Research On Martensitic Transformation And Microstructure Of NiMnGaFeCu And NiMnSnAl Ferromagnetic Shape Memory Alloys

As typical magnetic shape memory alloys,Ni-Mn-Ga and Ni-Mn-Sn alloys have advantages combining high frequency response of magnetostrictive materials and giant strain of shape memory alloy and so on.However,large study found that these alloys persist some shortcomings,doping elements can improve the performance of the alloys and improve their microstructures effectly.Therefore,this thesis has choose Fe,Cu doped Ni-Mn-Ga and Al doped Ni-Mn-Sn alloy as objects,and studied the characteristics of martensitic transformation and microstructure by using optical microscopy,DSC,SEM,XRD,TEM and HRTEM.There only exist the reverse martensitic transformation in Ni57Mn17Ga23-xFe1Cu1+x(x=0.5,1)alloys upon being subject to a circle of heating and cooling,The two alloys consist of martensitic phase and y phase at room temperature,and y phase content in alloys increases with the increase of Cu content.Martensitic transformation and reverse martensitic transformation occur in Ni55.5Mn17Ga24Fe1.5Cu1 alloys,while only reverse martensitic transformation were observed and y phase precipates in Nis7Mn17Ga22Fe1Cu2 alloy.Valence electron concentration in nominal composition is lower than y phase and higher than martensitic matrix phase.Martensitic structures in alloy mainly show NM tetragonal phase,? phase is a face-centered cubic structure.With the increase of Cu substituting for Ga,cell volume of the crystal deceases,while increasing Fe substituting for Ni,cell volume of the crystal increases.Ni57Mn17-zGa24Fe1+zCu1(z=0,0.5,1)undergoes martensitic and reverse martensitic transformation during a heat cycling process.Phase transition temperatures exceed room temperature,the thermal hysteresis tends to increase with the increase of Fe.These alloys exhibit a single matrix structure of NM martensite.With the increase of Fe and decrease of Mn,valence electron concentration rises and cell volumn of the crystal decseases.Ni57Mn16Ga24Fe2Cu1 alloy exhibits a majority of non-modulated tetragonal martensite(2M),with a very small 7M phase.The 2M martensite is internally twinned,the size of the internal twins is within the scope of the nanometer scale.Stacking fault emerges near some twin interface and the strain contrast stripes were also observed along<112>A direction.For Ni48Mn39.5Sn12.5-xAlx(x=1,1.5,2,2.5,3)alloys,all four martensitic phase transition temperatures are above room temperature,and with increasing content of Al,phase temperature shows a increase tendency,the thermal hysteresis decreases.Ni48Mn39.5Sn12.5-xAlx(x=0.5,1)alloys show austenitic parent phase;austenite and martensite coexist in Ni48Mn39.5Sn12.5-xAlx(x=1.5,2,2.5 alloy,and the second phase precipitated;Ni48Mn39.5Sn9.5Al3 alloy exhibits a single martensite phase.With the increase of Al content,the alloy microstructure yields L21-40-1OM-14M transformation sequenses.