Effect of Grain Size on Fatigue Strength and Subsurface Crack Initiation in Beta Ti-22V-4Al Alloy
Keiro Tokaji, Koji Ohya and Hiroyasu Kariya
Abstract: This paper describes the effect of beta grain size on fatigue strength and subsurface crack initiation of solution treated and aged materials (STA) and as-solution treated materials (ST) in beta Ti-22V-4Al alloy. Rotating bending fatigue tests have been conducted at room temperature in laboratory air using smooth specimens and the obtained results were discussed on the basis of crack initiation,small crack growth and fracture surface examination. The fatigue strength of STA materials increased with increasing grain size,reached the maximum at a certain grain size and then decreased with a further increase in grain size,while in ST materials,fine grained material showed a higher fatigue strength than coarse grained material,indicating that the obtained fatigue strength of STA materials depended on both grain size and alpha precipitate distribution. STA materials showed S-N curves composed of two parts which are short life regime less than 105 cycles and long life regime around or more than 107 cycles. In the latter regime,subsurface crack initiation occurred and a relatively flat facet was always seen at the crack origins. No subsurface crack initiation was observed in ST materials. The occurrence of this type of fracture is discussed based on microstructural and mechanical considerations. Key Words:Titanium alloy, Fatigue strength, Solution treatment temperature, Grain size, Subsurface crack initiation