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Non-destructive investigation on the effect of precipitation hardening on impact toughness of 7020 Al-Zn-Mg alloy

The aim of the study is to investigate the effect of precipitation hardening on the impact toughness of Al-Zn-Mg alloys, and to search for the relationship between sound velocity, electrical conductivity, and impact toughness in order to provide a rapid and non-destructive measurement of toughness. The specimens of 7020 alloy were solution treated at 723 K for 1 h and quenched, then, following a 96 h natural ageing at room temperature, artificially aged at 413 K for various times. For each specimen group, Charpy impact and hardness tests were performed, sound velocity and electrical conductivity was measured at room temperature. The results show that impact toughness decreases with increasing ageing time for the conditions from under-aged to peak aged, and the lowest toughness was obtained for the sample having highest hardness. The reason of the toughness reduction is thought to be the formation of metastable eta' precipitate during ageing. Improvement in hardness and yield strength by precipitation result in a reduction in the impact energy since less plastic work is done before the strain in the plastic zone is sufficient to fracture the test specimen. A correlation between impact toughness, hardness, sound velocity, and electrical conductivity of the aged specimens has been established.