NEW FERROMAGNETIC AND FUNCTIONALLY GRADE SHAPE MEMORY ALLOYS

The paper summarizes recent advances in shape memory alloy investigation with focus on the path used to find new ferromagnetic shape memory alloys and on the improvement of microactuation by thermoelastic control of the phase transformation in graded shape memory alloy fi lms. The first part of the paper describes experimental results and theoretical considerations concerning the search, fabrication and characterization of new Co-based Heusler-type ferromagnetic shape memory alloys with focus on the CoNiGa system. It is shown that CoNiGa alloys undergo a reversible thermoelastic martensitic transformation in a wide temperature range, depending on the composition and displaying a nickel-l ike saturation magnetization. The second part of the paper takes into consideration the case of layered and functionally graded microlayered films constructed by continuously changing the temperature during the deposition process of NiTi alloys on heated Si canti lever-type substrates. It is shown that – depending on the deposition temperature and sequence of deposition high actuation can be achieved. The discussion takes into consideration the martensitic transformation in layers under constraints from the substrate and/or the neighboring microlayers. The best actuation can be obtained when transformation starts and develops gradually from the top microlayer. This was achieved in NiTi films deposited continuously on heating.