EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

The Ni-Mn-Ga Heusler alloys have been widely investigated in the last few decades as a potential material for actuator and sensor devices. This alloy exhibits shape memory effect (SME) associating with the martensitic transformation, superelasticity, and magnetic field-induced strain (MFIS). In these materials, the functional properties can be tuned by chemical modification. Thus, the series of polycrystalline alloys of nominal composition Ni50Mn25Ga25-xCux (x = 1–10 at.%) was manufactured from high purity elements using conventional arc-melting method. Thereafter, to check the influence of heat treatment one part samples was slowly cooled with a furnace to ambient temperature when the other one was water quenched. The x-ray measurements revealed that the substitution of Ga by Cu introduces changes in the type of crystal structure and the tetragonality of the unit cell. Moreover, the four types of crystal structure were detected i.e austenite and five-layered, seven-layered, and non-modulated martensite. The DSC measurements showed that the addition of Cu instead of Ga affected the martensitic transformation temperature, which increased from 202 to 691 K. Using SEM the microstructure observation was taken, detected changes in microstructure corresponds to the changing type of crystal structure. In the case of the highest amount of Cu content, i.e., 9 and 10 at. % a two-phase microstructure containing martensite and ɣ phase was detected. To assess the influence of substitution of Ga by Cu on mechanical properties and shape memory effect, compression tests were carried out at room temperature. The addition of Cu brings significant improvement of room-temperature ductility of Ni-Mn-Ga-Cu alloys. On the other hand, the occurrence of ɣ phase strongly decreases the shape memory effect. In polycrystalline Ni50Mn25Ga17Cu8 upon heating to 833K, a full recoverable strain equal to 7 % was achieved.