Sciences (b) Effect of the Thickness of the Specimen on the Formation of GP Zones in an A1-4 wt%Zn Allov ( a ) and BY A. SAKAKIBARA ( a ) , M. YAMADA ( a ) , T. KANADANI ( b ) , and M. OHTA (a) It was found by hardness test that age-hardening in Al-Zn alloys was somewhat suppressed near the surface, and the result was thought to be due to interruption of GP zone formation caused by depletion of vacancies annihilating