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 treatmen t at 200 to 500 degrees 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 % alpha AlMnSi at equilibrium respectively. Microstructure a s-spun ranged from primary icosahedral phase nucleating radial cellular alp ha Al arrays to less regular duplex arrays of alpha Al and alpha AlMnSi wit h decreasing alloy content and decreased section thickness or reduced dista nce from the chill surface. Heat treatment in the range 200 to 500 degrees C transformed any icosahedral phase present to alpha AlMnSi along with sphe roidization and coarsening/coalescence of alpha AlMnSi, to produce isolated spheroids when volume fraction f was lower and very stable interlinked cha ins at higher f. Measured coarsening rates of alpha AlMnSi 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 Hal l-Fetch and matrix solid solution hardening as-spun supplanted by particle- radius dependent Orowan combined with matrix Hall-Fetch hardening for the e volution of hardness during prior long term heat treatment at 425 degrees C . (C) 1999 Kluwer Academic Publishers.