| With characteristics of low density, high specific strength, high specific stiffness and recoverability, magnesium alloy has been paid widespread atten-tion as a new generation of structural material. However, its poor strength, ductility and corrosion resistance have greatly restricted its application to aer-ospace, military project, automobile and other high-end application fields, ma-terials are usually required to simultaneously meet the two indicators of low density and extremely-high heat as well as mechanical properties. With lower absolute performance, magnesium alloy is still far from enough to replace steel, aluminum and titanium alloy. Thus, under the grim situation of increasingly depleting steel, aluminum and titanium materials, how to give full play to the resource advantages and performance advantages of magnesium alloy becomes an important topic of magnesium alloy study.The Hall-Petch relationship (σy=σ0+κyd-1/2) shows that:The greater the material slope constant ky is, the more obvious the effect of grain size on me-chanical property is. The κy value of magnesium alloy is very large, several times as large as the common body-centered cubic or face-centered cubic met- als. So grain refinement is the best way to improve the strength and plasticity of magnesium alloys. However, even with the currently most effective ECAE method, the grain size of the magnesium alloy is refined to about1μm to greatly improve the performance of magnesium alloy, still there is big gap compared with the performance of the titanium and steel.Studies showed that:If the grain size of magnesium alloy was refined to nanoscale, great improvement will happen to thermal and mechanical proper-ties of the material. Currently the methods used in the preparation of nano-crystalline metals include inert gas condensation method, high-energy ball milling, severe plastic deformation method and amorphous crystallization method. Although these methods have their advantages, there are still una-voidable shortcomings. Researches on magnetic material showed that when the metal material coarse powder was hydrogenated and disproportionation hap-pened, microstructure of the material was substantially refined to nanoscale. Further researches found that when the material went through dehydrogenation and restructuring, although the grain size became larger, still it could remain at the nanoscale.In this paper, cast AZ91magnesium alloy was used as the research object, and constant-volume method was adopted to measure the PCT curve of pow-der materials. Systematic analysis was conducted on absorption and desorption mechanism of hydrogen magnesium alloy. Absorption and desorption hydro-gen dynamic model was deduced under reasonable assumptions. In combina-tion with theoretical calculations of hydrogenation thermodynamics and kinet-ics of magnesium alloy, the paper was to comprehensively understand the quantitative impact of hydrogenated environment and temperature conditions on absorption and desorption hydrogen kinetic characteristics of magnesium alloy. On the premise of ensuring the speed of hydrogen absorption and de-sorption, experimental program was scientifically designed through orthogonal design, so that the influence rule of various factors in experimental links on the grain size was to be explored and that the best HDDR process parameters were to be selected. Powder samples from different HDDR process parameters were collected, and detailed analysis was made on phase composition and micro-structure of samples by using X-ray diffraction and transmission electron mi-croscope.The results showed that: AZ91magnesium alloy was an ideal material for hydrogen storage, after24hours of treatment at450℃and7MPa hydrogen pressure, AZ91magnesium alloy powder was basically completely hydrogen-ated, and the hydrogen content in the powder was about7.5mass%. Hydrogen absorbing speed and amount of the powder was proportional to the tempera-ture or pressure. Within a certain temperature range, dehydrogenation speed and amount of the hydrogenated powder was proportional to the temperature. Major effect on average grain size of hydrogenated AZ91magnesium alloy powder came from hydrogen pressure, followed by hydrogenation temperature. And the hydrogenation time had the minimal impact on the grain size. After complete dehydrogenation, despite the increase in grain size of the powder, yet the coarsening effect was not very obvious, still maintained at the nanoscale. Comprehensive analysis concluded that the optimum process parameters of HDDR refined cast AZ91magnesium alloy powder were:Hydrogenated the original powder for12hours at450℃and4MPa hydrogen pressure, followed by dehydrogenation process for4hours at high vacuum environment of350℃.With the optimum process parameters, the average grain size of the ful-ly dehydrogenated powder was about34nm. The HDDR process of AZ91magnesium alloy powder could be summarized as follows:During the hydro-genation stage, disproportionation happened; the solute atoms (Al, Zn, etc.) precipitated from the matrix, H atoms entered the interstitial void of Mg to form Mg [H] interstitial solid solution and then generated MgH2, and mean-while generated Mg0.42Al0.58.During the early dehydrogenation stage, solute atoms (Al, Zn, etc.) became solid solution again and entered Mg crystal lattice, and the Mg-rich metallic phase was reverted. And during the last period of dehydrogenation, Mg0.42Al0.58translated into Mg17Al12. |
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