Al-Zn-Mg合金の焼入感受性および析出物の核形成に対する添加元素の影響

Abstract

Influences of additions of Li, Be, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ge, Zr, Mo, Ag, Cd, In, Sn, Sb, Pb and Bi less than 0.5 at% on the quench-sensitivity, or the loss of strength on slow quenching, of Al-2.5 at%Zn-2 at%Mg alloys were studied from the measurements of hardness and electrical resistivity and also electron microscope observations, and proposed models on the nucleation of precipitates to explain the mechanism are discussed with particular attention to the role of lattice defects. The results are as follows:(1) The quench-sensitivity of Al-Zn-Mg alloy is most markedly increased by the addition of Cr, followed by that of Mo, V, Zr, Mn (group a), Cd, Ag, Be, Cu (group b) and the others, in the order named. Microstructural evidence and change in electrical resistivity indicate that the effects of quench-sensitivity are attributable to the precipitation, during slow quenching, of solute in an incoherent form in which it could make no contribution to subsequent age-hardening.(2) The effect of group (a) can be reasonably explained in terms of the increase in crystal defects such as dislocations, sub-boundaries, grain boundaries and especially misfit surfaces around microscopic insoluble compounds formed by these additional elements which act as preferential nucleating sites for the precipitates M(MgZn2). On the other hand, the atoms of group (b) in supersaturated solution may form clusters which act as the heterogeneous nucleation center for the precipitates, although a large quantity of Cu in solution may also have the effect of reducing the solubility of Zn and Mg.