Fatigue Crack Initiation and Propagation Dominated by Crystallographic Factors in TiB/near α-Ti Composite

Abstract

Discontinuously reinforced titanium matrix composites (DRTMCs) with a network structure have been extensively researched due to their superior combination of strength and ductility. However, their fatigue performance has remained unknown. In order to elucidate the fatigue behavior of DRTMCs, a tension–tension fatigue test was performed on a TiB/near α-Ti composite with network structure. The results showed that the variability of fatigue lifetime increased as the stress level decreased. Fractography analysis indicated that fatigue crack initiation was associated with facet formation, while the subsequent propagation was hindered by the network structure comprising TiB whiskers and silicides. Crystallographic characterization further revealed that facets formed due to a combination of shear and normal stress. The reduction in fatigue lifetime was attributed to an increase in the effective slip length, which was influenced by the orientation of grains near the crack-initiation sites toward basal slip in the life-limiting specimen. Quasi in situ observation suggested that the crack initiation was facilitated by both basal and prismatic slip of α-Ti as well as fracture of TiBw. Crack propagation was found to be associated with basal and prismatic slip systems with high Schmid factors, regardless of whether the crack was intergranular or intragranular.