Influence of duplex ageing on secondary alpha precipitates and mechanical properties of the near beta-Ti alloy Ti-55531

This paper presents the results of studies on the microstructure and mechanical properties of a near beta-Ti alloy processed with different ageing approaches. Influences of intragranular ultrafine alpha precipitates and grain boundary alpha layer on mechanical properties in Ti-55531 alloy were analyzed. The results indicate that the precipitation microstructure of the duplex aged alloy exhibits a uniform size and acicular alpha precipitated orientation shows an angle of approximately 60 degrees. While the morphology of secondary alpha precipitates in single-step aged samples appear the irregular distribution and heterogeneous size. With the final ageing time increasing, the morphology of intragranular alpha precipitates and grain boundary alpha phase brings out a distinct coarsening. The single-step aged alloy can yield a better combination of the ultimate tensile strength (1265 MPa) and ductility (9%). However, the ultimate tensile strength of the duplex aged alloy even reaches 1368 MPa but it breaks at the stage of elastic deformation. The fracture mode of Ti-55531 alloy changes from predominantly dimple fracture in single-step aged samples to predominantly faceted and cleavage type fracture in duplex aged samples causing by the differences of intragranular alpha precipitates and the interface between grain boundary alpha layer and intragranular microstructure.