The phase decomposition, magnetic and mechanical properties in Fe-30Mn-xC (x=0-6.21 at.%) alloy system have been investigated using optical microscope, X-ray diffractometer, vibrating sample magnetometer and universal testing machine.
In the Fe-30Mn alloy, single γ phase is found in solution treated state, but martensite is strain-induced in polished and mechanical tested specimen. The maximum elongation and the increase of the tensile and yield stress with temperature decreasing are observed, which imply that Fe-30Mn alloy has TRIP (Transformation Induced Plasticity) property. Concurrently, Md of the strain induced martensite is marked between 443 and 453 K. The lattice parameter increases linearly as $a_γ = 0.3608+0.00088×(at.%C)$ in nm unit and the Neel temperature $T_N$ decreases linearly from 420 to 287K as the carbon content increases from x=0 to 6.21 at.% in water-quenched γ Fe-30Mn-xC alloys. In this study, $T_N$ decrease is discussed due to the increasing of the effective atomic radius of γ-phase by the carbon addition, or due to the electron transfer conception if carbon donates electrons to the d-bands of transition metals.
The increase of the effective atomic radius decreases the exchange interaction in the Bethe-Slater curve. Upon ageing the carbon containing γ-phase alloys at the temperatures between 803 and 853K, cementite phase with $(Fe, Mn)_3C$ stoichiometry precipitates. It is observed that morphologies of cementite are different according to the composition and temperature. In the low carbon supersaturated condition, cementite precipitates as the allotriomorph in the grain boundary. That phase has the Widmanstatten or lath-like morphology in the relatively high carbon supersaturated condition.
