For thick plate and forgings of alloy 7075, the overaged T73 temper is sometimes chosen over T6, which gives the highest strength. Which is the main reason?

Answer: C. The strength is 10 to 15% lower than T6, but coarse, discontinuous grain-boundary precipitates raise resistance to stress corrosion cracking

The 7000 series, containing Zn, Mg and Cu, are the strongest aluminum alloys, but in the T6 condition, when tensile stress acts through the thickness of a thick part in a humid environment, stress corrosion cracking along grain boundaries readily occurs. T73 is an overaged temper in which aging is carried further than T6, at a higher temperature or for a longer time; the tensile and yield strengths are about 10 to 15% lower than T6, but the grain-boundary precipitates become coarse and discontinuous, so cracks along grain boundaries advance less easily. Overaging coarsens the precipitates, so the option in which they become finer and denser and raise the strength is backward. T7 indicates overaging; it does not skip aging, and finishing by natural aging alone corresponds to T4. Tempers such as T76 and T74, with other balances of strength and corrosion resistance, are also used.