LAUSR

Abstract 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.