Role of hydrogen embrittlement on resistance to crack extension of 2024 aluminium alloy for different artificial aging conditions under short corrosion exposure times

Abstract

The effect of precipitation sequence for various heat treatments simulating natural aging and subsequent corrosion exposure on cracking resistance (fracture toughness) of AA2024-T3 is investigated. Artificial aging heat treatment of fracture toughness specimens was investigated for three different temperatures and for all possible heat treatment conditions, including under-, peak- and over-aged tempers. Fracture toughness Kcr decreases continuously with isothermal aging up till peak-aged condition, showing the maximum Kcr decrease of approximate 36% for the lowest investigated temperature. A small recovery was noticed for the over-aged condition. The short, 2 h exfoliation corrosion exposure time, was selected to corrode the pre-heat-treated specimens and introduce hydrogen. Surface pits, intergranular corrosion and associated secondary cracking were noticed for some aging conditions and a pick-up of diffusible hydrogen was observed in all cases. The highest fracture toughness decrease (− 25%) due to corrosion exposure was noticed for the under-aged condition that was attributed to the GPB zones formation. The isothermal aging temperature proved to be extremely influential on the corrosion-induced fracture toughness decrease, with the lowest investigated temperatures associated with the strongest embrittlement. The highest Kcr decrease per investigated aging temperature was noticed at the under-aged condition and for all aging temperatures, namely −18% for the 170 °C, −16% for the 190 °C and − 12% for the 210 °C, respectively. The lowest fracture toughness decrease due to corrosion exposure was noticed for the peak-aged specimens, where S΄΄ precipitates, fully coherent with the matrix, are formed. For these conditions, surface pits, intergranular corrosion and associated secondary cracking were not prevalent. The effect of secondary cracks and pits on the primary crack tip stress field in fracture toughness tests, is explored and shown to be crucial in understanding the Kcr – embrittlement of microstructures susceptible to corrosion pits and intergranular corrosion. Corrosion exposure on extreme over-aged specimens plays a non-important role on the fracture toughness decrease. Diffusible hydrogen contents and spectra were determined for the various aged conditions. Bulk hydrogen levels were found to be relatively low, and no correlations were observed between hydrogen and fracture toughness reductions.