Multistep superelasticity of Ni-Mn-Ga and Ni-Mn-Ga-Co-Cu microwires under stress-temperature coupling

Martensitic and intermartensitic transformation induced by changing temperature and/or applying mechanical stress was investigated systematically in Ni50Mn30Ga20 and Ni46Mn24Ga22Co4Cu4 polycrystalline shape memory microwire. In particular, intermartensitic transformation induced by stress can be obtained in single crystal but difficult in polycrystalline materials that are usually intrinsically brittle due to the weak grain boundary cohesion. In this paper, two kinds of oligocrystalline microwires, containing free surface and bamboo-like grains, with austenite and martensite at room temperature exhibit multistep superelastic behavior with larger than 10% fully reversible strain from intermartensitic transformation. The two alloys undergo four kinds of phase transition sequence under the condition of stress-temperature coupling, and the later has smaller stress hysteresis and exhibits excellent high temperature mechanical properties during loading-unloading loops. Meanwhile, the transformation critical stress as a function of temperature phase diagram for the two kinds of alloys has been established. Utilizing the elements of Co and Cu replacing Ni and Mn in Ni50Mn30Ga20 alloys, phase transformation temperature is broadened and increased. For Ni-Mn-Ga microwire, continuous martensitic transformation process from 228 to 298 K is investigated by in-situ TEM observation, and the structure is confirmed to be five-layered modulated martensite at low temperature. Multiple martensites coexistence, including five-layered and seven layered modulated martensite, is explored in Ni-Mn-Ga-Co-Cu microwire at room temperature. Those multistep superelastic behavior and mechanical properties can be comparable but different from single crystal. This is an effective method to obtain small size, high performance shape memory alloys, which are of great significance for device miniaturization and intelligence.

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