Orientation relationships of phase transformation in α-Al12Mn3Si pseudomorphs after plate-like Al6Mn precipitate in an AA3004 Al-Mn based alloy

The phase transformation in the plate-like precipitates of Al6Mn in an AA3004 Al-Mn based alloy, which was prepared by a chemical extraction technique, was investigated using transmission electron microscopy. The boiling-phenol method observed the entire range of possible shapes of the precipitates and the habit planes of the plate-like Al6Mn were 〈001〉 in the sheet plane and 〈110〉 on its edges. The two orientation relationships between Al6Mn and α-Al12Mn3Si after the phase transformation is observed to be 001¯6//023¯αand1306//53¯2α and 001¯6//15¯2αand2006//311αparticles1and2 (the suffixes 6 and α indicate zone axises or lattice planes of Al6Mn and α-Al12Mn3Si phases, respectively). The lattice misfits in these orientation relationships were observed to be − 1.4% and 1.0%, respectively. Additionally, the thickness of the plate-like Al6Mn was kept constant after transforming into the α-Al12Mn3Si phase. Further, we did not observe any low index planes of the α-Al12Mn3Si parallel to the (001) plane of the Al6Mn, which means that the outlines of α-Al12Mn3Si were constrained by the original shape of Al6Mn, i.e., the transformed α-Al12Mn3Si may be considered as a pseudomorph after Al6Mn.