Preparation Of Nanocrystalline ZK60Alloy Powders By The Hydrogenation-desorption Treatment

For its excellent properties, Magnesium and Magnesium alloys have been considered asthe ideal choice for instead of Aluminum alloys, iron and steel materials in aerospace,automotive, electronic communications and other industries. However, the relatively poorstrength and plastic deformability of Magnesium alloys largely restrict their applications inthe industry. The mechanical properties of Magnesium alloys are greatly affected by grainsize, so grain refinement is a good choice to improve both its strength and plasticity. Thereare many methods to refine grain size, but it is difficult to prepare nanocrystalline Magnesiumalloys by these common methods. In our study, hydrogenation-disproportionation-desorption-recombination (HDDR) process is chosen to the grain refinement of ZK60Magnesium alloypowders, and nanocrystalline ZK60Magnesium alloy powders is obtained successfully.In this paper, the craft parameters of hydrogenation-desorption technology for the grainrefinement of as-cast ZK60alloy powders by HDDR was investigated first. Then the effectsof hydrogenation temperature, hydrogen pressure and hydrogenation time on the phasecompositions was studied respectively by powder X-ray diffraction(XRD) in the HDDRprocess. The microstructure evolution of the alloy powders was characterized by opticalmicroscopy(OM), scanning electron microscopy(SEM) and transmission electionmicroscopy(TEM). The nanocrystallization mechanism of ZK60Magnesium alloys in thisprocessing was described, too. At last, under the same craft parameters ofhydrogenation-desorption technology pure Mg was processed, and the role of disproportionation reaction in the HDDR process for Magnesium alloys was discussed.The results reveal that, in the hydrogenated process hydrogenation temperature is thechief factor that influences the degree of hydrogenation most. The higher hydrogenationtemperature is, the faster the reaction processes and the more degree of hydrogenation.Prolonging hydrogenation time would increase the degree of hydrogenation as well. Effect ofhydrogen pressure on the hydrogenation process is relatively small, and it presents that thedegree of hydrogenation decreases first and increases next. In the dehydrogenated process,the effects of dehydrogenation temperature and dehydrogenation time are the same as that ofhydrogenated process. In case of the refined grain growing up again, dehydrogenation timeshould not be too long. All factors considered, the optimized HDDR technology for grainrefinement of as-cast ZK60Magnesium alloys is selected as follows: hydrogenated at450℃under2MPa for12h and then thoroughly dehydrogenated at350℃for3h in vacuum.Through the treatment of HDDR with the optimized technology, the evolution ofmicrostructure of ZK60Magnesium alloys shows that the original powder particles breakinto many smaller particles under the inner stress after hydrogenation, and after the furtherdehydrogenation, the sizes of hydrogenated powder particles are hardly changed. While thegrain sizes of ZK60Magnesium alloys can be refined from150~200μm to around30nm,and the nanocrystalline sizes appear relatively uniform. As the results of pure Mg processedby HDDR, it proves that the influences of Zn、Zr on the reactions can be negligible. So themajor grain refinement of ZK60Magnesium alloys in HDDR process happens in the phasetransition and recombination process, and the disproportionation reaction dose not work onthe grain refinement.