Effect of Cooling Rate on Microstructure and Fracture Characteristics of β-Rich α + β Type Ti-4.5Al-3V-2Mo-2Fe Alloy

Abstract

This study investigated the effect of cooling rate on microstructure and fracture characteristics of β-rich a + 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 (FC) and slow furnace-cooling (SFC) from various solutionizing temperatures in the a + β field. The types of secondary phases are martensite a (orthorhombic α), acicular a and plate-like a 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 1C , 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 1C 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 1C , In a particular condition, J 1C decreases slightly with increasing width of acicular α for microstructure containing predominantly local acicular a, but increases monotonously with farther increases in the width of acicular α. J 1C increases considerably with increasing width of plate-like a. The increase of J 1C is mainly due to increasing the effect of extrinsic toughening mechanism.