Microstructural and Crystallographic Evolution of Ti-Al-B-(Y2O3) Powders with Varying Milling Durations

Abstract

This study investigates the mechanochemical synthesis and microstructural evolution of Ti₆₀Al₄₀+B₃(Y₂O₃)₅ powders subjected to high-energy mechanical alloying for 12, 24, and 48 hours. CALPHAD-based thermodynamic simulations predicted multiphase stability regions and quantified the distribution of Ti, Al, B, and Y in equilibrium phases, including the activities of each constituent, establishing a theoretical baseline for comparison with experimentally induced metastable states. Mechanical alloying produced progressive grain refinement, with median grain sizes decreasing from 0.07 μm (12 h) to 0.04 μm (48 h), consistent with severe plastic deformation and defect accumulation. Crystallite size analysis indicated a significant reduction from 18.054 nm at 12 hours to 7.042 nm at 48 hours, reflecting enhanced lattice strain and grain refinement induced by prolonged milling. SEM and EDS analyses confirmed improved homogenisation, enhanced Ti–Al mixing, and the evolution from discrete elemental regions to a chemically uniform nanostructure. X-ray diffraction revealed a clear reaction pathway characteristic of a fully reactive MA system. At 12 h, only elemental α-Ti, fcc-Al, and residual Y₂O₃ were detected, reflecting limited reaction. After 24 h, the emergence of TiAl, Ti₃Al, and TiB₂ peaks signalled the onset of intermetallic and boride formation. By 48 h, the powders fully transformed into a nanocrystalline TiAl/Ti₃Al matrix reinforced by TiB₂, while Y₂O₃ became largely amorphous and incorporated into a disordered Y–Ti–O phase. The complete disappearance of Ti and Al reflections confirms full consumption of reactants, consistent with a fully reactive mechanochemical process. It was observed that extended milling produced a highly refined, chemically homogeneous nanocomposite consisting of TiAl/Ti₃Al intermetallics, TiB₂ ceramic reinforcements, and an amorphous Y–Ti–O dispersoid phase. These findings provide insight into phase evolution mechanisms and offer valuable guidance for optimizing MA parameters for Ti–Al–B–Y₂O₃ powder systems aimed at high-performance structural applications.

© 2026 The Authors. Published by ELSEVIER Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0)

Peer-review under responsibility of the scientific committee of the NAMRI|SME.