Microstructure evolution and bending fracture mechanism of as-rolled gradient-laminated network titanium matrix composites

Abstract

Titanium matrix composites (TMCs) have drawn significant interest due to decent specific strength and heat resistance. Introducing gradient and laminated structure can effectively enhance the toughness of TMCs. To achieve sheet forming, the gradient-laminated network titanium boride reinforced Ti6Al4V (TiB/TC4) composite was fabricated by low-energy milling, powder stacking, vacuum hot pressing sintering and β-phase-region hot rolling. After hot rolling, the layer thickness decreased, the layer interface was kept straight, and no macroscopic defects were observed. In the meanwhile, the flattened network structure, rotation and fracture of TiB whiskers, and grain refining of the matrix were observed. TiB whiskers can also promote the dynamic recrystallization of nearby matrix and tend to maintain orientation relations. While retaining the gradient-laminated network structure, the bending strength of the composite was enhanced to 2205 MPa after hot rolling, which is 13.07% higher than that of the as-sintered composite. The strengthening was attributed to grain refining of the matrix and alignment-induced load transfer strengthening of TiB whiskers. Digital image correlation (DIC) revealed the upward movement of the neutral surface away from the center during the bending test, which led to increased tensile stress and crack initiation in the center layer, rather than the region with maximum tensile strain. Upon observing the fracture side face, the crack propagation path was determined to be influenced by layer interfaces, the network structure connectivity, and the outmost alloy layers.