  Despite the fundamental importance of solid-solid transformations in many
technologies, the microscopic mechanisms remain poorly understood. Here, we
explore the atomistic mechanisms at the migrating interface during solid-solid
phase transformations between the topologically closed-packed A15 and
body-centred cubic phase in tungsten. The high energy barriers and slow
dynamics associated with this transformation require the application of
enhanced molecular sampling approaches. To this end, we performed metadynamics
simulations in combination with a path collective variable derived from a
machine learning classification of local structural environments, which allows
the system to freely sample the complex interface structure. A disordered
region of varying width forming at the migrating interface is identified as a
key physical descriptor of the transformation mechanisms, facilitating the
atomic shuffling and rearrangement necessary for structural transformations.
Furthermore, this can directly be linked to the differences in interface
mobility for distinct orientation relationships as well as the formation of
interfacial ledges during the migration along low-mobility directions.
