Microstructure variation of TC4 alloy after hydrogen treatment was investigated by optical microscope （OM）

X-ray diffraction （XRD） and transmission electron microscope （TEM）. Effects of hydrogen content on hardness of TC4 alloy were researched by hardness testing. The results show that martensite α′ and hydride δ appear in the TC4 alloy after hydrogenation treatment with increase of hydrogen content

while only α and β phase existed in the alloy after dehydrogenation. Microstructure is refined in the alloy after hydrogenation and dehydrogenation treatment

which is attributed to the transformation of eutectoid （δ+α）

in which the δ phase parallel to the α phase

and the decomposition of martensite α′. Hardness of the TC4 alloy is increased with the increase of hydrogen content after hydrogenation treatment as a result of microstructure refinement in the alloy.