The conflicts between strength and ductility of bimodal Ti-5553 alloy with fine equiaxial prior β grains

In the paper, homogeneous bimodal Ti-5553 alloy with the prior-β grain size of 15 μm is successfully fabricated by elaborate thermo-mechanical processing in α+β field, solution and ageing, in order to explore the combination of strength and ductility accessible to β-titanium alloys. The strengthening curve of ageing at 500°C comprises the incubation period, the strengthening period and the stable period. The attainable yield strength of the alloy aged at 500°C is 1430 MPa, associating with a receivable tensile elongation of 9.5%. During the ageing at 620°C the strength of the bimodal alloy first increases with ageing time, then reaches a maximum value of 1079 MPa after 2 h and finally remains constant within the ageing period from 2 h to 32 h. It is unexpected that the tensile elongation of bimodal alloys aged at 620°C are the same as that of the solution-treated alloy. Interestingly, the ageing at 700°C dose not exhibit the strengthening effects but could make the alloy more ductile. Within the ageing temperature scope of 500°C∼700°C, the precipitating kinetics of grain boundary α (α_GB) precipitation is more active than that of the intragranular α (α_Intra) precipitation. However, α_GB does not have the precipitation strengthening effects. The yield strength of bimodal Ti-5553 alloy depends on the dislocation density of β phase and the size effects of prior β grains. By exploring the tensile damage sources, we find that the ductility of bimodal Ti-5553 alloy of is influenced by the plasticity of α_GB and the stress concentration of β/α_Intra interface. The combination of strength and ductility of high-strength bimodal Ti-5553 alloy might be further improved by optimizing the thickness of α_GB.