Crystal Analysis of Nonequilibrium δ_non-Phase in Al-Li-Mg Alloys

The nonequilibrium δnon-phase was originally investigated in the work[1] when studying the ageing processes of the 1424 alloy ( Al-Li-Mg-system). As shown in the work[2] this phase may also be formed during the cooling from the temperature of SSHT. The δnon-phase precipitates are also present in 1420 alloy of the same system[3]. The basic structure investigations were carried out on the 1424 alloy sheets aged in accordance with the regime: 125˚C, 32 hrs. The investigations were performed by TEM using the JEOL JEM 200CX microscope. The diffraction patterns from the δnon-phase on the crystallographic axes of zones close to <110>, <111>, <112> were obtained. It can be seen that the regular reflex networks appeared after the deviation from axes of zones by 3-4˚. It was established that the lattice parameters of δnon-phase and its orientation relation with the matrix can be approximately described by the following way: aδnon≈ aα/2[112] b δnon ≈a α/2[110] cδnon≈ a α [111] where aα- the period of the FCC- lattice of solid solution. The model of the crystallographic structure of the δnon-phase precipitates is proposed. On the basis of this model the mechanism of the δnon-phase formation is discussed. The late ageing stages are analyzed and it was shown that the δnon-phase particles are the nuclei for S1(Al2LiMg)- phase.