Investigation On Surface Modification Of AZ91D Magnesium Alloy By Severe Deformation And Rare Earth Diffusion At Low Temperature

Magnesium and its alloys have a lot of good characteristics, such as low density, good damping property, high specific strength, high specific stiffness, excellent heat conductivity and so on. And magnesium and its alloys have been used for a number of applications, including aviation and space industries, transportation system and communication electronic industries. Magnesium alloys are acknowledged as the 21 st century green engineering material. However, its anticorrosion property is poor, therefore, it has practical significance to widespread its applications by surface treatment technique.Firstly, the AZ91 D magnesium alloy was surface nanocrystalled by surface mechanical attrition treatment via changing balls diameter and handling time. The microstructure was characterized by optical microscopy(OM), X-ray diffraction(XRD) and transmission electronic microscopy(TEM). In the mean time, surface deformation grain refinement mechanism was investgated. Then, molten salt diffusion was used by changing diffusion time and temperature of the treated AZ91 D magnesium alloy with Ce、Nd、Y. The microstructure was characterized by scanning electron microscope(SEM).The phase was analysised by energy dispersive sectrometer(EDS) and X-ray diffraction(XRD). The anticorrosion property was tested in the electrochemical workstation by immersing the alloys into the 3.5wt% Na Cl solutions. The following results were achieved,1) The grains of AZ91 D magnesium alloys can be refined by surface mechanical attrition treatment. Grains become smaller with balls diameter or handling time increasing.2) When balls diameter is 16 mm and handling time is 180 min, the surface mechanical attrition treatment has optimal results, and achieve thicker deformed layer in depth, and surface grain size can reach 50 nm.3) Twinning and dislocation slip are the grain refinement mechanism of AZ91 D magnesium alloy during surface mechanical attrition treatment. In the low strain plastic deformation zone, the major deformation pattern is twinning under short peening time. However, dislocations slipping and snarling appeared as the handling time extending, and with the stress and strain increasing.Then more slip system entrained, which conformation more dislocation cells and more subgrains.The ultrafine grains are formed after the dynamic recrystallization of subgrains.4) After severe plastic deformation, AZ91 D magnesium alloy were treated by surface molten salt diffusion treatment with Ce, Nd, Y at low temperature, and the anticorrosion property of AZ91 D magnesium alloy was improved.5) A rare-earth diffusion layer formed on AZ91 D magnesium alloy at the appropriate diffusion condition. And α-Mg, Mg17Al12, and Al2 RE or Al RE3 are the main phase components of the diffusion layer.6) The microstructure and properties of AZ91 D magnesium alloy were treated by rare-earth diffusion at low temperature. In the case of same diffusion temperature, the permeation layer becomes thicker with the diffusion time increasing, and the permeation layer’s continuity and compactness improve. However, too long diffusion time cann’t improve the layer’s thickness significantly. Even worse, it decreses the compactness of the diffusion layer, which in fact will ruin the substrate’s overall performance.In the case of same diffusion time, the permeation layer becomes thicker with the diffusion temperature increase. What’s more, the temperature affects more prominent than time. And too high temperature may entrain cracks and decrese the substrate’s completion.It is practicable to form a consecutive and compact diffusion layer with rare-earth by diffusion process control. The optimum process of Ce diffusion is 420℃×2.5h. The optimum process of Ce-Nd diffusion is 440℃×2.5h. And the optimum process of Ce-Nd-Y diffusion is440℃×2.5h.