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.

关于硼（B）改性的Ti合金的一个问题是，合金中形成的TiB可能引起早期疲劳裂纹萌生，尤其是当其拉伸强度大大高于1100 MPa时。因此，进行了本研究以确定在高强度的0.1 pct B改性的β型Ti-6.8Mo-4.5Fe-1.5Al合金中，TiB是否确实可以成为疲劳裂纹萌生的起点。对具有或不具有B的合金进行不同类型的热处理，以产生各种微结构。所得的抗张强度为1217至1564MPa。无硼和B改性合金在进行相同的热处理时均显示出几乎相同的拉伸强度。当抗拉强度水平低于〜1200 MPa时，与无B合金相比，B改性合金显示出更高的HCF强度。在这些情况下，疲劳裂纹源自晶界。相反，当抗拉强度水平高于1250 MPa时，B改性合金的HCF强度低于无B合金。疲劳裂纹既不是从晶界也不是从微观结构单元开始的，而是从TiB和基体之间的界面开始的。总而言之，B改性合金的HCF行为和疲劳裂纹萌生的起点高度依赖于其微观结构，因此，其拉伸强度（包括在〜1250 MPa时的临界拉伸强度水平）会导致疲劳恶化TiB的存在导致强度升高。B改性合金的HCF强度低于无B合金。疲劳裂纹既不是从晶界也不是从微观结构单元开始的，而是从TiB和基体之间的界面开始的。总而言之，B改性合金的HCF行为和疲劳裂纹萌生的起点高度依赖于其微观结构，因此，其拉伸强度（包括在〜1250 MPa时的临界拉伸强度水平）会导致疲劳恶化TiB的存在导致强度升高。B改性合金的HCF强度低于无B合金。疲劳裂纹既不是从晶界也不是从微观结构单元开始的，而是从TiB和基体之间的界面开始的。总而言之，B改性合金的HCF行为和疲劳裂纹萌生的起点高度依赖于其微观结构，因此，其拉伸强度（包括在〜1250 MPa时的临界拉伸强度水平）会导致疲劳恶化TiB的存在导致强度升高。