Microstructure and Hardness of 7055 Aluminum Alloy Processed by Circular Cryogenic-Aging Treatment

7055 aluminum alloy was processed by circular cryogenic-aging treatments for four times. Some characters such as precipitates, dislocation density, crystal structure and micro hardness were studied in detail. It is showed that the sizes of precipitates are decreased from several hundred nanometers to 100nm with the increase of cycle index. The main components are AlZn or AlZnMg phases. The hardness test result illustrates that the micro hardness arrives at 232HV after circular cryogenic-aging treatment for twice. The corresponding precipitates exhibit high density and complex sizes. The sizes are in the range of 50-200nm.