From Ilmenite to High-Purity Ti4Al(C,N)3/Al2O3 Composites: A Clean Pathway to MAX-Phase Materials with Tailored Lattice Integrity

The high cost of conventional titanium feedstocks has severely constrained the industrial application of Ti–Al-based MAX-phase composites. This work presents a clean, cost-effective pathway for synthesizing Ti 4 Al(C, N) 3 /Al 2 O 3 composites directly from natural ilmenite concentrate. The synthesis encompasses two sequential stages: (i) the reduction and carbonitridation of pre-oxidized ilmenite in a hydrogen-rich atmosphere to yield a Ti(C, N, O) precursor, and (ii) pressureless sintering of the precursor with Al and Ti powders, followed by acid leaching to remove intermetallic byproducts. Through systematic parametric optimization, the precursor synthesis was established at 1150 °C for 70 min in a 4 vol pct CH 4 –80 vol pct H 2 –16 vol pct N 2 atmosphere, achieving a carbonitridation ratio exceeding 90 pct while suppressing detrimental carbon deposition. Subsequent pressureless sintering at 1500 °C for 4 h with a molar ratio of 2.2Ti(C, N, O):2Al:1Ti yielded the target composite. Mechanistic investigation, combining experimental characterization and density functional theory (DFT) calculations, reveals that the Al 3 Ti intermediate phase plays a dual critical role: it promotes deep deoxygenation of Ti(C, N, O) with in situ formation of Al 2 O 3 reinforcements, while simultaneously healing N vacancies in the MAX-phase lattice. Aberration-corrected transmission electron microscopy (AC-TEM) confirmed that the synthesized Ti 4 Al(C, N) 3 solid solution possesses a pristine hexagonal structure identical to that of phase-pure Ti 4 AlN 3 , with no observable lattice distortion or extended defects. This work presents a novel strategy for the economical and sustainable synthesis of high-performance Ti 4 Al(C, N) 3 /Al 2 O 3 composites.