| Served as lightweight material for heat treatment reinforcement,Al-Zn-Mg alloys rival the traditional steel when undergo certain preparation and heat treatment process in strength for 600MPa?800MPa,making a significant material in the field of defense equipment and civilian facilities.A great number of nano-sized precipitates formed in Al matrix contribute to the high strength.Under a condition of an externally applied load,the precipitates will restrict the motion of dislocation by cutting and bypass mechanism,which enhances the resistance of plastic deformation for lattice in a certain degree.Adjusting the content of main alloy elements(Zn and Mg included)in pure aluminum and optimizing the concrete aging process are viewed as effective method for improving alloy performance.Taking the fundamental and critical factor in consideration respectively,material workers lay focus on alloy composition,process flow and parameter regulation of artificial aging.To a certain extent,natural aging lacks of controllability and bears the limitation of characterization methods.In the meanwhile,there is insufficient systematic research for the impact of alloy composition on alloy precipitate behavior.In recent years,the successive emergence and development of advanced characterization techniques have made it possible to further explore problems described above.Based on the study of natural aging in 7N01 alloys,four self-made alloys with various Zn/Mg atoms are designed.The hardness curves of each alloy in different aging conditions were obtained by micro-vickers hardness tester.The corresponding precipitation phase concerned with the hardness characteristic points have been detected statistically by a series of analytical detection method,including 3DAP,HAADF-STEM,TEM,XRD,etc.The principle conclusions are as follows.(1)In natural aging and artificial aging of 120? and 150?,solid solution strengthening is the main mechanisms for alloys with Zn/Mg atomic ratio ?1.What's more,there is natural aging effect only when Zn/Mg atomic ratio ?1.The strengthening mechanism will transform to precipitation strengthening when Zn/Mg atomic ratio>1.47.(2)?' phase can be the major hardening phase in 3#alloys,but 4#alloys is ?p phase artificial aged at 150?/110h.With time going by in 7N01 alloy,the composition of the precipitation phase tends to be convergent in natural aging,and large size clusters possess of lower Zn/Mg atomic ratio.The equilibrium phase will be ? phase in 7N01 and 3#?4#alloys,there is no T phase detected in XRD analysis.(3)The decomposition path of the supersaturated solid solution in natural aging process of 1 year and 2 months can be summarized as:SSSS? Clusters(with a wide spread of Zn/Mg atomic ratios)? Chemistry adjustment of a long time ? GP?.zones(Zn/Mg?1.5),a special component adjustment behavior during the period of 1 month to 3 months related to the increase of Mg atom has been detected and unstable clusters will redissolve into A1 matrix under aging temperature above 120 ?.The metastable ? phase haven't been observed in this experiment.(4)If time is long enough of natural aging for the first stage in the NA+AA mode of two-stage aging,the precipitation sequence follows the regulation:SSSS ? Clusters(with a wide spread of Zn/Mg atomic ratios)? Chemistry adjustment of a long time ? GP?.zones(Zn/Mg?1.5)? ?' ? ?-MgZn2.In the AA+NA mode of two-stage aging,the incomplete precipitation process will trigger the follow-up natural aging. |
PDF Full Text Request