Systematical investigation of field controrable shape memory alloys

点击翻译按钮获取中文摘要

Large magnetic-field-induced strains have been observed in Heusler alloys with a body-centered cubic ordered structure and have been explained by the rearrangement of martensite structural variants due to an external magnetic field. These materials have attracted considerable attention as potential magnetic actuator materials. We have investigated systematically the intelligent materials exhibiting magnetic shape memory effect. Research results are as follows :1.Intermetallic compounds MnCo_<1-x>Ge(0.06【less than or equal】x【less than or equal】0.08) exhibit the diffusionless thermoelastic martensitic transformation at about 300 K. The martensitic phase crystallizes in the orthorhombic TiNiSi-type and transforms to the parent one with the hexagonal Ni_2In type crystal structure at T_M with increasing temperature. These phase transformations are analyzed thermodynamically using a Landau-type thermodynamical potential.2.The phase diagram of ferromagnetic shape memory alloys Ni_<50+x>Mn_<12.5>Fe_<12.5>Ga_<25-x> was obtained. The Curie temperature decreases gradually with x. On the other hand, the martensitic transformation temperature T_M increases steeply until x=3.0, after then they reach to the value 380 K at x=5.5. The good ductility was confirmed for the samples with x【greater than or equal】3.0.3.Magnetic and structural properties of new ferromagnetic shape memory alloys Ni_2Mn_<1+x>Z_<1-x>(Z=In and Sn) have been investigated. In the parent phase these alloys exhibit ferromagnetic behavior. On the other hand, the spontaneous magnetization of the martensitic phase drops significantly compared with that of the parent phase, indicating that these alloys are ferrimagnetic or antiferromagnetic in the martensitic phase. We have observed a field-induced reverse martensitic transition for Ni_2Mn_<1.44>Sn_<0.56>.