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

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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.