Mo clustering-induced β precipitation in α″ martensite in α + β titanium alloys

The decomposition of α″ martensite in α+β titanium alloys strongly influences the mechanical properties of these alloys. Although α″ martensite is known to decompose into an α matrix and β precipitates with two crystallographic variants, the mechanism governing β precipitation within α″ martensite remains unclear. In this study, we demonstrate that solute Mo clustering within α″ martensite precedes and induces β precipitation. Using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and first-principles calculations, we systematically elucidate the clustering-assisted nucleation and growth mechanisms of the β phase in α″ martensite of a Ti–9wt%Mo model alloy. Mo clusters are observed for the first time within α″ martensite prior to β phase nucleation. First-principles calculations reveal that their formation arises from a strong mutual attraction between Mo atoms, with clustering initiated by the pairing of two adjacent Mo columns. These Mo clusters introduce local lattice distortion that closely resembles the β structure, thereby providing a structural template for β formation with an unconventional orientation relationship of [011] β // [001] α″ and ( 01 1 ¯ ) β // (010) α″ . During subsequent growth, the β phase undergoes a clockwise or anticlockwise lattice rotation of approximately 3.6°, enabling coherent edge-to-edge matching with the surrounding α″ martensite. This rotation increases the number of β variants from one to two and drives the orientation relationship towards the conventional Burgers relationship. These findings provide new mechanistic insight into the decomposition of α″ martensite and are expected to be applicable to a broader class of α+β titanium alloys.