Prospects of Additive Manufacturing of Co-Ni-Ga High-Temperature Shape Memory Alloys
Generally, shape memory alloys (SMAs) are known for their unique material properties. However, in terms of high temperature applications, binary Ni Ti, currently being the most commonly used alloy system, suffers from limited transformation temperatures. In order to overcome these limitations, high-temperature (HT-) SMAs have been introduced. Ternary Ni-Ti-Hf is the most promising HT-SMA, since substantial progress has been made lately with respect to both processing and functional performance.
Among alternative HT-SMA candidates, the Heusler-type Co-Ni-Ga alloys received considerable attention owing to their excellent functional properties in single crystalline state. However, polycrystalline structures suffer from intergranular constraints. Microstructural features, i.e. crystallographic texture and grain boundary character as well as morphology play an important role in terms of functional reversibility. Whereas grain boundary triple junctions promote functional degradation, oligocrystals, i.e. so-called bamboo-structures, and columnar-grained microstructures promote superior functional performance. In this regard, additive manufacturing (AM) comes into focus as a technology allowing for direct microstructure design, i.e. realization of globular as well as columnar-grained structures and distinct crystallographic texture.
The present study focuses on additively manufactured Co-Ni-Ga SMA obtained by powder-based AM techniques, i.e. selective laser melting (SLM) and direct energy deposition (DED). Detailed microstructure analysis using in-situ techniques and electron microscopy was conducted allowing for correlations between the phase transformation characteristics and microstructural features.