Effect of cryogenic pre-deformation on the stress relaxation response and mechanical/corrosion properties in Al–Zn–Mg–Cu alloy

Abstract

The stress relaxation ageing behavior and mechanical/corrosion properties of the alloys pre-treated by conventional T4 temper without pre-deformation and cryogenic pre-deformation with different levels (20%–80%) have been investigated and compared by hardness test, stress relaxation ageing test, tensile test, intergranular corrosion test and microstructure characterization. Pre-deformation by cryo-rolling could increase the steady-rate relaxation rate at 120 °C by 35%–360% and effectively prolong the duration of the variable-rate relaxation stage compared with the T4 treatment. The dislocation movement facilitated by the recovery is responsible for the significant increase in the stress relaxation rate. Increasing the pre-deformation level markedly improves the strength and localized corrosion resistance but lowers the elongation in the stress relaxation aged alloy. Cryogenic pre-deformation promotes the heterogeneous nucleation of relatively large η2-type phases at dislocations apart from the uniform formation of fine η2-type phases in the matrix. High density dislocations retain after stress relaxation ageing at the investigated temperature. The dislocation hardening could offset the softening due to the precipitate coarsening and leads to a higher strength in the alloy pre-deformed by 50–80%. The modified grain boundary precipitation consisting of narrower precipitation-free zones and discontinuous grain boundary precipitates contribute to the better corrosion resistance in the cryo-rolled alloy.

Keywords

• Stress relaxation ageing
• Cryo-rolling
• Precipitation
• Dislocation hardening
• Al–Zn–Mg–Cu alloy