Austenite decomposition in Fe-Mn-Al-C alloys

The austenite decomposition of Fe-Mn-AI-C alloys with manganese contents up to 10 wt.% is studied by means of differential thermal analysis (DTA), optical microscopy and transmission electron microscopy. Both continuous cooling and isothermal heat treatments are investigated. At temperatures between 1000 and 1200°C isothermal heat treatments show that alloys with aluminium contents below 10 wt.% present a duplex structure of ferrite and austenite. A subsequent austenite decomposition into a mixture of ferrite and carbide, (Fe,Mn) ₃AIC_x (K-carbide), with a perovskite-type structure follows on cooling during non-isothermal treatments with the progression of this decomposition depending on the manganese to aluminium ratio. Microstructural analysis by optical and electron microscopy indicates that the decomposition of the austenite proceeds by the formation of a ring of K-carbide surrounding untransformed austenite that on further cooling decomposes into a mixture of phases containing the K-carbide with two different morphologies. A mottled contrast microstructure with very fine domains of austenite and K-carbide and a lamellar mixture of ferrite and K-carbide are observed in DTA samples cooled from the melt.