Ductility loss of 7075 aluminum alloys affected by interaction of hydrogen, fatigue deformation, and strain rate

The ductility loss phenomenon of the 7075 alloys affected by the interaction of the hydrogen, pre-fatigue deformation, and strain rate was elucidated. In order to investigate the tensile properties of pre-fatigued material in varying humidity environment, quasi-static and impact tensile tests on pre-fatigued materials were conducted, which were prepared in environments with different humidities. The flow stress obtained by tensile test was not affected by pre-fatigue deformation with varying humidity and frequency, regardless of the strain rate. In contrast, pre-fatigue deformation resulted in a loss of ductility. The ductility loss phenomenon, owing to pre-fatigue deformation, was most significant for a combination of long pre-fatigue time, high humidity, and low strain rate. Therefore, the microstructure of the materials was observed in order to determine the main factor. The microstructure of pre-fatigued materials using TEM indicated that dislocations accumulated in some regions inside the crystal grains near the Al7Cu2Fe (local dislocation field). Thus, the ductility loss phenomenon is believed to occur as this local dislocation field develops during tensile deformation, thereby promoting the formation and growth of voids, and in turn promoting ductile fractures. According to the hydrogen-enhanced localized plasticity theory, accumulated hydrogen promotes the formation and multiplication of dislocations near Al7Cu2Fe, and increases the density of the local dislocation field, which in turn results in the progression of the ductility loss. Furthermore, the hydrogen transport mechanism owing to dislocations operates at low strain rates; thus, hydrogen affects the dislocation activities during tensile deformation. This results in a faster density increase of the local dislocation field, which in turn promotes a loss of ductility. This phenomenon can therefore be described as a type of hydrogen-induced ductility loss.