Pre-strain-dependent creep ageing behavior of an Al Cu alloy

Abstract

Pre-strain treatment is widely used in combination with many forming technologies to achieve better formability and higher performance. The pre-strain-dependency of the creep age behavior was systematically investigated in this paper. When pre-straining is introduced, the ageing strengthening of the AlCu alloy is significantly enhanced, which saturates when pre-strain amount exceeds 9%. The formability is also greatly improved due to the multiplication of the creep strain, along with an alteration in creep mechanism from diffusion creep to dislocation creep. Applying stress was still found to be detrimental to the ageing response but the degree is much smaller. Stress exceeding the yield strength was found to generate the same effect with certain amount of pre-strain. The microstructure including dislocations and precipitates was characterized with X-ray diffractometer, scanning/transmission electron microscopy and differential scanning calorimetry to explain the macroscopic evolution of the studied alloy. The typical bi-modal distribution of precipitates, where θ' heterogeneously grows around dislocation and θ" homogenously disperses in the matrix, was detected due to the pre-strain-induced massive dislocation. Increasing the pre-strain amount leads to a larger proportion of θ’ and a higher strength, which also saturates when pre-strain amount exceeds 9%. The present study revealed the relationship between the macroscopic behavior and the microstructural evolution in a large range of pre-strain amount and stress level, which provides valuable guidance in the development of the creep age forming technology.