微结构和应力比对Ti-6Al-4V高周和超高周疲劳行为的影响 *
摘要
Titanium alloys have been widely used as superior engineering materials because of their high specific strength, high temperature resistance and high corrosion resistance. In their engineering applications such as used in aircraft engines, titanium alloys may experience even 10(10) fatigue cycles. Recently, faceted crack initiation was observed in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes of titanium alloys, which resulted in a sharp decrease in fatigue strength. Therefore, the HCF and VHCF of titanium alloys have both scientific significance and engineering requirement. In this work, the effects of microstructure and stress ratio (R) on HCF and VHCF of a Ti-6Al-4V alloy have been investigated. Fatigue tests were conducted on a rotating-bending fatigue machine and an ultrasonic fatigue machine. All the fatigue fracture surfaces were observed by SEM. The results show that the HCF and VHCF behaviors of the fully-equiaxed and the bimodal Ti-6Al-4V alloy are similar. The observations of fracture surface indicate that two crack initiation mechanisms prevail, i.e. slip mechanism and cleavage mechanism. With the increase of stress ratio, the crack initiation mechanism switches from slip to cleavage. The S-N curves present the single-line type or the bilinear type. For the cases of rotating-bending and ultrasonic axial cycling with R= -1.0, -0.5 and 0.5, the S-N curves are single-line type corresponding to the slip mechanism or cleavage mechanism. For the cases of R= -0.1 and 0.1, the S-N curves are bilinear type corresponding to both slip and cleavage mechanisms. A model based on fatigue life and fatigue limit is proposed to describe the competition between the two mechanisms, which is in agreement with the experimental results.