α- and β-Mn precipitates in the spinodal Fe₃₀Ni₂₀Mn₂₅Al₂₅ alloy

Abstract

Precipitates in the spinodal alloy Fe₃₀Ni₂₀Mn₂₅Al₂₅, produced by drop-casting followed by hot extrusion, have been investigated using transmission electron microscopy. The as-extruded microstructure consisted of alternating, coherent body-centred cubic and B2 (ordered body-centred cubic) rods aligned along ⟨100⟩ directions with a wavelength of ∼60 nm. Upon annealing at 550°C, precipitates were formed having two distinct morphologies: relatively spherical precipitates with the α-Mn crystal structure and more elongated precipitates with the β-Mn crystal structure. Both precipitates existed simultaneously after annealing at this temperature for up to at least 99 h. The compositions of the precipitates and the orientation relationships between the precipitates and the spinodal matrix were determined.

Acknowledgements

Special thanks are given to Dr R.D. Noebe of the NASA–Glenn Research Centre in Cleveland, OH for casting the Fe₃₀Ni₂₀Mn₂₅Al₂₅ ingot and to Dr B. Bewlay of the General Electric Global Research Centre in Niskayuna, NY for extruding the alloy. The author would also like to thank James Hanna for his insightful discussions concerning certain diffraction patterns, which helped identify the α-Mn phase. This research was supported by the National Science Foundation's Graduate Research Fellowship Program, National Institute of Standards and Technologies Grant 60NANB200120 and National Science Foundation Grant DMR 0552380.