Unusual precipitation induced by solute segregation in coherent twin boundary in titanium alloys

Heterogeneous precipitation and hence the distribution of α and β phase in α+β titanium alloys can impact strongly on alloy mechanical properties. The β phase is commonly observed to precipitate heterogeneously at boundaries between α′ martensite plates and dislocations inside the martensite, especially in the additively manufactured α+β titanium alloys, with a Burgers orientation relationship with the surrounding matrix phase. Here, we report an interesting phenomenon in a Ti-4wt%Mo alloy where the β phase precipitates heterogeneously on Mo-segregated {101¯1} fully coherent twin boundaries (CTBs) within α′ martensite, with an unusual Potter orientation relationship with α′. We find that the CTB has a structure resembling that of the β phase and that Mo segregation in the CTB leads to an unusual out-of-plane shift of the segregated atomic columns that makes the local structure almost identical to the β structure, hence serving as a template for β precipitation with the unique orientation relationship. Our findings reveal a novel mechanism via which the distribution of β precipitates can be manipulated effectively for better mechanical properties and could be extended to a group of Ti alloys including the current workhorse alloy Ti-6Al-4V and more broadly to other engineering alloys.