Hydrogen absorption behavior and microstructual change by thermochemical processing (TCP) with hydrogen in Ti-6Al-4V (Ti-6.4) alloy were investigated. Mechanical properties and fracture toughness of Ti-6•4 and Ti-5Al-2.5Fe(Ti-5•2.5) alloys subjected to various TCP were, then, investigated.
Microcracking is observed at a hydrogen flow rate of over 12.3 ml/s in Ti-6•4 alloy when the starting microstrucure is Widmanstätten α. The microstructures of Ti-6•4 and Ti-5•2.5 alloys are refined by both newly proposed and conventional TCP. Tensile strengths of the both alloys are also improved by various TCP. Newly proposed TCP where hydrogenation and dehydrogenation are conducted below the hydrogenated β transus temperature, that is, BTH (Below Transus Hydrogenation), is found to be effective for achiving a good balance of tensile strength and elongation in both Ti-6•4 and Ti-5•2.5 alloys. Fracture toughness of Ti-6•4 alloy is decreased by TCP while that of Ti-5•2.5 alloy is increased.
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