Preparation Of Nanocrystalline Magnesium Alloy Powders By Hydrogen Treatment Process

Magnesium alloys are widely used in the fields of automobile,3C and aerospace engineering, because of its low density, high specific strength and environmental-friendly. But the relatively poor absolute strength and ductility of Magnesium alloys limits their further applications. Grain refining is a sensible choice to improve both the strength and ductility of the metal material. However, it is difficult to prepare nano-crystalline Mg alloys by the common grain refining methods. In our study, nano-crystalline AZ91Mg alloys powders were successfully prepared by HDDR process.In this paper, the effect of the different HDDR parameters on the phase transformation of AZ91Magnesium alloy powders was investigated and an optimized HDDR technique was acquired to refine the Mg alloy grain to nanometer scale. The microstructure evolution of the alloy was characterized by powder X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The grain refining mechanism of AZ91Magnesium alloy in HDDR process was also discussed.In hydrogenated process, the degree of hydrogenation increases with the increase of the hydrogenation temperature, hydrogen pressure and hydrogenation time. The hydrogenation reactions take place when the temperature is higher than300℃. When the hydrogenation is complete at450"C under4MPa hydrogen pressure for12hours, further raising the hydrogen pressure has no obvious effect to the phase composition of the hydrogenation products. In dehydrogenated process, the dehydrogenation reaction also occurs when the temperature is above300℃. The degree of dehydrogenation increases with the increase of the dehydrogenation temperature. Considering various factors, the optimized HDDR technology of AZ91alloy is selected as the combined parameters of hydrogenation temperature of450℃, hydrogen pressure of4MPa, hydrogenation time of12hours, dehydrogenation temperature of350℃and dehydrogenation time of4hours in vacuum.The hydrogenation of Mg17Al12shows that Mg17Al12is difficult to disproportionation, so the disproportionation of y-Mg17Al12in AZ91magnesium alloy is small and can be ignored. Under the optimal parameters, pure magnesium can be also hydrogenated and dehydrogenated as AZ91alloy. The microstructure analysis was conducted in the hydrogenation and dehydrogenation process of AZ91Magnesium alloy and the results showed that the original powder surface is tend to uneven or broken lamellar after hydrogenation, and after further dehydrogenation the powder surfaces tend to smooth. The particle sizes of the dehydrogenaed powders are close to the original powders. The grain sizes of AZ91alloy can be refined to20nm though an optimized HDDR process. The major grain refinement of AZ91magnesium alloy in HDDR process happens in the phase transition and recombination process.