Scripta METALLURGICA Vol. 23, pp. 983-988, 1989 Pergamon Press plc Printed in the U.S.A. All rights reserved A STUDYOF AI-Li ALLOYSUSING SMALL ANGLE NEUTRONSCATTERING B.C. Pike, S. Messoloras and R.J. Stewart J.j. Thomson Physical Laboratory, University of Reading, Whiteknights, P.O. Box 220, Reading, RG6 2AF, U.K. (Received December 27, 1988) (Revised April S, 1989) Introduction Aluminium lithium alloys promise a weight saving potential in aircraft structures (1,2,3,4). The density of aluminium is reduced by about 3% for each weight per cent addition of lithium, while the Young's modulus is increased by about 6% (5). The strengthening of the AI-Li alloys in the range 20-200°C is associated with the metastable 6' precipitates. The 6' phase is a superlattice with L12-type structure (6) and composition Al3Li. Because of the low misfit the precipitates have a spherical morphology. The coherent 6' solvus has been determined by x-ray small angle scattering (7). No extensive precipitation in alloys containing less than 7.3at% Li has been observed (8). At high temperatures the 6' phase transforms to an incoherent AILi phase (5) the so called 6 phase. The binary aluminium lithium alloys suffer a pronounced loss of ductility and toughness during ageing to a high strength (g). The addition of Mg leads to a marginal improvement in fracture resistance but leads to detrimental Al2MgLi grain boundary precipitates (10). The addition of copper improves the fracture toughness. Zirconium when added to AI-Li system inhibits recrystallization and results in higher yield stress. Different multicomponent AI-Li alloys have been developed commercially in order to improve toughness and ductility (11,12). The aim of the research described in this paper was to follow the growth of the 6' precipitates as a function of ageing time at different temperatures in bulk samples and examine the effect of small additions of Cu or Zr on the precipitation. Samples of two binary alloys, a ternary AI-Li-Zr alloy and two commercial alloys with different Cu contents were investigated. The method of small angle neutron scattering (SANS) was employed, since SANS can provide structural information on the 1-200nm scale from a bulk specimen (4 Icnhl). The addition of Zr or a higher amount of Cu resulted in smaller 6' precipitates. The growth kinetics of the 6' precipitates in all the alloys studied can be described by the theory of coarsening. All the precipitate sizes determined for the different ageing times and temperatures are parametrized on a single linear plot for all the alloys studied. From the growth of the 6' precipitates the energy of diffusion and interfacial tension are determined. Experimental Procedure In this work two binary alloys with Li content 3.78at% (alloy BI) and 9.74at% (alloy B2) were examined. In addition a ternary alloy containing zirconium (AI-9.07at%Li-O.O4at%Zr, alloy TI) and two commercial alloys (CI and C2) supplied by RAE were measured (Table I ) . The commercial alloys were in sheet form and of about Imm thick, while the remaining three were in plate form 25mm thick. Alloy specimens of 20x20 mm and Imm thick were cut. All the samples were solution treated in air using a vertical furnace for either 15min at 540°C (C2) or 15min at 530°C for the rest, followed by gravity quenching into iced water. Then the samples were aged for different times. Mainly two ageing temperatures were used 150 and 185°C. Three of the five alloys (CI, C2 and BI) were extensively studied for different times at these two temperatures (lmin to 235h at 150°C and Imin to 77h at 185°C). In addition the alloy CI was treated for Ih in the temperature range 200-360°C. For ageing times between Imin to lh an oil-bath furnace was used to ensure rapid heating to the required temperature. For longer ageing times a conventional tube furnace was 983 0036-9748/89 $3.00 + .00 Copyright (c) 1989 Pergamon Press plc