Formation of a new phase after high-temperature annealing and air cooling of an Fe-Mn-Al alloy

Abstract

The observation of a new phase precipitated in the bcc matrix of a Fe-Mn-Al alloy heated at 1573 K and air cooled to room temperature is reported. The composition of the alloy was Fe-24.1 wt pct Mn-7.6 wt pct Al-0.03 wt pct C. These precipitates had a morphology of Widmanstatten side plates distributed uniformly in bcc grains, and some precipitates resided along the bcc grain boundaries. From transmission electron microscopy (TEM) and X-ray analyses, the crystal structure of the new phase was identified as a simple cubic (SC) Bravais lattice and was related to an ordered fcc phase. The new phase is similar to the L12 crystal structure. The orientation relationships between the SC Widmanstatten side plate and the ferrite matrix are [011]SC//[111]bcc and $$(1\bar 11)$$ SC// $$(10\bar 1)$$ bcc, which correspond to the well-know Kurdjumov-Sachs (K-S) orientation relationship. The formation of SC annealing twins from the ferrite phase at the initial stage of the nucleation during air cooling was observed for the first time in the Fe-Mn-Al alloys. It is noted that both twin grains grow within the ferrite matrix and maintain both the K-S orientation relationship with the parent ferrite phase and the twinning orientation relationship between them.