u2033 wall thickness and aged by a 2 step practice of 8 hr at 250 f plus 4 hr at 305 f had yield and tensile strengths of 100 9 ksi and 102 6 ksi respectively similarly the subject alloy can be over aged beyond peak strength in a second step at temperatures in the 310 360 f temperature range for 2 to 24 hours to provide a desirable combination of strength and corrosion resistance another preferred embodiment includes a final aging treatment in a third step at a lower temperature in the range 175 300 f for 1 to 30 hours which provides an additional strength benefit with no loss in corrosion properties as yet another alternative the alloy can be subjected only to the aforementioned second and third aging steps by skipping the first step to compare the invention alloy with other commercial high zinc alloys such as aa7036 aa7056 and aa7449 which have higher mg cu ratios in the range 1 0 to 1 4 the following alloys were prepared as described in example 1 fig4 compares the toughness levels of these alloys on the basis of mg cu ratio with the invention alloys using those compositions that have similar strength levels 93 95 ksi and total mg cu contents 4 0 4 2 as noted earlier it is important that undissolved second phases do not remain after processing so that fracture toughness can be.