Tailoring the configuration of grain boundary α for further improvement of impact toughness in Ti-652 titanium alloy

Dual-phase titanium alloys usually suffer from the continuous distribution of grain boundary α (GBα) phase which can deteriorate the plasticity and toughness. It has been proven in our previous work that the special dot-dash configuration of grain boundary α (α_GB-d) in hierarchical microstructures induced by a two-step solution heat treatment can improve the impact toughness of Ti-652 (Ti-6Al-5Mo-2Sn-0.3Si-0.5 V). In this work, to map the precipitation process of α_GB-d, microstructural features of the samples were captured by water quenching from different heat-treated stages. It was found that the size of α_GB-d was highly affected by the position of undissolved primary α (α_p) after the first-step solution. Orientation relationship (OR) analysis shows that α_GB-d precipitated during the second-step solution generally owned non Burgers orientation relationship (non-BOR) with the β matrix. On this basis, the optimization of α_GB-d was successfully achieved via decreasing the applied first-step solution temperature. By quantifying the microstructural information of α_GB-d, the amount of α_GB-d was identified to be increased by 66 % while its size got decreased by near 47 %. Moreover, statistics analysis revealed the linear positive correlation between the impact toughness and the area fraction of α_GB-d. In the sample with the highest impact toughness, α_GB-d has the largest area fraction of and smallest size. Therefore, the key to improving the impact toughness of Ti-652 is to disperse a large number of small-sized α_GB-d in the microstructure. This rule could provide guidance for future microstructural regulation for superior impact toughness in dual-phase Ti alloys.