An in-situ study of low thermal expansion and internal stress evolution in FeMn alloys

The mechanism of low thermal expansion phenomenon in antiferromagnetic FeMn alloys needs to be revealed. In this work, the evolution of lattice thermal expansion and internal stress in FeMn alloys was investigated by in-situ synchrotron X-ray diffraction during thermal cycling. Preceding the martensitic transformation, the coefficient of lattice thermal expansion (CLTE) decreased and the full width at half maximum (FWHM) of the diffraction peaks broadened abnormally. The evolution of the lattice thermal expansion and the FWHM were both reversible and isotropic. The additional reflection spots around the fundamental diffraction spots in the electron diffraction patterns of austenite and the dispersed nano-scale martensite embryos in austenite observed in the dark field TEM images proves the existence of premartensitic transformation. Similar phenomena were found in FeMn Invar alloy, which indicate that the Invar effect is closely related to the internal stress evolution caused by premartensitic transformation.