Effect of hydrogen content on microstructures and room-temperature compressive properties of TC21 alloy

Highlights: • The amounts of β and δ phases increase with the increase of hydrogen content. • Ultimate compression of TC21 alloy increases by 30% after hydrogenation. • Flow stress decreases about 150–200 MPa after hydrogenation. • The optimum hydrogen content for cold forming of TC21 alloy is 0.9 wt.%. Mechanical tests were carried out at room temperature to reveal the effect of hydrogen content (0.2–1.2 wt.%) on the compressive properties of TC21 alloy. Results show that different hydrogen contents have distinct effects on microstructures and compressive properties of TC21 alloy. With the increase of hydrogen content, the amounts of α phase and acicular α′ martensite phase decrease, while the amounts of β phase and δ hydride increase, δ hydrides distribute along phase/grain boundaries first and then precipitate in β phase. As hydrogen content increases, the yield strength and microhardness decrease first and then increase, and the ultimate compressive strength decreases. The ultimate compression changes little first, and then increases up to a maximum, and finally decreases with the increase of hydrogen content. The optimum hydrogen content is about 0.9 wt.%, with which TC21 alloy exhibits higher plasticity and lower flow stress. Moreover, the fracture mode of TC21 alloys with hydrogen content less than 1.0 wt.% is a ductile fracture, while the fracture mode of the alloy with 1.2 wt.% hydrogen is a transgranular brittle fracture caused by hydride. Reasons for the distinct effect of hydrogen content on the room-temperature compressive properties are discussed.