Effect of Cold Work on Fatigue Behaviour in Beta Ti-22V-4Al Alloy
Keiro TOKAJI, Kohji OHYA, Kiyoji MORI, Shinzaburo TAKAFUJI and Yozo KATO
Abstract:In order to investigate the effect of cold work on the fatigue behaviour of beta Ti-22V-4Al alloy, rotary bending fatigue tests have been performed using smooth specimens aged after cold work of two different reductions (CWA). The fatigue behaviour of the CWA specimens was evaluated and compared with that of solution treated and aged specimens (STA). It was found that the CWA specimens showed two different crack initiation modes, surface and subsurface crack initiation, depending on stress level, as similarly seen in the STA specimens. In the surface fracture regime, the fatigue strength of the CWA specimens increased remarkably compared with that of the STA specimens and the CWA specimen of higher reduction exhibited higher fatigue strength than the counterpart. Since the crack initiation process had a considerably large fraction of fatigue life, the increase in fatigue strength due to cold work was attributed to increased crack initiation resistance. In the subsurface fracture regime, fatigue strength was almost similar regardless of cold work. A smooth facet was seen at the subsurface crack initiation site. The sizes of the facets were one to two times as large as the average grain size of each CWA specimen and the subsurface crack origin became deeper as stress level decreased. Furthermore, fatigue life tended to increase with decreasing maximum stress intensity factor for facet. Key Words:Fatigue strength, Beta titanium alloy, Cold work, Crack initiation, Small crack growth, Subsurface fracture