Unravelling the competitive effect of microstructural features on the fracture toughness and tensile properties of near beta titanium alloys

The competitive effect of microstructural features including primary α (α_p), secondary α (α_s), grain boundary α (α_GB) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes. The relative effect of β grain size and STA (solution treatment and ageing) processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method. These results were further explained via correlating microstructure with the fracture toughness and tensile properties. It was found that large numbers of fine α_s precipitates and continuous α_GB played greater roles than other features, resulting in a high strength and very low ductility (<2%) of STA process samples. The β grain size had a negative correlation with fracture toughness. In the samples prepared by BASCA (β anneal slow cooling and ageing) process, improved ductility and fracture toughness were obtained due to a lower density of α_s precipitates, a basket-weave structure and zigzag morphology of α_GB. For this heat treatment, an increase in prior β grain size had an observable positive effect on fracture toughness. The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained. It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties. This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties. A novel table was proposed to quasi-quantitatively unravel these competitive effects.