A new type of FCT martensite phase in single-crystalline Fe3Pt Invar alloy

Martensitic transformation in a highly ordered Fe3Pt has been investigated by magnetization and X-ray diffraction measurements. We confirmed that a new type of face-centered tetragonal (FCT) martensite phase appears below 60 K in Fe3Pt with a degree of order S = 0.88. The tetragonality c/a gradually increases with decreasing temperature, and is approximately 1.005 at 10 K. This is in contrast to the tetragonality c/a < 1 generally observed in the FCT martensite with a degree of order less than 0.8. The spontaneous magnetization increases in association with the transformation.

• TEM observations at room temperature reveal that the specimen is a single phase of L12-type structure.
• The spontaneous magnetization and magnetic susceptibility curves of Fe3Pt with S = 0.88 have bend points at 60 K.
• XRD measurements reveal that this alloy exhibits a martensitic transformation to FCT and the transformation temperature is 60 K.
• The tetragonality c/a is larger than unity in the FCT martensite.
• This tetragonality is in contrast to the tetragonality c/a < 1generally observed in the FCT martensite with S < 0.8.