Hydrogen Embrittlement in the overaged aluminium alloy 7040-T7651

Abstract

Aluminium alloys 7xxx series are widely used in structural components in the aerospace industry as they have high strength at low densities. However, it is well known that in moist-air environments the material fails at lower strengths, and the fracture surface displays macroscopic brittle features attributed to atomic hydrogen, this is known as hydrogen embrittlement. Although many attempts have been made in previous researches, the mechanisms under which atomic hydrogen weakens the material is still not completely understood. The objective of the present research is to characterize the effects of atomic hydrogen in the microstructure and to correlate surface observations to possible hydrogen embrittlement mechanisms which promote premature failure of aluminium alloys when they are exposed to humid air environments. For this objective, the commercial over-aged aluminium alloy AA7040-T7651, which is a new generation 7xxx series alloy, in the “as-received” conditions has been selected for its high strength associated with a high Zn content. After a careful surface preparation, and controlling the experimental conditions during the study to prevent Intergranular Type-1 EIC initiation followed by series of interrupted slow strain rate tensile tests at different humidity compositions (0%, 50% and 90%RH) and two strain rates (10-5 and 10-6 /s) to compare the embrittlement degree under different conditions (humidity composition and exposure times). The main techniques provided in-situ, and post-mortem high-resolution scanning electron images and EBSD maps that provided qualitative and quantitative data of the microstructure differences when the humidity levels were varied. It was found that atomic hydrogen: i) play an essential role in the nucleation and propagation of voids and cracks, ii) increased deformation and damage at grain boundaries, iii) produce changes in the morphology of voids and cracks diminishing the roundness and increasing the aspect ratio of features, and iv) fracture initiation sites were identified on the surface, which were associated with featureless grain-boundaries separation. Finally, it was concluded that a hybrid hydrogen embrittlement mechanism is responsible for the failure observed, in which the leading mechanism diffuses atomic hydrogen to grain boundaries, crack tips and zones of high hydrostatic stresses ahead of crack tips (HELP) causing featureless grain-boundaries separation by decohesion (HEDE) which act as initiation sites

Date of Award	31 Dec 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Timothy Burnett (Supervisor)