Effect of cooling rate on microstructure and fracture characteristics of β-rich α + β type Ti-4.5Al-3V-2Mo-2Fe alloy

This study investigated the effect of cooling rate on microstructure and fracture characteristics of β-rich α + β type Ti-4.5Al-3V-2Mo-2Fe alloy rolled plate. Particular attention was paid to the roles of the local and continuous secondary phases within prior β grain in the static fracture toughness. A variety of microstructures containing different types, morphologies, sizes and volume fractions of secondary phases were obtained in matrix β (within prior β grain) with varying cooling rates; namely, water-quenching (WQ), air-cooling (AC), furnace-cooling and slow furnace-cooling (SFC) from various solutionizing temperatures in the α + β field. The types of secondary phases are martensite α (orthorhombic α″), acicular α and plate-like α observed in WQ, AC and FC specimens, respectively. While, there is no or lack secondary phase observed in matrix β for SFC specimen. Deformation-induced martensite (α″), DIM, was observed in WQ specimens after testing. The results showed that the fracture toughness, J_IC, and calculated flow stress, σ_f, of the microstructures containing a secondary phase depend mainly on the type and width of the secondary phase. The J_IC of microstructures containing a secondary phase, in general, is superior to that of the microstructures lacking secondary phases. Both the martensite α and DIM appear to increase J_IC. In a particular condition, J_IC decreases slightly with increasing width of acicular α for microstructure containing predominantly local acicular α, but increases monotonously further increases in the width of acicular α. J_IC increases considerably with increasing width of plate-like α. The increase of J_IC is mainly due to increasing the effect of extrinsic toughening mechanism.