Precipitation of β-Mn in the Form of Widmanstätten Side-Plates in the Ferrite Matrix of an Fe–28.6 Mn–10.9 Al Alloy Steel

The microstructural evolution and phase stability in Fe–Mn–Al alloys play a decisive role in determining their mechanical performance and potential applications. This study investigates the precipitation behavior and crystallography of the β-Mn phase in an Fe–28.6Mn–10.9Al (wt.%) alloy subjected to annealing at 1100 °C, followed by water quenching and subsequent isothermal holding at temperatures between 500 °C and 900 °C for 20 h. Microstructural analysis using X-ray diffraction, optical and electron microscopy revealed a single body-centered cubic (BCC) ferritic matrix above 850 °C and the formation of β-Mn precipitates with Widmanstätten side-plate morphology at lower temperatures. The β-Mn phase was thermally stable between ~500 °C and 850 °C, with the volume fraction increasing with temperature and reaching a maximum near 650 °C. The β-Mn precipitates coarsened progressively with increasing temperature and were found to be richer in Mn than the surrounding Fe-rich BCC matrix. Crystallographic analysis established an orientation relationship (OR) of (<inline-formula>
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              <mml:semantics>
                <mml:mrow>
                  <mml:mn>02</mml:mn>
                  <mml:mover accent="true">
                    <mml:mrow>
                      <mml:mn>1</mml:mn>
                    </mml:mrow>
                    <mml:mo>&#xAF;</mml:mo>
                  </mml:mover>
                </mml:mrow>
              </mml:semantics>
            </mml:math>
          </inline-formula>)_β // (100)_α and [ ]_β // [012]_α, where // denotes nearly parallel alignment, signifying a semi-coherent interface between the two structures. These findings clarify β-Mn precipitation, its interfacial relationship with ferrite, and its thermal stability in high-Mn Fe–Mn–Al alloys, offering guidance for microstructural design in next-generation lightweight steels.