Orientation dependence of Portevin–Le Châtelier plastic instabilities in depth-sensing microindentation

Abstract

Plastic instabilities were investigated in an Al–Zn–Mg–Cu alloy by depth-sensing microhardness testing in Vickers geometry. The alloy investigated showed strong age hardening as a consequence of Guiner–Preston zone formation at room temperature. The orientation dependence of the Portevin–Le Chátelier (PLC) effect was investigated by microindentation tests in differently oriented grains. If the direction of the indentation was close to the 〈100〉 crystal axis and the diagonal of the Vickers indenter coincides with the 〈110〉 crystal direction, the PLC effect was more pronounced. Under these conditions the instabilities could be observed even after 5 h of natural aging, while the PLC effect disappeared in grains with other orientations after 2 h of aging. The orientation dependence of the indentation curves was observed up to the maximal measured imprint size (d ≈ 80 μm). It is suggested that the initialization of the PLC bands takes place in the close vicinity of indenter/sample contact surface. Considering only a uniaxial compressive stress component in the sample/indenter contact planes, in the vicinity of the indenter single sliplike and multiple sliplike conditions are attained depending on the orientation of the indenter relative to the sample. Changes of the slip conditions correlate with changes in the observation regime of instability which explains the orientation dependence.