Fatigue crack initiation and growth in titanium alloy composites reinforced with TiC and TiB particles were investigated. The composites were Ti-6wt%Al-4wt%V reinforced with 10vol% of TiC particles and Ti-8wt%Al reinforced with 8vol% of whisker like TiB particles. They were produced in-situ by the vacuum arc remelting process. Fatigue strength and life of the composites in notched specimens are significantly less than that of Ti-6wt%Al-4wt%V alloy, while the static strengths are higher in both composites. On the other hand, fatigue crack growth rate is dependent on the type of the reinforcement. The particulate TiC reinforcement increases the growth rate through their cracking, while the TiB particles decreases the growth rate through the higher crack closure level and the crack deflection. Direct observations in scanning electron microscope by in-situ fatigue tests revealed that the fatigue cracks in composites initiated from the cracking of the reinforcing particles and the early cracking and its growth to matrix reduced the fatigue lives of the composites in low cycle region. At the stress level above the fatigue limits, fatigue cracks in both composites initiated at the matrix or the interface between the matrix and the particles. Degradation in fatigue limit might be caused by the difference in microstructure of matrix.
The titanium alloy itself shows higher static and fatigue strength. However, the reinforcements are not so tough as to bear their high shared stress. The reinforcements are eventually damaged due to high applied stresses and those provide occasionally fatigue crack initiation site.
