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

Abstract

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. • In this paper, many abnormal phenomena were observed in the Fe Mn alloys. The lattice thermal expansion behaviors and internal stress evolution in FeMn alloys was investigated by in situ synchrotron X-ray diffraction. The correlation between low thermal expansion property and premartensitic transformation were revealed by comparing the similar abnormal phenomena during Invar effect and the premartensitic transformation. The highlights are as follows: • During the Invar effect of Fe 27Mn alloy and preceding the martensitic transformation of Fe 24Mn alloy, the coefficient of thermal expansion decreased and the FWHM of diffraction peaks of austenite increased abnormally. The evolution of thermal lattice strain and FWHM was reversible and isotropic. • Transmission electron microscopy observations confirmed that there are massive martensite embryos embedded in austenite phase during Invar effect and premartensitic transformation. The existence of martensite embryos caused the abnormal increase of FWHM and may be related to the low thermal expansion behavior of FeMn alloy. • These similar phenomena imply a close relationship between the mechanism of Invar effect and the premartensitic transformation in FeMn alloy. • Our results may help to reveal the relationship between Invar effect and premartensitic transformation in antiferromagnetic Invar alloys, and lead to an in-depth understanding of the mechanism of low expansion property.