Non-martensitic α-phase And Promoted Martensitic Transformation Of Fe-Ni Nanoparticles

Fe-Ni binary alloy system is one of the classical model materials instudying martensitic transformation (MT). In Fe-Ni nanoparticles, MTbehaviors are quite controversial since both the size effect and structuralcharacteristics of materials have impact on the MT. Generally, decreasingthe particle size will cause a suppression of the MT due to the influence ofsize effect. However, Synthesis method is closely associated with MTbehaviors since it is considered to influence the structural characteristics ofthe as-prepared nanoparticles. An unusual MT behavior was reported inFe-33.0wt%Ni nanoparticles prepared by sol-gel&reduction method, ofwhich the Mstemperature was apparently higher than the bulk alloy of thesame composition. To investigate and analyze this abnormal phenomenonwill be a compliment of the matensitic nucleation mechanism.In this dissertation, Fe-Ni nanoparticles were prepared by Sol-Gel&Reduction method and XRD, TEM, MPMS were used to investigate themicrostructure and the MT behaviors of the specimens. The Fe-Ninanoparticles were detected to be a compostion of fcc phase and some bccphase. And the bcc phase is considered to be the main reason why the Mstemperature is abnormal high in the nanoparticles.The bcc phase formed because of preparation is defined to benon-martensitic α-phase, which is distinct from the bcc phase formed inMT. The non-martensitic α-phase is considered to be formed in thereduction process of the Sol-Gel&Reduction method. Evolution of thenon-martensitic α-phase was investigated in different reduction process.It’s revealed that the non-martensitic α-phase is ascribed to be formed asan intermediate meta-stable phase during reduction of the iron-nickel oxides because of the lower energy barrier between the oxides and the bccphase, though fcc phase is thermodynamically stable. And thenon-martensitic α-phase has a uniform distribution in the fcc parent phase.The impact of the non-martensitic α-phase on consequent MT behaviorsin Fe-Ni nanoparticles was revealed. It’s suggested that the presence of thenon-martensitic α-phase could dramatically decrease the energy barrier formartensitic nucleation and lead to the promotion of the MT behavior.Conbining with the classical matensitic nucleation mechanism， theunderlying mechanism were further discussed.