Tensile, impact and fatigue properties of ultra-fine grained alpha+beta type titanium alloys prepared by hydrogen treatment

Recently, the refinement into ultra-fine grains in the field of metallic materials has been attracting a great deal of attention. One of the few methods available for creating ultra-fine grains is the hydrogen treatment, which easily produces grain sizes of 1 to 3 mu m in titanium materials. This paper describes the characteristics of mechanical properties, such as tensile strength and elongation, of ultra-fine grained alpha+beta type titanium alloys utilizing this method. Several test specimens having grain size varing between ultra-fine (1 to 3 mu m) and coarse (approximately 20 mu m) were prepared and subjected to tensile, impact and fatigue tests. The results are as follows. It was found that the yield strength of materials with a grain size of 1-3 mu m increased linearly in proportion to the grain size to the -1/2 power. The ultra-fine grained materials show higher elongation considering their high strength. In particular, it showed extremely large uniform elongation. Even for the fatigue strength which is similar to the tensile strength, a high value is shown. The impact test indicated that the ultra-fine grained materials had slightly low toughness. The high strength of ultra-fine grained materials can be explained by the Hall-Petch relationship. The high uniform elongation is considered to be produced by dispersion of the highly deformable beta phase throughout the materials, which creates uniform deformation, and thereby prevents stress from concentrating locally when tension is applied.