Pre-precipitation in AlMgGe and AlMgSi

Abstract

Single crystals of high-purity aluminium-base alloys (Al-0.7% Mg 2Si, Al-1.0% Mg 2Ge and Al-1.4% Mg 2Ge) have been studied by a crystal monochromated X-ray technique. The pre-precipitation stage of these alloys has been investigated in detail during isothermal ageing at 135, 150, 200, 250 and 300°C. Diffuse X-ray scattering observed during the early ageing period shows that the needle-shaped G.P. zones are probably formed in at least two steps. When first detected these segregates do not seem to possess any internal periodicity whatever. X-ray scattering effects are also observed which seem to indicate that these primitive zones possess a high vacancy concentration. The second part of the process is associated with the establishment of a linear periodicity along the segregate axis which lies parallel to one of the matrix cube directions. The periodicity is identical to that of the matrix—4.04 Å—along this direction. During this stage, the vacancy concentration of the primitive zones abruptly decreases. Hardness measurements of single crystal specimens of these alloys isothermally aged at the same temperatures as above reveal that the G.P. zones are the only hardening agent.