Effect of carbon and aging treatment on precipitation of ordered α2 in Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd alloy

The effect of carbon and heat treatment on microstructural stability of a near-α titanium alloy Ti–5.6Al–4.8Sn–2Zr–1Mo–0.35Si–0.7Nd during high-temperature exposure was investigated. Aging leads to the precipitation of α2 phase from α after both β- and (α + β)-solution treatment. The α2 precipitates are uniformly distributed throughout the α matrix for alloys aged at 700 °C and 750 °C, but the precipitation is heterogeneous, preferring dislocations or other substructures during aging at 800 °C. Carbon has little effect on α2 precipitation for fully lamellar microstructure because there is no partitioning of alloying elements. For bimodal microstructure with ∼15% primary α (αp), however, increasing carbon addition reduces α2 particle size and increases interparticle spacing in αp after exposure at 700–800 °C for 168 h, reducing and delaying α2-related embrittlement. This effect is attributed to carbon decreasing the Al concentration of the αp and increasing the partitioning of Al and Mo to the transformed β.