1144 Materials Science and Engineering, A134 ( 1991 ) 1144-1147 The microstructure and properties of high pressure gas-atomized AI-Li-Hf alloy powders A. F. Norman and P. Tsakiropoulos Department of Materials Science and Engineering, University of Surrey, Guild]brd, Surrey GU2 5XH (U.K.) Abstract Powders of the Al-2.2Li-1.23Hf alloy were prepared by high pressure gas atomization and the sub- 45 ¢¢m size fraction was consolidated by extrusion. Continuous and discontinuous precipitation of the a' (A13LixHf j x) phase occurred during solution treatment at 450 °C. The d' phase precipitated during ageing at 190°C either at the a'-matrix interface or as fine 6' particles after prolonged ageing. Peak strength was reached after 1 h at 190 °C. The contributions of the various strengthening mechanisms have been calculated. 1. Introduction Binary A1-Li alloys offer high specific prop- erties.but suffer from low ductility and toughness due to slip localization and work softening on planes containing the sheared d' precipitates [1]. Property improvements have been achieved by alloying additions which co-precipitate with d' or tie up lithium in complex precipitates [2-5]. In this work resistance to precipitate shear has been increased by altering the 6' precipitate morphol- ogy and composition through additions of haf- nium. The paper discusses the microstructures and tensile properties of the extruded powders of an AI-Li-Hf alloy which has been subjected to solution and ageing treatments. 2. Experimental The Al-2.2wt.%Li-1.23wt.%Hf alloy was atomized by high pressure gas atomization using helium as the atomizing gas at 3.5 MPa static gas pressure and a ratio of metal to gas mass fluxes of 0.82. The sub-45/~m size fraction of the powder was sieved in an argon atmosphere, canned, degassed at 350°C for 2 h and extruded at 350°C with an extrusion ratio of 25:1 using a ram speed of 20 mm min-1. Tensile specimens were encapsulated under argon in silica tubes to minimize oxidation during heat treatment. Vickers hardness was measured with a 5 kg load. Specimens for electron microscopy of the extruded powders were prepared in a solution of 30 vol.% nitric acid in methanol at - 30 °C using a potential of 20 V. 3. Results The gas atomized powders were spherical in shape with an oxide thickness of 2 nm [6] and had a mass median diameter 10.7/~m. The powders had a cellular solidification microstructure with both lithium and hafnium retained in solid solu- tion upon atomization. In the extrusions the grain size was 0.7/~m and primary (AI3Hf-DO23) and/ or metastable phases (L 12-A1 Hf and 3 L12-AI3Li ) were not detected. There was no evidence of oxide stringers along the prior par- ticle boundaries. The tensile properties in the as- extruded condition are given in Table 1. The density of the as-extruded material was 2.46 g cm -3, a 12% reduction over the density of conventional aluminium alloys. TABLE 1 Tensile properties for the extruded powders after solution treatment at 450 °C for 2 h and ageing at 190 °C Property As Time (h) extruded 1 10 100 1000 o4(MPa) 295 416 372 349 341 UTS (MPa) 343 460 433 403 405 %El 7.5 4.9 3.3 4.7 2.8 0921-5093/91/$3.50 © Elsevier Sequoia/Printed in The Netherlands