Strength and toughness of microstructually controlled α + β type titanium alloys by thermochemical processings with hydrogen

Hydrogen absorption behavior and microstructural 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 microstructure is Widmanstaetten α. The microstructures of Ti-6·4 and Ti-5·5.2 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 alloys is decreased by TCP while that of Ti-5·2.5 alloy is increased.