Investigation of the impacts of artificial ageing, re-ageing and cryogenic cycles on multiplex second phase precipitation and life-time prediction in 7075 alloy

In this study, artificial ageing, re-ageing and multiple levels of cryogenic treatments were practiced on 7075 aluminium alloy. Microstructural changes in the material as a result of the final cryogenic cycles were characterized by SEM and TEM, and the resulting phases with successive upsurge stages of nucleation sites were observed. Throughout crystallographic and X-ray analyses; dislocation densities, strain values of the planes, texture coefficient and plane orientations were determined. Micro-hardness measurements were also carried out to mechanically support the data attained from these characterization methods. According to the results, dislocation density and hardness values particularly after −40 °C cryogenic treatment, achieved the highest value. Heating rate and activation energy-based phase life time predictions against temperature, together with nucleation and growth rates of precipitates were evaluated by thermal analyses. Life time calculations pointed out that the −40 °C was the critical point of sub-zero progression where, the longest useful life expectancy at temperatures of between 80 °C and 300 °C could be reached out meticulously.