Gradient microstructure of TC21 alloy induced by hydrogen during hydrogenation

Through melt hydrogenation, a gradient microstructure (α″ + α′)/(α + β_H) has been observed in TC21 alloy. The addition of hydrogen induces martensite transformation and increases the volume fraction of β. It is found that the absorption process of hydrogen atoms can be divided into melting and cooling stages. During cooling, the continuous absorption of hydrogen and the corresponding decrease of freezing point of melt extend solidification time of melt and lead to hydrogen enrichment in the upper of the specimen, which induces the formation of the gradient structure. The hydrogenated TC21 alloy shows higher thermoplasticity compared with the unhydrogenated TC21 alloy. The flow stress of the upper part of the hydrogenated alloy is lower than that of the center part. A gradual variation has been observed in the microhardness along the gradient direction due to variation in the microstructure. The microhardness of the upper surface drops about 45% with 14.6 at.%H.