Structure and properties of rapidly solidified Al rich Al-Mn-Si alloys : Part I : Melt spun ribbons

The evolution of microstructure as-spun and during subsequent heat treatment at 200 to 500°C for up to 1000 h has been studied for Al-6.3 Mn-3.3 Si, Al-8.3 Mn-3.7 Si and Al-14.5 Mn-5.8 Si (wt %) alloys, containing 17, 26 and 48 vol % αAlMnSi at equilibrium respectively. Microstructure as-spun ranged from primary icosahedral phase nucleating radial cellular αAl arrays to less regular duplex arrays of αAl and αAlMnSi with decreasing alloy content and decreased section thickness or reduced distance from the chill surface. Heat treatment in the range 200 to 500°C transformed any icosahedral phase present to αAlMnSi along with spheroidization and coarsening/coalescence of αAlMnSi, to produce isolated spheroids when volume fraction f was lower and very stable interlinked chains at higher f. Measured coarsening rates of αAlMnSi were a factor of 10 below predictions of LSW theory at lower f but were within a factor of 2 of prediction at highest f. Hardness was governed by a combination of Hall-Petch and matrix solid solution hardening as-spun supplanted by particle-radius dependent Orowan combined with matrix Hall-Petch hardening for the evolution of hardness during prior long term heat treatment at 425°C.