MICROSTRUCTURE AND PROPERTY DEVELOPMENT IN LOW-DENSITY RAPIDLY SOLIDIFIED AL-LI ALLOYS

The development of microstructure and properties during the consolidat ion and heat treatment of low density rapidly solidified Al-Li alloys is discussed in this paper. The effects of Zr and Mn as ternary additi ons to a binary Al-3.5Li base are investigated, the additions being ma de in an attempt to improve the strength and fracture toughness of the base alloy. Microstructural evolution was closely monitored and the m echanical properties have been related to the changes in microstructur e. Three rapidly solidified inert gas atomised powder alloys were prep ared, namely, Al-3.5Li, Al-3.5Li-0.3Zr, and Al-3.5Li-0.7Mn. These allo ys were consolidated via cold compaction followed by hot extrusion. Mi crostructural development from the powder through consolidation to the final heat treated product was investigated using a combination of X- ray diffraction, differential scanning calorimetry, and transmission e lectron microscopy. Tensile properties and fracture toughness were eva luated in the as extruded and heat treated conditions. It was found th at by careful selection of the solution treatment conditions a desirab le distribution of cross-slip promoting hard particles could be achiev ed in both the transition element containing alloys. These dispersoids were identified as being the metastable ZrAl3 and stable MnAl6 phases . The addition of transition elements was found to enhance the yield s trengths of the alloy with increases up to 20% exhibited in the undera ged condition. With respect to the fracture toughness of the materials , the Mn additions had the most pronounced effect, producing an improv ement of almost 100% in this property. (C) 1993 The Institute of Mater ials. Manuscript received 23 June 1992. At the time the work was carri ed out the authors were in the Department of Materials, Imperial Colle ge of Science, Technology and Medicine, London. Professor Sheppard is now a private consultant, Poole, Dorset.