Study Of Phase Transformation And Propties In Rare Earth Ni-Mn-Ga Magnetic Shape Memory Alloys

Ni-Mn-Ga alloy is the only one that both ferromagnetic and shape memory effect among Heusler alloy family. In most cases shape memory alloys are nonmagnetic and the options to influence their shape and dimensions are restricted to temperature and stress. In Ni-Mn-Ga, however, the change in the shape and dimensions can be influenced by an external magnetic filed in addition to temperature and stress. The alloy demonstrates large magnetic-field-induced strains (MFIS) based on reorientation of martensite variants in magnetic field. This characteristic will make the alloy a candidate of ideal smart driving materials. Thus Ni-Mn-Ga alloy has been received much attention. However, all the complexity of phase transition, unstable MFIS, mechanical property and process ability, and lower Curie temperature of the alloy greatly limit its application. This paper base Zhao Zengqi etc on bare earth Ni-Mn-Ga alloys, and considering locality and state of bare earth in alloys , and effect of rare earth on Phase Transformation and property of magnetism have been studied. The major research efforts as follows:As x≥0.2, Tb precipitated mostly lied in grain boundary and in the form of the fine particle in grain, and with bare earth Tb addition, Tb precipitated become bigger.With the addition of Tb, It was a little alter of the matensite phase transformation temperature and magnetism of Ni₅₀Mn₂₉Ga₂₁Tb_x(x=0,0.05,0.1,0.2,0.4,1.2)alloys.Phase transformation behaviors were studied in the alloys of Ni₅₀Mn_25+xGa_25-xTb_0.5(x=0,1.5,3,4.5,6,7.5). Matensite phase transformation temperature could be considerly increased by increasing Mn and decreasing Ga. The reason: Chemisal free energy of nucleation have been reduced by increasing Mn and decreasing Ga, accordingly matensite phase transformation temperature have been increased.Measurement of magnetic-field-induced strains (MFIS) have been made for Ni₅₀Mn_25+xGa_25-xTb_0.5(x=0,1.5,3,4.5,6,7.5). The symmetry of crystal structures have been decreased and magnetocrystalline anisotrop have been increased with increasing Mn and decreasing Ga, it resulted in augment of MFIS.In short, the addition of Tb could remarkably improve properties of mechanic, at the same time, others properties is constant; It is found phase transition temperature is 484K and MFIS is -2.868% of Ni₅₀Mn_32.5Ga_17.5Tb_0.5 alloy by increasing Mn and decreasing Ga, therefore the alloys showed a prospective practice future.