Disorder in MAX phases at the atomic scale.

Atomic disordering in materials alters their physical and chemical properties and can subsequently affect their performance. In complex ceramic materials, it is a challenge to understand the nature of structural disordering, due to the difficulty of direct, atomic-scale experimental observations. Here we report the direct imaging of ion irradiation-induced antisite defects in MAX phases using double C-corrected scanning transmission electron microscopy and provide compelling evidence of order-to-disorder phase transformations, overturning the conventional view that irradiation causes phase decomposition to binary fcc-structured MX. With the formation of uniformly distributed cation antisite defects and the rearrangement of X anions, disordered solid solution γ-(MA)X phases are formed at low ion fluences, followed by gradual transitions to solid solution fcc-structured (MA)X phases. This study provides a comprehensive understanding of the order-to-disorder transformations in MAX phases and proposes a method for the synthesis of new solid solution (MA)X phases by tailoring the disorder.