Abstract
The solid solubility of Hf in Al has been extended to 2wt%Hf (0.3at%Hf) by chill casting into a wedge shaped copper mould. This is also the limit of solubility of Hf in sub-45mum size high pressure gas atomised powders. Primary precipitation of the metastable Li2-Al3Hf phase in the form of cuboid or petal like particles occurred upon exceeding the solubility limit. It is suggested that in the Al-Hf system the solubility limit is controlled by absolute stability rather than solute trapping. The cooling rate required to suppress the nucleation of the metastable and equilibrium Li2-Al3Hf phases increased with alloy composition. The metastable phase grew from a Hf rich centre along the directions and was responsible for grain size refinement from 480mum to about 4mum. Hafnium increased the lattice parameter of alpha-Al at a rate of 0.003A/wt%Hf and the micro-hardness of the solid solution by 167.75MPa/wt%Hf (17.1kgmm-2/wt%Hf). Clustering of Hf in clusters <10A diameter is envisaged as a possible mechanism for the strength of the solid solutions. Two modes of decomposition of Al-Hf solid solutions have been identified; continuous precipitation of fine spherical Li2-Al3Hf precipitates and discontinuous precipitation of rod/filament like precipitates of the same phase. Precipitate spacings in the range 0.16mum-0.3mum are required to account for the peak hardness increments of 225.6-382.6MPa (23-39kgmm-2) for Al-(0.32-1.04)wt%Hf alloys aged at 400°C for up to 1000h and are in reasonable agreement with the observed spacings of 0. 03mum-0. 5mum. Powders of the Al-1.6wt%Hf alloy were prepared by gas atomisation and the sub-45mum powder size fraction was consolidated by extrusion. The as extruded grain size was fine (~1mum) and the Hf was retained in solid solution. Continuous and discontinuous precipitation of spherical (1nm to 30nm) and filamentary (aspect ratio ~25, diameter <0.1mum) type particles respectively of the Li2-Al3Hf phase occurred after 10h ageing at 300°C and 400°C. Precipitation of this phase was accompanied by an increase in the tensile strength, yield strength and hardness. The equilibrium Al3Hf phase did not precipitate after 1000h ageing at both temperatures. The contributions of grain size, solid solution strengthening and of the mechanisms for precipitate shearing and precipitate looping by moving dislocations have been calculated and related to the alloy microstructure. The fine grain size contributed about 60% and 50% towards the strength of the alloy in the as extruded and aged conditions respectively. After 100h at both ageing temperatures precipitate looping was the dominant mechanism and the solid solution contribution to strength was diminished. Discontinuous precipitation contributed about 20% towards the strength of the alloy. The experimental relationship ass=0. 15Hv relates the micro-hardness of solid solutions of Al-Hf alloys to the solid solution strengthening contribution to the yield strength of the alloys. Powders of the Al-2.25wt%Li alloy reached peak strength after 10h at 190°C. This is attributed to the precipitation of the delta (Al3Li) phase. The PM Al-0.91Li-0.63Hf, M-1.96Ll-l.0Hf, Al-2.2Ll-l.23Hf and Al-2. 64Li-1. 6Hf (concentration in wt%) alloys have been studied. Precipitation in the Al-2.2Li-1.23Hf and Al-2.64Li-l, 6Hf alloys was controlled by a two step heat treatment. During solution treatment at 450°C the alpha-(Hf)-Al3 (Li, Hf) phase formed by continuous and discontinuous precipitation. During subsequent ageing at 190°C the delta phase formed in the alpha-Al matrix and at the alpha'/matrix interface and grew giving complete shells around both the spherical and filamentary alpha'(Hf) precipitates. Properties have been measured in the solution treated condition and after ageing. 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% 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 prior to heat treatments is attributed to precursor phase/s to delta'.