Mechanical destabilization enables defect-driven phase transformation in layered MAX phases

Mechanical processing is widely used to refine powders and control microstructure, yet its impact on the intrinsic phase stability of layered ceramics remains poorly understood. Here, we demonstrate that high-energy ball milling drives the MAX phase Ti 3 AlC 2 into a metastable, defect-rich state that fundamentally alters its thermally activated phase evolution. Mechanical destabilization activates thermally-driven solid-state transformation pathways, leading to the formation of TiC x and Al 2 O 3 during subsequent heat treatment. This transformation is initiated by outward diffusion and surface oxidation of Al, while coherent TiC x laths form in situ within the MAX grains through a defect-assisted solid-state mechanism. Leveraging this mechanically induced metastability, we establish a route to fabricate dense Al 2 O 3 /TiC x /Ti 3 AlC 2 nanocomposites with a refined microstructure, high hardness of 13 GPa, and retained fracture toughness of 7.6 MPa·m 1/2 . These findings provide a mechanistic framework for defect-driven phase transformations in layered materials and introduce mechanical activation as a deliberate design tool for tailoring phase evolution and functional performance.