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

Heterogeneous precipitation and hence the distribution of alpha and ,B phase in alpha+ ,B titanium alloys can impact strongly on alloy mechanical properties. The ,B phase is commonly observed to precipitate heterogeneously at boundaries between alpha' martensite plates and dislocations inside the martensite, especially in the additively manufactured alpha+ ,B 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 ,B phase precipitates heterogeneously on Mo-segregated { 10 1 over line 1 } fully coherent twin boundaries (CTBs) within alpha' martensite, with an unusual Potter orientation relationship with alpha' . We find that the CTB has a structure resembling that of the ,B phase and that Mo segregation in the CTB leads to an unusual outof-plane shift of the segregated atomic columns that makes the local structure almost identical to the ,B structure, hence serving as a template for ,B precipitation with the unique orientation relationship. Our findings reveal a novel mechanism via which the distribution of ,B 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.(c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

Heterogeneous precipitation of the β phase at boundaries within α' martensite and dislocations greatly affects the mechanical properties of α+β titanium alloys. A novel phenomenon was observed in a Ti-4wt%Mo alloy where the β phase selectively precipitates on Mo-segregated twin boundaries within α' martensite. The unique orientation relationship between the β phase and α' is formed due to the similar structure of the twin boundary and Mo segregation, providing a template for β precipitation and potential improvement in mechanical properties.