| 7xxx series (AlZnMg) aluminum alloys are among the age-hardening aluminum alloy, widelyused as structural materials in transportation, and other military or civil sectors. A homemadeAl-4.6Zn-0.9Mg alloy with thermal treatments samples and mechanical property was used in thepresent study, analyzed by scanning transmission electron microscopy (STEM), energy dispersiveX-ray (EDX) analysis and X-ray diffraction.According to the experiment, the hardness distribution of Al-4.6Zn-0.9Mg (extruded shape) andAl-4.6Zn-0.9Mg (plate) welding joint is made, and the relation of the fatigue and tension propertywith microstructure is analyzed. The fatigue mechanism and the affecting factors of Al-4.6Zn-0.9Mgis investigated intensively.It was found that the Al-4.6Zn-0.9Mg (plate) alloy is occupied by equilibrium phase η-MgZn2.The fuse zone have lots of dislocation but without notable precipitation..Al-4.6Zn-0.9Mg (extrudedshape) alloy is identified by precipitation hardening. Fracture surface of the fatigue sample appearsdifferent topographyby the stress.After extensive analysis, it has revealed that stress concentration isthe keyreason which leads to the sample fatigue failure in a high stress，meanwhile，the particle whichcontains Fe, Si and Mn elements, usually become the site for the crack propogation.The precipitation behaviors in a triple-step-aged AlZnMg alloy were studied in detail，usingscanning transmission electron microscopy (STEM), energy dispersive X-ray (EDX) analysis andX-ray diffraction in association with thermal treatments and hardness measurements. It is shown thatfor one-step ageing higher hardness values can be achieved if the alloy is aged at100℃rather than120℃. The retrogression, i.e., the second step of the triple-step-ageing process, plays a crucial role indetermining the precipitate microstructure of the alloy. It is demonstrated that if the retrogressiontemperature is at the range between300℃and400℃nearly all the precipitates formed in the first-stepageing will be dissolved and further more will lead to more severe boundary segregation of soluteatoms in the next step of re-ageing. However, the precipitate dissolution only destroys the precipitatestructure and dissolves some of the solute atoms of the original precipitates. The remained soluteatoms can form the solute cluster that would not re-transform back to original precipitate structures.As such the number of effective nuclei of the hardening precipitates during re-ageing will decrease,but they can grow (or coarsen) rapidly. Upon re-ageing, leading to a triple-step-aged alloy with lowstrengh. |
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