| High-performance aluminum alloys have been widely used in the field of food handling and chemical processing industries because of their attractive comprehensive properties such as low density, medium strength, high ductility and excellent corrosion resistance. However, in order to broaden the application in automotive and aerospace industries, alloys with higher mechanical strength are required. An effective approach to improve the strength of aluminum alloys is the addition of scandium as an alloying element.In the current paper, Al-Mg-Sc-(Zr) alloys containing different presents of Sc and Zr were prepared using ingot metallurgy processing. The effect of Sc and/or Zr on the microstructures and mechanical properties of Al-Mg were investigated based on optical microscopy, electron back-scatter diffraction, transmission electron microscopy and properties testing. In addition, the morphologies, inner substructure and spatial distribution of primary particles formed in as-cast Al-Mg-Sc and Al-Mg-Sc-Zr alloys have been investigated. The existing forms and acting mechanisms of Sc and/or Zr in Al-5Mg alloy were discussed in detail. The results are as follows:(1) In Al-5Mg-Sc alloy, the addition of 0.2 wt.% Sc has not resulted in grain refinement significantly. However, a remarkable grain refinement effect was observed for Sc content greater than 0.4 wt.% Sc, changing the typical dendritic microstructure into fine equiaxed grains and decreasing on the average grain size from 259.1 μm to 77.2μm. A further reduction in the average grain size (50.3μm) with increasing the Sc content (0.6 wt.%), thus leading to a greater benefit in grain refining. When the content of Sc reaches 0.2 wt.%,0.4 wt.% and 0.6 wt.%, the tensile strength of the Al-Mg alloy has been increased by 4%,30% and 40% respectively; the yield strength has been increased by 18%,78% and 111% respectively; however the elongation has been decreased by 12%,33% and 44% respectively. The attractive comprehensive property of Al-5Mg-Sc alloy was obtained while the Sc content reaches 0.4 wt.%, that the tensile strength, yield strength and elongation of the Al-5Mg-0.4Sc alloy were 280.2 MPa, 153.6 MPa and 14.8%, respectively.(2) When Sc additions are greater than the eutectic composition (0.4 wt.%), minor Sc mainly exists in the form of primary Al3Sc particles. Due to the similar crystal structure (αAl=0.4050nm, αAl3Sc=0.4106nm) and the consistency in orientation between the matrix and the particle, these Al3Sc primary particles located at the grains inside and grain boundaries nearby have two effects of hindering the undesirable diffusion and subsequent crystal growth and behaving as heterogeneous nucleation cites for α-Al matrix, which reduce the grain size and refine the microstructure. However, not all of the particles will necessarily act as nucleants for the matrix phase. On the other hand, the Sc resides exclusively in the a-Al matrix and leads to the formation of supersatration solid solution while the Sc content below the eutectic composition. The nanoscaled coherent Al3Sc dispersoids that precipitated from supersaturated matrix during thermo-mechanical processing have a much effective in pinning grain and subgrain boundaries to inhibit recrystallization grain growth, thus resulting in intense substructure strengthening and dispervise strengthening.(3) Primary particles formed in Al-Mg-Sc alloy nucleate heterogeneously on oxides within the melt and subsequent grow to the formation of cubic shaped ones with a cellular-dendritic substructure probably formed by{100} faceted primary arms growing in (111) directions with (110)and (100) side branches. Finally, these primary particles due to the solute diffusion and microsegregation exhibit an ’Al3Sc+α-Al+Al3Sc…’ multilayer structure.(4) A calculating method, based on EBSD results, was introduced for the spatial distribution of these particles in matrix. By calculating, the three habit planes of the observed square, rectangle and triangle particles that show an intersection plane close to (100), (110) and (111) respectively, are close to (3,2,1), (2,3,0), (1,1,4), (2,0,3), (0,1,0), (3,0,2) and (3,1,4), (3,1,2), (0,3,1) respectively. The results show that primary particles can be deemed to randomly distribute in the matrix, rather than simple {100},{110}or{111} planes.(5) The addition of 0.2 wt.% Sc and 0.1wt.% Zr imposed a remarkable grain refining effect onAl-5Mg alloy for as-cast condition. This is a consequence of the primary particles due to the the similar crystal structure and the consistent orientation act as heterogeneous nucleants for α-Al matrix. In addition, these primary particles found in Al-5Mg-0.2Sc-0.1Zr alloy were believed to be the eutectics consisted of Al3(Sc,Zr) and α-Al phases, and could be represented as an " Al3(Sc,Zr)+α-Al+Al3(Sc,Zr) " multilayer structure, with a "cellular-dendritic" mode of growth. The strengthening effect of the addition of Sc and Zr to Al-5Mg alloy is much more effective that that of Sc additions. |
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