STRENGTHENING MECHANISMS IN POWDER-METALLURGY AL-LI-HF ALLOYS

The microstructure and mechanical properties of powder metallurgy Al-2.2Li-1.23Hf and Al-2.64Li-1.6Hf (wt-%) alloys have been determined. Precipitation was controlled by a two step heat treatment. During solution treatment at 723 K the alpha'(Hf)-Al3 (Li,Hf) phase formed by continuous and discontinuous precipitation. During subsequent aging at 463 K the delta' phase precipitated in the alpha-Al matrix and at the alpha'/matrix interface and grew forming complete shells around both the spherical and filamentary alpha'(Hf). Properties were measured in the solution treated condition and after aging. Considerations of the various strengthening mechanisms suggest that solid solution and Orowan strengthening are the two major contributors towards the strength of the alloys. Grain size strengthening accounts for only 20 to 30% of the strength of the peak aged alloys while solid solution strengthening is diminished in the peak aged condition. The high level of solid solution strengthening before heat treatments is attributed to precursor phase(s) to delta'. (C) 1993 The Institute of Materials. Manuscript received February 1992; in final form 16 September 1992. The authors are in the Department of Materials Science and Engineering, University of Surrey, Guildford.