Relationship Among Tensile Strength, High Cycle Fatigue Strength, and Origin of Fatigue Crack Initiation in a Minor Boron (B)-Modified β-Type Ti-6.8Mo-4.5Fe-1.5Al Alloy

One concern regarding boron (B)-modified Ti alloys is that TiB formed in the alloy could cause early fatigue crack initiation, especially when its tensile strength is considerably higher than 1100 MPa. Therefore, the present study was undertaken to determine whether TiB could indeed become an origin of fatigue crack initiation in a high strength 0.1 pct B-modified β-type Ti-6.8Mo-4.5Fe-1.5Al alloy. An alloy with or without B was subjected to different types of heat treatments to produce various microstructures. The resultant tensile strengths ranged from 1217 to 1564 MPa. Both the B-free and B-modified alloys showed almost the same tensile strength when they were subjected to the same heat treatment. When the tensile strength level was below ~1200 MPa, the B-modified alloy showed higher HCF strength compared to the B-free counterpart. In these cases, the fatigue crack originated from grain boundaries. In contrast, when the tensile strength level was above 1250 MPa, the HCF strength of the B-modified alloy was inferior to the B-free counterpart. The fatigue crack initiated neither from the grain boundaries nor from the microstructural unit but rather from the interface between TiB and matrix. In sum, the HCF behavior and the origin of the fatigue crack initiation of a B-modified alloy was highly dependent on its microstructures, and, thus, on its tensile strength including the critical tensile strength level at ~1250 MPa, where deterioration of fatigue strength occurred due to the presence of TiB.