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

CoNiAl型形状记忆合金（SMA）的特征在于在单晶状态下具有优异的形状记忆效应（SME），包括单向SME、双向（TW）SME和超弹性（SE）。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品。由于单晶的工业应用和生产存在各种困难和限制，因此需要功能性多晶基SMA。然而，根据现有技术，由于材料的脆性，CoNiAl多晶中的可逆马氏体相变导致沿着晶界断裂，结晶各向异性和相变应变的取向依赖性。本研究项目的目的是克服晶界失效的问题，从而产生多晶SMA基于两个-相CoNiAl合金具有高功能稳定性。其策略包括在晶界处相对延展性第二相（γ相）的受控生长以及支持该相可逆转变成其低温改性（ε相）。因此，通过吸收韧性相中的转变应变来防止晶界失效。由于该相也要以这样的方式调节，即优选通过马氏体转变（γ/ε）实现延展性，因此防止了晶界相的机械疲劳。在晶界处的韧性相的生长是通过晶界工程（GBE）方法实现的，并且由CoNiAl基底的化学成分的修改和特殊的热处理组成。随后的应力诱发马氏体（SIM）时效导致细沉淀物的形成，所述细沉淀物遵循马氏体针的取向并且通过稳定ε相来支持完全的γ-ε转变。SIM老化的主要任务还包括类似于已知的单晶SMA机制的材料基质中L10马氏体的稳定化。这样可以调整转变温度，理想情况下，可以实现TWSME。在计划工作的框架内，将生产合金并进行上述措施。功能（SME，TWSME和SE）和磁性能将被详细表征。随后，对最有前途的材料进行循环测试，以确定其功能稳定性。利用光学显微镜和电子显微镜的方法，深入研究了CoNiAl-SMA中γ相的控制形成、γ-ε-转化以及与基体的相互作用机制，以了解CoNiAl-SMA中的相相互作用。

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.