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

The development of microstructure and properties during the consolidation and heat treatment of low density rapidly solidified Al-Li alloys is discussed in this paper. The effects of Zr and Mn as ternary additions to a binary Al-3.5Li base are investigated, the additions being made in an attempt to improve the strength and fracture toughness of the base alloy. Microstructural evolution was closely monitored and the mechanical properties have been related to the changes in microstructure. Three rapidly solidified inert gas atomised powder alloys were prepared, namely, Al-3.5Li, Al-3.5Li-0.3Zr, and Al-3.5Li-0.7Mn. These alloys were consolidated via cold compaction followed by hot extrusion. Microstructural 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 electron microscopy. Tensile properties and fracture toughness were evaluated in the as extruded and heat treated conditions. It was found that by careful selection of the solution treatment conditions a desirable distribution of cross-slip promoting hard particles could be achieved 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 strengths of the alloy with increases up to 20% exhibited in the underaged condition. With respect to the fracture toughness of the materials, the Mn additions had the most pronounced effect, producing an improvement of almost 100% in this property. (C) 1993 The Institute of Materials. Manuscript received 23 June 1992. At the time the work was carried out the authors were in the Department of Materials, Imperial College of Science, Technology and Medicine, London. Professor Sheppard is now a private consultant, Poole, Dorset.