Studies On The Mechanism Of Solid-state Diffusion-alloying On The Surface Of Magnesium And Its Alloy

In this paper, surface-alloying modification treatment was used to treat pure magnesium and its alloy(ZM5) through solid-state diffusion-alloying, resulting in alloyed layers on the surface of magnesium(Infiltrate is zinc powder) and its alloy(Infiltrate is zinc & aluminum powder). The formation mechanism of the alloyed layers and the interrelationship among various process conditions such as the diffusion temperature and time and so on were discussed, which offered evidences for the optimization of the process parameters of the surface-alloying modification treatment on magnesium and its alloy.The microstructure and morphology of the alloyed layers obtained in different conditions were observed through Optical Microscope(OM) and Scanning Electron Microscopy(SEM), and the chemical composition and structure characteristics were analyzed through Energy-dispersive Spectroscopy(EDS) and X-ray Diffraction(XRD). The alloying mechanism of the diffusion process in different process conditions was studied, and on the basis of the corresponding diffusion mathematical model, the effect of the diffusion temperature and time on the chemical composition , thickness , microstructure and morphology of the alloyed layers was analyzed, and methods of homogenizing the alloyed layers were brought forward for the further study.The results show that: (1) The chemical composition, phase structure, morphology and thickness of the surface diffusion layers varies with the different process conditions(mainly the diffusion temperature and time). The surface alloyed layers are composed of a unitary solid solution phase subzone at the beginning of the diffusion process, and then develops to a solid solution phase subzone + a reacting phase(intermetallic compound) subzone with the process prolonging. In order to obtain the optimum alloyed layers, the diffusion temperature and time both have an optimum value range in which the alloyed layers are all made up of a solid solution phase subzone + a reacting phase subzone. For pure magnesium the optimum diffusion temperature is 400℃~420℃, the optimum time is 8h~12h; for ZM5 the optimum temperature is 430℃~450℃, the optimum time is 4h~12h; (2) The formation process of thealloyed layers can be roughly divided into the following steps: ①The active atoms of the infiltrate was absorbed to the surface of the base metal; ②The active atoms of the infiltrate break through the contact interface into the surface of the base metal resulting in the solid solution phase subzone with definite thickness; ③With the diffusion time lengthening, the active atoms of infiltrate diffuse continuously into the inside of the base metal, as a result, a small quantity of uncontinous intermetallic compound formed on the surface of the solid solution phase subzone when the concentration of the infiltrate atoms is high enough; ④The diffusion process going on, the continous intermetallic compound formed ultimately and grew inward; (3) In the above stages, the diffusion process is the crucial one. As the process parameters varies, the effect of the body diffusion and the grain boundary diffusion on the diffusion process varies correspondingly. When the diffusion temperature is low, the diffusion is slow, and the effect of the grain boundary diffusion is more evident than that of the body diffusion, the diffusion process goes along the grain boundary firstly; in the condition that the diffusion temperature is higher, the diffusion is faster, the difference of the effect between the grain boundary diffusion and the body diffusion is not distinct, the extents of the body diffusion and the grain boundary diffusion are similar in the diffusion process.