Study On Microstrcture Evolutions In Hydrogenation Of Mg-Al And Mg-Nd Alloy Powders/Bulks

Magnesium and its alloys are currently the lightest metal structurematerial in the engineering application. Besides, magnesium and its alloyare also a potential and efficient hydrogen storage material. TheHydrogenation-Disproportionation-Dehydrogenation-Recombining(HDDR) process is a new method to prepare materials with ultra-fine/nanocrystals. However, the high-thermodynamic stability, high desorptiontemperature, low plateau pressure of MgH2preclude the application ofHDDR in Mg alloys. Another efficient approach to producenano-structured metal hydride powders is mechanically assisted hydridingvia ball milling at room-temperature.Based on these considerations above, in the present work, asystematic study has been carried out on microstructure and phaseevolutions of Mg-Al powders ball milled in hydrogen atmosphere. Both inMg-3wt.%Al and Mg-9wt.%Al systems, β-MgH2phase was observed upon a short milling time, i.e.4h, and reached its maximum percentage(~80%) after32h. Neither in the two systems do as-milled powders containMg17Al12. While heating the milled powders of the Mg-9wt.%Al system to350oC results in the precipitation of Mg17Al12. DTA/TG analyses of thosepowders milled for from8h to40h showed that, either well-developedpeak doublets or shoulders were observed, which quite plausiblycorrespond to the separate hydrogen desorption from smaller and largerparticles of β-MgH2.Besides, in the present work, hydrogenation induced microstructuremodifications in a bulk Mg-Nd alloy and its effect on mechanicalproperties have also been carefully investigated. The hydrogenationtreatments are carried out on the as-cast, solution treated and aged Mg-4wt.%Nd alloy samples at250oC~450oC under1~3MPa hydrogen pressure.It is shown that a sub-stoichiometric NdH2.61is formed in all the treatedalloy samples preferentially at the Mg12Nd grains or precipitates. Themicrostructure and mechanical properties of the Mg-Nd alloy before andafter hydrogenation treatments were systematically investigated. Based onthe above analyses, the mechanisms of hydrogenation and its effect on themechanical properties of the Mg-4Nd alloy are proposed and discussed.The results have shown that after hydrogenation treatment at400oC under 3MPa for20h, the ultimate tensile strength (UTS) and yield strength (YS)of the Mg-Nd alloy are reduced while the elongation is increased evidently.