| AZ serial magnesium alloys as the most widely used commercial magnesium alloy system have favorable casting formability,wide range of application and low cost.However,some properties of AZ serial alloys are lower than that of aluminum alloys,especially for mechanical properties.Thus,improving the mechanical properties is meaningful for enlarging the application of AZ serial alloys.In this study,AZ80 alloy was selected to be study subject due to its excellent combination properties,and Nd,Gd elements were added as alloying elements.The researches about Al-RE phase morphology control have been carried out to improve the microstructure and mechanical properties of RE modified AZ80 alloys.At the beginning of this paper,the microstructure and mechanical properties evolution of AZ80 alloys with single addition of Nd or Gd have been investigated.The results revealed that the microstructure differences among RE-contained AZ80 alloys were the construction and morphology of Al-RE phases.For the Nd modified AZ80 alloys,Al-RE phases were successively in the form of needle-like Al11Nd3 phase and block-shaped Al2Nd phase with the increase of Nd.In Gd modified alloys,Al-RE phases were mainly in the form of block-shaped Al2Gd phase.The change of Al-RE morphology had great influence on the mechanical properties of the RE-contained alloys:the Al-RE phases with smaller size and dispersive distribution could contribute to improve the mechanical properties,and the block-shaped Al-RE phases were more effective in strength enhancement than the needle-like Al-RE phases.However,the block-shaped Al-RE phases easily became coarse with the increasing of RE.The coarse Al-RE phases damaged to strength and resulted in the decline of mechanical properties.Therefore,morphology control of Al-RE phases was important for the mechanical properties enhancement of RE-contained AZ80 alloys.In this study,new preparation technology and combination alloying were used to control the morphology of Al-RE phases.In the studies of new preparation technology,AZ80-3.0RE(RE=Nd or Gd)alloys with coarse Al-RE phases were considered as the representational research object to discuss the preparation technology of Al-RE phases morphology control.Firstly,the precipitation opportunity of Al-RE phases was discussed.The melt of AZ80-3.0RE at different conditions was frozen via sub-rapid solidification.The results indicated that the block-shaped Al-RE phases precipitated with the fusion of alloying materials.Based on this phenomenon,the assumption was put forward to control the morphology of Al-RE phase:the melt was applied stirring during alloying process.The Al-RE phases precipitated with the function of stirring power to suppress the stocking of Al,RE elements on Al-RE phases.The growth of Al-RE phases was impeded during this process.According to this assumption,a new magnesium preparation technology was put forward-melt mixing and stirring casting method.Adopting the idea of grouping alloy materials melting,Al and RE elements were segregated in two group of melt,and then melt stirring was applied during melt mixing to ensure the precipitation of Al-RE at the melt stirring.It was found that the melt mixing and stirring casting method could control the morphology of Al-RE phases effectively and impede the Al-RE phase become coarse,and the microstructure was improved obviously.The maximum tensile strength and elongation of the AZ80-3.0RE alloy is up to 245MPa and 10.8%,respectively.Compared with normal processed AZ80-3.0RE alloy,the tensile strength and elongation is increased by 34%and 50%.separately.However,at the present melting environment,the melt mixing and stirring casting method created much oxide inclusion in the sample,which caused instability of mechanical properties.Thus,another research approach is carried out,which is combined adding Nd and Gd in AZ80 alloy to control the Al-RE phase morphology.It is aimed to prepare the alloys with high strength and stable mechanical properties at normal melting process.In the AZ80 alloys with combined addition of Nd,Gd,needle-like and block-shaped Al-RE phases could be formed.With the change of Nd,Gd content,the needle-like Al-RE phase was suppressed as the block-shaped phase increased,and vice versa.According to structural analysis,the needle-like Al-RE phase was based on the Al11Nd3 crystal structure.Some of Nd in Al11Nd3 was substituted by Gd to form Al11Nd(Gd)3 phase.The block-shaped Al-RE phase was based on the Al2Gd or Al2Nd crystal structure,and the substituted phenomenon occur between Gd and Nd.The adjustment of Nd content could control the precipitation of needle-like Al-RE phase,and the adjustment of Gd content could control the precipitation of block-shaped Al-RE phase.Combined the suppression relationship between needle-like and block-shaped Al-RE phases,the amount and size of Al-RE phase could be controlled.According to above researches,the composition design scheme of Nd.Gd combined addition was proposed.The results indicated that in the experimental alloys.AZ80-1.2Gd-0.9Nd alloy exhibited the most excellent mechanical properties,in which the tensile strength and elongation were 219±5MPa and 9.1±0.5%,respectively.The average mechanical properties of the alloy,especially for the stability of mechanical properties,were much better than the alloys prepared by melt mixing and stirring casting method.It could be expected that integrated the approaches of melt mixing and stirring casting and combined addition of Nd,Gd could prepare the AZ80 alloys with higher strength and stability. |
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