Microstructure and mechanical properties of as-cast Ti2AlNb alloys

• Cooling rate influences chemical ordering progression from CMRO to CQLRO to LRO in Ti 2 AlNb alloys. • “Phase transition zone” at BCC/O interface reveals complex B2 → B19 → O phase transformation pathway. • Quantitative relationships established between cooling rate and dendrite size/second dendrite arm spacing. • Ti 2 AlNb with CMRO shows optimal ductility; CQLRO exhibits highest tensile strength. A Ti-22Al-25Nb (at.%) alloy was solidified in molds made of copper, graphite, alumina ceramics, and quartz sands with decreasing cooling rates, to investigate the as-cast microstructure and mechanical properties. The alloy solidifies merely into a body-centered cubic (BCC) phase in the copper mold, while additional O or α2 phases precipitate in the other samples. As the cooling rate decreases, the BCC matrix undergoes ordering transitions from chemical medium-range order (CMRO), quasi-long-range order (CQLRO), to long-range order (CLRO, i.e., B2 phase). The formation of secondary phases depends strongly on solute concentration and crystal defects, as indicated by their preferential precipitation in Al- and Ti-rich regions and at grain boundaries. Detailed TEM analyses reveal a “phase transition zone” at the BCC/O phase interface, indicating a metastable transition structure from BCC to O phase. In response to cooling rate and resulting microstructure, the alloy solidified in the copper mold exhibits optimal ductility up to 10.49 %, while the graphite mold yields the highest tensile strength of 1145 MPa. This research provides insights into the relationships between solidification conditions, microstructure, and mechanical behavior in as-cast Ti 2 AlNb alloys, offering a foundation for optimized alloy design and processing strategies