Insight into the complex ternary phase behavior in Al-Mn-Ce alloys

The subtle change in the space group that occurs in the Al12-xMnxCe τ phase as the Al/Mn ratio is varied in the Al-Mn-Ce system was characterized using scanning electron microscopy and analytical transmission electron microscopy and the atom positions associated with this change were refined using x-ray diffraction. In addition, the reaction temperatures were identified using differential scanning calorimetry. Understanding the nature of this phase is of particular importance because of its presence in a series of high strength, high temperature alloys that are being pursued in this system. It is shown that there are two distinct space groups for the τ phase that are related to one another and depend on the Al/Mn ratio – at lower ratio, the space group is I4//mmm but at higher Al/Mn ratio, the Mn and Al atoms order on one of the Wyckoff sites in the I4/mmm structure and the ordering plus slight distortions away from the symmetry positions causes a loss of the body centering and a change to the P4/nmm space group with a unit cell approximately twice that of the I4/mmm cell. The results provide clarification to the conflicting reports in the literature and lead to a clearer assessment of the phase equilibria in the Al-rich portion of the Al-Mn-Ce system. [Display omitted] •The structure of two Al-rich phases, Al10Mn2Ce and Al8Mn4Ce, were determined.•The composition and temperature range in which they exist was identified.•An ordering reaction between these phases was found in a narrow composition range.•The structure changes gradually with composition even before the ordering reaction.