Influence Of Al And Zn Contents On The Microstructure And Mechanical Properties Of Extruded And As-rolled Mg-Al-Zn Alloys

Magnesium alloys are lightweight structural materials of current interest in transportation applications, because of specific strength and stiffness, excellent damping properties, etc., compared with the existing structural alloys. However, Mg alloys possess limited room temperature ductility and formability due to the hcp crystal structure. At present, AZ and AM magnesium alloys, which were the most widely used commercial magnesium alloys, have good strength and machining properties as well as good casting performance. However, its plasticity around room temperature is not desirable, which restricts their further applications.Non-basal slip or twinning systems have to be activated to satisfy the geometrical requirement of five independent deformation components for homogeneous deformation without crack formation. Now, one approach to activate non-basal slip is to reduce the c/a ratio of Mg through alloying. With an increase in Al or Zn contents, the unstable stacking fault energy(γus) continuously decreases. what’ s more, more slip positions with a low γus emerge in the Mg matrix for the slip systems. The probabilities to slip increase with an increase in Al or Zn contents, which improve the plastic performance in Mg–Al–Zn alloys at given compositions. So, influence of Al and Zn contents on the microstructure and mechanical properties of extruded and as rolled Mg-Al-Zn alloys was investigated.The main research content of this thesis are the composition optimizing of extruded and as rolled Mg-Al-Zn system magnesium alloys, and the conclusions are as follows:(1) The results show that the tensile strength of the extruded Mg-Al-Zn system magnesium alloys enhances with the increase of Al or Zn content. Which was attributed to the increasing volume fraction of secondary phases, grain refinement and solid solution strengthening. While, the plasticity of the extruded Mg-Al-Zn magnesium alloys becomes bad with the increase of Al content because of residual course eutectic phase. We have been optimized the composition optimization of extruded Mg-Al-Zn magnesium alloys, the extruded Mg-9wt.% Al-1.2wt.% Zn alloys exhibited high ultimate tensile strengths and tensile strains, which were 370 MPa and 18.6%, respectively.(2) The optimum heat treatment process is 275 oC 1.5 h for the as–extruded Mg-8wt.% Al-1wt. % Zn alloys. In comparing extruded Mg-8wt.% Al-1wt.% Zn alloys, the ultimate tensile strength of the extruded Mg-8wt.% Al-1wt.% Zn alloys with 275 oC 1.5 h was increased from 317 MPa to 347 MPa, the yield Strength was increased from 121 MPa to 163 MPa. What’s more, elongation-to-failure basically remain unchanged.(3) We investigated four rolled AZ91 alloys with different pretreatment, the results show that pretreatment process have more influence on Mg17Al12 phase than grain sizes. The optimum pretreatment process is 300℃1h+425℃3h. The ultimate tensile strength and elongation-to-failure of as-rolled Mg-9wt.% Al-1wt.% Zn alloys were 364 MPa and 15%, respectively.(4) The composition of as rolled Mg-Al-Zn magnesium alloys was optimized, the as rolled AZ81 and AZ91 alloys exhibited high ultimate tensile strengths and tensile strains, which were 349 MPa and 19% as well as 364 MPa and 15%, respectively. The ultimate tensile strength of as rolled Mg-9wt% Al-1.2wt% Zn alloys was 382 MPa. The elongation–to–failure of rolled AZ81 alloy was 120.7%.