Mo equivalent guided microstructural regulation and strength-toughness synergy of Fe-doped near β titanium alloy

The intrinsic strength-toughness trade-off, coupled with an inadequate understanding of microstructure regulation, restricts the further development of as-cast near β titanium (Ti) alloys with ultra-high strength and toughness. In this work, a series of Ti-7Mo-4Al-4Zr-3Nb-2Cr- x Fe alloys with different Fe contents ( x =0wt.%, 1wt.%, 2wt.%) were designed with the Mo equivalent (Mo [eq] ) ranging from 9 to 16. Results show that owing to the solute segregation and increased undercooling during solidification, the β grains of the 1Fe alloy are refined by 45%, while excessive Fe leads to slight grain coarsening. Meanwhile, the continuous decline of α phase content from 18.3% for 0Fe alloy to 4.2% for 2Fe alloy diminishes the number of α/β interfaces. The original high-density dislocation pile-up at phase boundaries is alleviated, and the dislocation density decreases significantly. The 1Fe alloy with Mo [eq] =13.24 achieves the optimal strength-toughness matching, whose tensile strength is 990 MPa and toughness is 75 MPa·m 1/2 . The combined effects of grain refinement strengthening, modulated precipitation strengthening, and optimized dislocation distribution change the deformation mode from localized strain concentration to more uniform plastic flow, which fundamentally resolves the strength-toughness trade-off. This work reveals the comprehensive mechanism of Fe-mediated microstructure evolution and performance regulation, verifies the validity of Mo [eq] design criterion, and provides a theoretical basis for the development of high-performance as-cast Ti alloy ingots.