Phase stability in a powder-processed Al–Mn–Ce alloy

There has been considerable interest in Al-rich Al–Mn–Ce alloys due to the variety of crystalline and quasi-crystalline metastable phases that can be formed. Here we report a study of the effects of heat treatment on an Al–5Mn–2Ce (at.%) alloy processed by gas atomization and consolidated by warm extrusion. Characterization using X-ray diffraction and electron microscopy showed that the powder microstructure consists mainly of an amorphous phase, FCC Al, and a previously unreported phase, Al₂₀Mn₂Ce. The extrudate is fully devitrified and contains a mixture of FCC Al, Al₂₀Mn₂Ce, and Al₆Mn, with a small amount of Al₁₂Mn and Al₁₁Ce₃. Upon heat-treatment at up to 450 °C, the Al₂₀Mn₂Ce and Al₆Mn phases decompose to give a hard stable phase mixture with 72–73 % Al₁₂Mn plus 13–14 % each of Al₁₁Ce₃ and FCC Al. Heat treatments at 500 °C give a much softer phase mixture consisting of 60 % FCC Al, 22 % of an unknown Al₃(Mn,Ce) phase, 9 % Al₁₂Mn, 8 % Al₆Mn, and 1 % Al₁₁Ce₃. The formation of large volume fractions of Al₁₂Mn for heat-treatments at up to 450 °C suggests that the presence of Ce may stabilize this phase, and that more dilute Al–Mn–Ce compositions could form the basis for new high-strength, low-density Al-based alloys with enhanced elevated temperature properties.