| Summary: | The element oxygen is expected to be a low-cost, strengthening element of titanium alloys due to its strong solid solution strengthening effect. High cycle fatigue behaviors of Ti-6Al-4V alloys with different oxygen contents (0.17%, 0.20%, 0.23% wt.%) were investigated in this paper. The results illustrated that Ti-6Al-4V-0.20O alloy possesses the highest fatigue strength and the lowest fatigue crack propagation rate. The fatigue fracture morphology verified that the fatigue cracks propagated transgranularly in both Ti-6Al-4V-0.17O and Ti-6Al-4V-0.20O alloys, and the fatigue cracks tended to extend intergranularly in the Ti-6Al-4V-0.23O alloy. The maximum nano-hardness varied from the <0001> direction to the <inline-formula><math display="inline"><semantics><mrow><mo><</mo><mover accent="true"><mn>1</mn><mo>¯</mo></mover><mn>2</mn><mover accent="true"><mn>1</mn><mo>¯</mo></mover><mn>0</mn><mo>></mo></mrow></semantics></math></inline-formula> and <inline-formula><math display="inline"><semantics><mrow><mo><</mo><mn>01</mn><mover accent="true"><mn>1</mn><mo>¯</mo></mover><mn>0</mn><mo>></mo></mrow></semantics></math></inline-formula> directions with the increasing oxygen content, which suggested that the dominant slip system varied from prismatic slip to pyramidal slip. The number of the <inline-formula><math display="inline"><semantics><mrow><mo><</mo><mover accent="true"><mi>c</mi><mo stretchy="false">→</mo></mover><mo>+</mo><mover accent="true"><mi>a</mi><mo stretchy="false">→</mo></mover><mo>></mo></mrow></semantics></math></inline-formula> type dislocations increased with the oxygen content, which indicated that the number of the first-order pyramidal and the second-order pyramidal <inline-formula><math display="inline"><semantics><mrow><mo><</mo><mover accent="true"><mi>c</mi><mo stretchy="false">→</mo></mover><mo>+</mo><mover accent="true"><mi>a</mi><mo stretchy="false">→</mo></mover><mo>></mo></mrow></semantics></math></inline-formula> slip systems increased. The oxygen can significantly change the fatigue fracture mechanism of Ti-6Al-4V alloy: From transgranular fracture to intergranular fracture. These results are expected to provide valuable reference for the optimization of the composition and mechanical properties of titanium alloys.
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