Fabrication And Properties Of Magnetic Shape Memory Alloy CoNiGa

Systematical investigations have been carried out on the structure, magnetic properties and martensitic transformation behaviors of Co50Ni20Ga32 polycrystalline samples treated by the different conditions. It was observed that the Co50Ni20Ga32 polycrystalline samples exhibit martensitic transformation at different temperature range because of different heat treatment conditions. The L21 structure and the Y phase type structure usually coexist in the CoNiGa alloys and the coexistence of the two phases can be changed by the various heat treatments conditions. Y phase affects the martensitic transformation and magnetism greatly. These results indicate that the magnetic properties and martensite behaviors can be adjusted just by controlling the contents of the phases, which is important for CoNiGa alloys to be ferromagnetic shape memory alloys.Character Temperatures (Martensitic phase transformation temperature(TM), reversed transformation temperature(TA), hysteresis T (TA-TM), Curie temperature (Tc)) of CoNiGa polycrystals change obviously with the conditions of annealing time, annealing temperature and quencher. Tc of CoNiGa polycrystal is not unique compared with CoNiGa single crystal by reason of polycrystalline samples' speciality and the effect of annealing at 700℃ for 5 days. The AC magnetic susceptibility of CoNiGa polycrystal was measured as a function of temperature. The curves show that magnetization of CoNiGa polycrystal changes regularly during the repeated experimental processes. Pre-martensitic transformation that is similar to NiMnGa single crystals was found in some Co50Ni20Ga32 polycrystals.Martensitic transformation occurs in a wide range from 202K to 343K in Co50Ni22Ga28, Co50Ni21Ga29 and Co50Ni20Ga30 single crystals. By applying a bias magnetic field of up to 2.0T, the shape memory strain of Co50Ni22Ga28 single crystal can be continuously controlled from negative 2.3% to positive 2.2% giving it a total strain of 4.5%. Temperature of whole austenitic phase doesn't exist in Co50Ni24Ga26 single crystal in ferromagnetic state, and it is about 394K that the austenitic phase and martensitic phase coexist.