Aging Precipitation Behavior Of Al-Zn-Mg-Cu Alloy And Its Effects On Mechanical Properties

In this paper,a series of thermal deformations and heat treatments are carried out on the self-melting Al-Zn-Mg-Cu alloy.Small angle X-ray scattering(SAXS),transmission electron microscope(TEM)and X-ray diffraction(XRD)are employed.The radius,radius distribution,volume fraction,varieties of precipitate and changes of Vickers hardness are studied quantitatively with the temperature and time of isothermal aging.JMA equation is upgraded by using the relative volume fraction of precipitates.In addition,a new aging strengthening model is established by considering various precipitation strengthening mechanisms.The main conclusions are shown as follow:1)The changes of precipitate and hardness are studied quantitatively in the isothermal aging process,when the temperature is 80,120 and 160? respectively.The results show that the radius of precipitate increases with aging time,and the volume fraction of precipitate increases and then becomes constant.The higher aging temperature is,the faster solute atom diffusion rate is,the stronger driving force for precipitate growth is,and the faster volume fraction reaches maximum value.Vickers hardness increases first and then decreases as the aging time extension.Hardness value is affected by size,volume fraction and transition of precipitate.Transition of ?' phase to ? phase is the decisive factor leading to a drop in hardness.2)JMA equation is upgraded based on concentration field and stress field.The modified JMA equation can reflect the change rules of precipitate relative volume fraction efficiently,which increases continuously and then becomes stable with the aging time and accelerates with the aging temperature increasing.3)The volume fractions of the GP zone,?' phase and ? phase are calculated quantitatively in each state,and the isothermal aging strengthening model is established based on varieties of precipitates transition and strengthening mechanisms.The volume fractions of GP zone and ?' both show a rapid rise and then a rapid decrease.However,the volume fractions of ? presents a change pattern of first slow,then fast and final slow rise starting at 0.The mean deviation of the yield strength just is 20 MPa between the prediction by strengthening model and the result by hardness conversion,which indicates that the model achieves accurate prediction of yield strength.