Studies On The Phase Transformation Materials In Fe-Based And Ni-Based Heusler Alloys

In this thesis,the phase transformation materials in some Fe-based and Ni-based Heusler alloys were investigated both theoretically and experimentally.The Heusler alloys investigated in the thesis include Ni2COZ(Z=Al,Ga,In,Si,Ge,Sn,Sb)?Ni-Fe-Mn-In(Sb)?Fe51Mn22Ga27-xSix?Fe2MnGaCx and Mn2NiGaCx.The site preference of transiton metal elements,magnetic properties,electronic structure and martensitic transfromation in these alloys are discussed in detail.The site preference of Co,electronic structure,magnetic properties and martensitic transformation in Ni2CoZ alloys have been investigated by first-principles calculations.It is found that in Ni2CoZ alloys,apart from Ni2CoIn,Co tends to occupy C site to form XA-type structure.Theoretical calculation reveals that the tetragonal martensitic phase has a lower total energy compared with the cubic austenitic phase.Therefore,a structural transition from cubic to tetragonal is likely to happen in these Ni2CoZ alloys.It should be noted that Ni2CoSi is paramagnetic in austenitic state,while it is ferromagnetic in martensitic state.This leads to a large change in the total moment,which is meaningful for the realization of magnetic field-induced martensitic transformation in this alloy.The influence of the substitution of Fe for Ni on the magnetic structure and martensitic transformation is investigated theoretically in Ni2Mn1.5Z0.5(Z=In,Sb)alloys.It is found that Fe-doping does not change the general magnetic structure of Ni2Mn1.5Z0.5.In both Ni2Mn1.5In0.5 and Ni1.75Fe0.25Mn1.5In0.5 the austenitic phase is ferromagnetic and martensitic phase is antiferromagnetic,which leads to the large magnetization difference observed experimentally.Meanwhile,the substitution of Fe for Ni can lower the martensitic transformation driving force and lead to the decrease of phase transformation temperature.The single BCC phase of Fe561Mn22Ga27-xSix alloys is obtained by melt-spun method.The martensitic transformation is not observed in ribbon sample,which is different from the results on bulk sample with the same composition in literature.Analysed by first-principles calculations,it is found that the martensitic transformation can be strongly influenced by atomic ordering,and it is likely to happen martensitic transformation only in 50%Fe(A)-Mn(B)atomic disordering structure.The calculated results also reveal that the magnetization increases when the martensitic transformation occurs,which agrees well with the previous report.In addition,Si-doping can change magnetic properties in these alloys.With increasing Si content,the saturated magnetization at 5K and Curie temperature decrease gradually.Fe2MnGaCx alloys were prepared by arc-melting method,and the experimental results reveal that the FCC structure does not change with the doping of C,and with the content of C increasing,the lattice constant,the saturated magnetization at 5K and Curie temperature increase.Additionally,the magnetic structure is transfered from antiferromagnetism into ferromagnetism with increasing C content.Mn2NiGaCx alloys were prepared by arc-melting method,and it is found that the doping of C does not change the BCC structure of Mn2NiGa.And as the content of C increases,the total magnetization of martensite increases,the martensitic transformation temperature decreases and thermal hysteresis becomes larger.Meanwhile,the increase of C content can suppress the martensitic transformation in Mn2NiGa alloy.