Study of the Correlation between Premartensitic Transformation and Invar Effect in Femn Alloys by Synchrotron High-Energy X-Ray Diffraction

The correlation between Invar effect and premartensitic transformation in antiferromagnetic FeMn alloys was investigated by comparing the microstructure evolution and thermal expansion behaviors of the Fe-27Mn and Fe-24Mn (at.%) alloys. The thermal expansion behaviors were studied by thermal dilatometer. The full width at half maximum (FWHM) evolution of the diffraction peaks during thermal cycling was studied by synchrotron high-energy X-ray diffraction. The microstructures of these alloys were characterized by transmission electron microscopy (TEM). The results show that the Fe-27Mn alloy exhibited Invar effect with a coefficient of thermal expansion (CTE) of 8.8×10-6/°C between -100 °C and 170 °C. The Fe-24Mn alloy underwent martensitic transformation at 75 °C following the premartensitic transformation that started at about 130 °C. The Fe-24Mn alloy showed a low CTE of 11×10-6/°C during premartensitic transformation, which is close to that of the Fe-27Mn alloy. X-ray results show that the FWHM of the austenite diffraction peaks broadened upon cooling in Fe-27Mn alloy. During the premartensitic transformation of Fe-24Mn alloy, the FWHM of the austenite diffraction peaks also had a strong temperature dependence, which is similar to that of Fe-27Mn alloy. TEM observations of these two alloys show that the diffraction patterns of the austenite contained some diffuse additional reflection spots around the fundamental reflections. Dispersed nano-scale martensite embryos were observed in the austenite. Therefore, the mechanism of Invar effect in FeMn alloy is considered to be closely related to the premartensitic transformation.