Hydrogen-induced embrittlement of fine-grained twin-roll cast AZ31 in distilled water and NaCl solutions

Twin-roll cast magnesium has a fine-grained microstructure and exhibits relatively high strength and ductility. However, hydrogen-induced embrittlement in magnesium alloys can limit the mechanical properties substantially and can lead to sudden and unpredictable failure. In this study, embrittlement is determined by means of slow strain rate tests (SSRT) and instrumental indentation tests (IIT). The latter enables analysis of embrittlement under the suppression of crack propagation mechanisms and surface influences (e.g. notch effects due to pitting) and is a new technique for the investigation of hydrogen embrittlement (HE). In this study, the influence of immersion time and elastic deformation on the HE of twin-roll cast and heat-treated magnesium sheets (AZ31) was analysed. Distilled water and two NaCl solutions (0.05 and 0.5 wt%) served as analysing media. It is shown that both influencing factors significantly affect the rate and the degree of HE. Relatively short immersion in distilled water led to a substantial decrease in strain and an increase in brittleness, even without external mechanical loading. In contrast, strength was not affected by immersion in water and the resulting HE. In comparison to immersion in distilled water, immersion in sodium chloride solution led to a total loss of plasticity and a decrease in strength during SSRT without a distinct increase in embrittlement during IIT. Analysis by X-ray diffractometry did not determine a detectable amount of magnesium hydrides in the embrittled AZ31 structure.