Characterization of plastic instability steps occurring in depth-sensing indentation tests

Plastic instabilities in solid solution Al-Mg and supersaturated Al-Zn-Mg-(Cu) alloys were investigated by depth-sensing microhardness tests. Experimental results show that in the case of Al-Mg alloys the characteristics of the plastic instability depend only on the solute concentration. In the case of age-hardenable Al-Zn-Mg-(Cu) alloys, however, although plastic instabilities were observed during indentation following the quenching, the development of the instability steps is strongly influenced by Guinier-Preston zone formation in the early stage of natural aging. After a certain time, t(i), of aging the steps disappear completely. The value of ti obtained for the Cu-containing alloy was found to be considerably longer than that obtained for the ternary Al-Zn-Mg alloy. This fact - together with the smaller initial rate of hardening in the Cu-containing alloy - indicates that the addition of Cu retards the early part of the decomposition process at room temperature in this alloy system. (C) 2002 Elsevier Science B.V. All rights reserved.