能化(SME、TWSME和SE)和磁性。随后，对最有希望的材料进行循环测试，以确定其功能稳定性。利用光学和电子显微镜等手段，深入研究了γ相的控制形成、γ-ε转化以及与基体的相互作用机制，以期对CoNiAl-SMA中的相相互作用有更深入的了解。

Shape memory alloys (SMA) of the CoNiAl type are characterized by excellent shape memory effects (SME) in the single crystal state, including one-way SME, two-way (TW)SME and superelasticity (SE). They can also be used at high temperatures and represent a cost-effective and promising alternative to other SMAs. As industrial application and production of single crystals is associated with various difficulties and restrictions, there is a need for functional polycrystal-based SMAs. According to the current state of the art, however, the reversible martensitic transformation in CoNiAl polycrystals leads to fracture along the grain boundaries due to the brittleness of the material, the crystalline anisotropy and the orientation dependence of the transformation strains.The present research project aims to overcome the problem of grain boundary failure and thus to produce polycrystalline SMAs based on two-phase CoNiAl alloys with high functional stability. The strategy for this includes the controlled growth of a comparatively ductile second phase (γ-phase) at the grain boundaries and the support of a reversible transformation of this phase into its low temperature modification (ε-phase). Thus, grain boundary failure is prevented by absorbing the transformation strains in a ductile phase. Since this phase is also to be adjusted in such a way that ductility is preferably achieved by a martensitic transformation (γ/ε), mechanical fatigue of the grain boundary phase is prevented. The growth of the ductile phase at the grain boundaries is achieved by grain boundary engineering (GBE) methods and consists of a modification of the chemical composition of the CoNiAl base and special heat treatments. Subsequent stress-induced martensite (SIM) aging results in the formation of fine precipitates that follow the orientation of the martensite needles and support the complete γ-ε transformation by stabilizing the ε phase. The main tasks of SIM aging also include the stabilization of the L10 martensite in the material matrix in analogy to already known mechanisms for monocrystalline SMAs. This allows the transformation temperatures to be adjusted and, ideally, TWSME to be achieved.Within the framework of the planned work, the alloys will be produced and subjected to the above-mentioned measures. The functional (SME, TWSME and SE) and magnetic properties will be characterized in detail. Subsequently, the most promising materials are cyclically tested to determine their functional stability. Using light and electron microscopic methods, the mechanisms of controlled formation of the γ-phase, the γ-ε-conversion and the interaction with the matrix are studied in depth to gain an understanding of the phase interactions in the CoNiAl-SMA.