On the low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing

Abstract

The low cycle fatigue behaviour of an Al-Zn-Mg-Cu alloy processed via non-isothermal ageing (NIA) was examined at different strain amplitudes. We showed that NIA improved the low cycle fatigue life (more than 7000 cycles) by optimising the precipitate configuration within 5.5 h while maintaining comparable mechanical properties (570 MPa for tensile strength) and conductivity (nearly 39% IACS) to conventional isothermal ageing, simultaneously. Experimental observation combined with molecular dynamic simulation revealed that precipitation configuration manipulated by NIA had a crucial effect on fatigue resistance. A great number of repeatedly sheared and locally destructed GP zones enhanced co-planar slip and slip localisation in the under-aged alloy during the early stage of NIA, responsible for the dramatic displacement steps on the surface and resultant poor fatigue performance. As the NIA further proceeded, moderately coarsened precipitates with an average dimension of 6.0 nm and elevated number density effectively impeded the dislocation movement and weaken the slip localisation to a great extent, improving the fatigue performance within a few hours.