Effect of Cu Alloying on the Microstructural and Functional Evolution of NiTiCu Shape Memory Alloys

NiTi-based shape memory alloys (SMAs) are gaining prominence as ternary alloying and advanced processing techniques offer improved control over microstructure and functional behavior. This study examines the effect of copper (Cu) alloying on the microstructural and thermoelastic properties of (NiTi)_100−xCu_x alloys with x = 0, 5, 8, 12, and 20 wt.%, synthesized via vacuum induction melting (VIM). Microstructural analysis showed that increasing Cu content significantly altered the phase composition, lowering the onset of martensitic transformation from 70 °C in the binary alloy to as low as − 60 °C. A clear correlation was observed between Cu concentration, secondary phase formation, and the balance of austenite and martensite. Cu induced both B19 and B19ʹ martensitic structures. However, excessive Cu content led to the formation of intermetallic compounds, grain boundary arrest, and partial suppression of the transformation. Hardness increased with Cu addition up to a certain point, then slightly declined due to increased martensite. The 20 wt.% Cu alloy showed the highest B19 content within a complex intermetallic matrix, while intermediate Cu levels produced a balanced B19/B19ʹ microstructure. These findings clarify the composition–microstructure–property relationships in NiTiCu SMAs and guide future design strategies for functional alloy systems.