初始弹性能对磁致形状记忆合金记忆行为的影响规律

A systematic substitution of Ni for Mn, Ga, or both Mn and Ga, and Mn substitution for Ga in the non-stoichiometric NiMnGa alloys is performed. The relationship among the composition, structure and martensitic transformation temperatures was studied in detail for the Ni and Mn excessive NiMnGa Alloys. The martensitic transformation temperatures almost linearly increase, up to near 400 oC, with increasing Ni content in all the former three series, and Mn substitution for Ga. these kinds of NiMnGa alloys were pecified according to the sequence of the tructural and magnetic transition where the structural transition temperatures are lower, same and higher than the magnetic transition temperatures, respectively. A co-occurrence of magnetic and structural transitions was observed in a non-stoichiometric Ni53Mn25Ga22 alloy, which undergoes a reverse martensitic transformation from ferromagnetic martensite to paramagnetic austenite with a magnetic transition temperature (134oC) higher than the Curie temperature of the stoichiometric Ni2MnGa alloy (103 oC).The effect of magnetic field on the phase transition temperature was found to be two orders of magnitude greater in the present alloy than in Ni2MnGa due to the absence of the ferromagnetic state in austenite. Superhigh strains of 15% for the Ni53Mn25Ga22 and 13.5% for the Ni54Mn23Ga23 with the nonmodulated martensite structure were achieved by variant reorientation. A strong magnetic anisotropy exists in the prefabricated single-variant with the easy axis along the contraction [001] direction of the high-temperature cubic phase. The same order of magnitude for the saturation magnetization Ms, magnetic anisotropy constant ku, and the twin reorientation stress stw, namely, 57.6 Am2/kg, 1.4 ′ 105 J/m3, and approximately 20 MPa, respectively, as those in 5M martensite indicate that it is reasonable to expect a much higher magnetic field-induced-strain (MFIS) in the nonmodulated NiMnGa alloys

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