Cobalt-mediated mechanical properties in as-deposited Ti alloys: From solid solution to eutectoid strengthening

To evaluate the influence of the eutectoid-forming element cobalt (Co) on microstructure evolution and mechanical properties of titanium alloys for aerospace applications, a series of Ti-Co alloys with varying Co contents (4.0, 8.4, 12.0 wt%) were designed and fabricated via directed energy deposition (DED). Scanning electron microscopy (SEM) revealed that the as-deposited Ti-4.0Co alloy exhibited continuous grain boundary α-phase, coarse proeutectoid α laths (average width: 9.64 μm), and a fine eutectoid microstructure of α-Ti and Ti 2 Co phases. Increasing Co to 8.4 wt% induced significant microstructural refinement. Electron backscatter diffraction (EBSD) analysis indicated a 75 % reduction in α-phase size compared to Ti-4.0Co. This refined microstructure in Ti-8.4Co alloy yielded a substantial 27 % increase in yield strength, primarily attributed to synergistic grain boundary and precipitation strengthening, while maintaining reasonable ductility. However, further increasing Co to 12.0 wt% led to continuous segregation of brittle proeutectoid Ti 2 Co phase along grain boundaries, diminished grain refinement effects, and the formation of coarse Ti 2 Co precipitates within grains. Although the Ti-12Co alloy achieved an approximate 30 % higher yield strength relative to Ti-4.0Co due to intense precipitation strengthening, its ductility deteriorated severely. Consequently, the Ti-8.4Co alloy demonstrates the optimum balance of strength and ductility among the studied compositions.