The structure and martensitic transformation of nano-sized particles in Fe-Ni films

The structure and martensitic transformation of the particles in Fe1-xNix (0 less than or equal to x less than or equal to60 at %) granular films prepared by magnetron sputtering were investigated by TEM and TEM dynamic observations respectively, TEM observation shows that the particle size in the as-sputtered state is less than 10nm, but the structure depends on the Ni content, i.e., bcc martensite is stable when 0 less than or equal to x less than or equal to 36 at % and fec austenite becomes stable when x=60 at %, while both bee and fec can co-exist when 36 at % less than or equal to x< 60 at %. The existence of a single bcc structure with high Ni content in the present specimens is significantly different from that in the bulk specimen and also different from that in the specimen prepared by vacuum-deposition method. The structure evolution during the temperature change indicates that martensitic transformation of nano-sized particles is quite different from that of bulk alloys. When the specimens were heated Lip to 573 similar to 773K in TEM, the bcc-->fcc reverse transformation occurred. The reverse transformation temperatures are roughly estimated as the same as those of the corresponding bulk alloys. However, the martensitic transformation did not occur when the specimens were cooled down to room temperature, even to 77K, indicating that the size reduction of particles results in the stability of austenite. In view of the stability of nano-sized austenite particles, it is Suggested that the martensite in as-sputtered Fe-Ni films is not a product of martensitic transformation, but directly originates from the collision of atoms during the sputtering.