In this study, Mg-GdH2 composite was prepared by internal hydrogenation treatment (IHT) of Mg-5Gd (wt) alloy at 300 and 325 degrees C for 36 and 48 h under hydrogen pressure of 0.34 MPa. Phase transformations during the hydrogenation process and mechanical properties of the composites were investigated. During the IHT, Gd in the alpha-Mg matrix reacted with hydrogen and formed GdH2 precipitates. The results indicated that Mg metal matrix composite reinforced with uniformly dispersed GdH2 hydride particles could be obtained after IHT. The strength of the Mg-GdH2 composite was inferior to that of the Mg-5Gd alloy in which Gd is dissolved in alpha-Mg solid solution. This implied that the formation of GdH2 precipitates did not result in further strengthening of the as-cast alloy. Analysis of strengthening mechanisms indicated that the strengthening effects brought by GdH2 precipitates were not able to compensate the loss of solid solution strengthening effect due to Gd dissolved in alpha-Mg. The decrease in strength was accompanied with the increase in ductility, which was due to the grain growth in Mg matrix and annihilation of defects. From these results, the potential of IHT to improve the strength of Mg-RE alloys is inferred, and recommendations for future studies are presented.
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