Improvement of Fatigue Strength by Two-Step Aging in Beta Ti-22V-4Al Alloy
Keiro TOKAJI, Kohji OHYA and Hiroyasu KARIYA
Abstract:In the present paper,the effect of two-step aging on fatigue strength in beta Ti-22V-4Al alloy is studied on the basis of observation of crack initiation and small crack growth and fractographic analysis of fracture surfaces. Rotating bending fatigue tests have been conducted at room temperature in laboratory air using smooth specimens prepared with materials subjected to different two-step aging treatments (STDA), and the obtained results were compared with the data of material solution treated and aged at a single temperature (STA) and as-solution treated material (ST). The fatigue strength of STDA materials aged at the first-step temperatures close to the second-step temperature was slightly lower than, or nearly the same as,that of STA material. Particularly, the STDA material aged at high temperature followed by low temperature showed no subsurface crack initiation which was seen in STA material in the long life regime around or more than 107 cycles, thus the fatigue strength was superior to that of STA material in that region. Fatigue cracks were initiated at beta grain boundaries in STDA materials aged at low temperature followed by high temperature, which was the same behaviour as in STA material, while were generated within beta grains in STDA materials aged at high temperature followed by low temperature, but small crack growth was not affected by the aging condition. Based on the above experimental results, the role of alpha precipitate distribution in fatigue behaviour was discussed and it was indicated that bi-modal microstructure composed of coarse and fine alpha precipitates was one of the recommended microstructures of the present alloy. Key Words:Titanium alloy, Two-step aging, Microstructure, Fatigue strength, Crack initiation, Small crack growth