Influence of Guinier-Preston zone formation on plastic instabilities in depth sensing indentation tests

Guinier-Preston zone formation and its effect on plastic instabilities in supersaturated Al-Cu and Al-Zn-Mg-(Cu) alloys were investigated by tensile tests and depth sensing microhardness measurements. It has been shown that plastic instabilities occur in depth sensing microhardness tests performed on supersaturated solid solutions immediately after quenching. In these measurements the load-depth (F-h) indentation curves are not smoothly changing functions, but contain characteristic steps indicating the oscillation of dynamic microhardness. Experimental results show that the occurrence of the instability steps depends on the composition of the alloy. Furthermore, the formation of GP zones suppresses the instabilities, therefore the plastic instabilities disappear at a certain state of decomposition. In this paper some characteristics of the dynamic Vickers indentation instabilities and the effect of GP zone formation on these parameters are investigated. An interpretation is given for the occurrence of instability steps from the aspect that, this phenomenon is a form of Portevin-Le Chatelier type plastic instabilities which is associated with a negative strain rate sensitivity. Results of indentation tests are compared with those of constant loading rate tensile tests.