Impact of dispersoids’ distribution on portevin-le-chatelier effect and surface quality in Al–Mg-Sc-Zr alloys

Abstract

The 5xxx-series Al–Mg alloys are susceptible to plastic instabilities visible as stress serrations during uniaxial tensile testing and known as the Portevin Le Chatelier (PLC) effect. This effect also results in stretcher strain markings on the surface of the deformed sheet which limits the application of 5xxx-series Al–Mg alloys in the automotive industry. Sc and Zr have recently been shown to efficiently suppress the plastic instabilities in Al–Mg alloys due to the formation of Al3(Sc,Zr) precipitates that affect the dynamics of dislocations pinning and unpinning and reduces the severity of PLC. The impact of the size distribution of Al3(Sc,Zr) on PLC and related surface quality is currently unknown. In this work, we investigate the PLC and surface quality of an Al–Mg-Sc-Zr alloy and an Al–Mg alloy free of Sc and Zr. The size and distribution of the Al3(Sc,Zr) dispersoids are varied by subjecting the alloys to different homogenization treatments. The PLC behaviour is quantitatively examined with tensile tests and surface roughness of deformed samples is assessed with optical profilometry. In general, the presence of dispersoids resulted in a decreased magnitude and number of stress drops compared to the Sc- and Zr-free Al–Mg alloys. The lowest PLC severity and optimal surface quality was obtained for dispersoids with an average size of 6–7 nm. This optimal size range is rationalized by discussing the combined impact of dispersoids and Mg solute on the waiting time between dislocation pinning and unpinning.