Promoting in-situ martensite decomposition in PBF–LB α+β Ti alloys by Mn addition

Powder Bed Fusion–Laser Beam (PBF–LB) enables the fabrication of complex geometries but usually produces undesirable α’ martensitic microstructures in α+β Ti alloys. To address this issue through alloy design, we investigated the effect of Mn on the facilitation of in-situ martensite decomposition. As-built Ti-2Mn developed α+β microstructures at lower volumetric energy densities (VEDs) than as-built Ti-6Al-4V, despite their comparable β stability at martensite decomposition temperatures. Moreover, in Ti-2Mn, the α phase showed partial Mn depletion at a VED of 152 J/mm 3 and nearly reached equilibrium in Mn content at a VED of 357 J/mm 3 . Meanwhile, Ti-6Al-4V retained an intermediate V content in the α phase between the equilibrium and α’ martensite states, even at 357 J/mm 3 . Due to the facilitated martensite decomposition, the uniform elongation of Ti-2Mn improved from 6.3 to 7.8%, while its ultimate tensile strength decreased only slightly (928 to 857 MPa). Although its strength was lower than that of Ti‑6Al‑4 V, Ti‑2Mn still achieved approximately 2.5‑times higher strength than as‑built commercially pure Ti while maintaining sufficient ductility without post‑heat treatment. These results demonstrate that Mn addition provides a practical alloy‑design strategy for enabling in‑situ formation of α+β microstructures in PBF–LB Ti alloys, offering a complementary pathway to conventional process‑parameter‑based microstructure control. • Ti-2Mn and Ti-6Al-4V have similar β stability at the decomposition temperature. • Martensite decomposition was analyzed via microstructures and solute partitioning. • In-situ martensite decomposition was more efficient in Ti-2Mn than in Ti-6Al-4V. • Ti-2Mn shows ~2.5 times higher strength than pure Ti. • Ti-2Mn has excellent uniform elongation even in as-built conditions.