Observing the D03 phase in Fe–Mn–Al alloys

Two Fe–Mn–Al alloys with compositions of Fe–23.0 wt.% Mn–7.4 wt.% Al–0.03 wt.% C (alloy A) and Fe–28.5 wt.% Mn–8.8 wt.% Al–0.09 wt.% C (alloy B) were studied throughout the quenching and aging process. After the specimens were heated at 1050 °C for 1 h, and then quenched in room temperature water, both alloys have a small amount of austenite distributed randomly along the ferrite grain boundary. In the TEM study, alloy B shows D03 precipitates exist within the ferrite grains while alloy A does not. Samples in alloy B were heated at 1050 °C for 1 h, and then quenched directly into a salt bath at lower temperatures. The upper-limit temperature for the existence of the D03 phase in the ferrite matrix for alloy B is between 500 and 510 °C. After quenching alloy A from 1050 °C and aging at temperatures below 430 °C, we observed that the D03 phase precipitated homogeneously inside the ferrite grains. This result is consistent with the classical nucleation and growth mechanism of D03 precipitates growing within the ferrite matrix. The upper limit temperature for the existence of the D03 phase in the ferrite grains of alloy A is between 430 and 440 °C. Measuring the Al contents in the ferrite grains of these alloys, we found that the 14.5 at.% Al in the ferrite grains of alloy A is, so far, the lowest Al content yielding the possible existence of a D03 phase in both binary Fe–Al and ternary Fe–Mn–Al alloys.