Martensitic Transformation In Fe-Ni Nano-particles Prepared By A Sol-gel Process

Fe-Ni alloy has the typical martensitic phase transformation of face-centered structure austenite fcc (γphase) to body-centered structure martensite bcc (α-phase). The size effect on MT behavior in Fe-Ni and other alloys drew a lot of attention during past decades. It is well-know that decreasing the particle size (in micrometer-scale) will cause a suppression of the MT. But in the case of nano-structured alloy, the effect of particle size on MT behavior is still controversial.Previous works suggested that the MT in nanoparticles might be a very complicated problem and should be studied in connection not only with the particle size but also with the microstructure correlated with different preparation methods.In this paper, the Fe-29.5wt%Ni, Fe-33.0wt%Ni and Fe-35.9wt%Ni nanoparticles with narrow size distribution were prepared via a sol-gel process followed by a reduction treatment. The MT (martensitic transformation) of Fe-Ni nanoparticles was studied by Magnetic Property Measurement System (MPMS) and X-Ray Diffraction (XRD). It was found that a considerable amount of low temperature martensite in Fe-29.5wt %Ni and a small amount of martensite in Fe-33.0wt.%Ni nanoparticles were detected by XRD at room temperature, while martensite was not detected in the Fe-35.9wt.%Ni nanoparticles, showing apparently compositional dependence of MT in Fe-Ni nanoparticles. Moreover, the Ms (defined as turning point of the increasing rate of magnetic susceptibility) of Fe-33wt.%Ni nanoparticles with an average particle size of 19.4nm was tested as high as 246K by both MPMS and XRD, which was nearly 100K higher than the Ms of counterpart Fe-33wt.%Ni bulk specimen. The possible reasons for high Ms of nanoparticles were discussed.Furthermore, an isothermal martensitic transformation was observed in Fe-33wt%Ni nano-particles after an incubation time of less than 1000s at 255K and without obvious incubation time at 77K.