Evidence of the transition from ordered to disordered growth during rapid solidification of an intermetallic phase

The dendrite growth velocity during solidification is measured on liquid drops of the intermetallic compound Ni50Al50 undercooled by levitation up to 265 K. A sharp increase of the growth velocity is found at a critical undercooling ΔT*≈250 K. In situ diffraction of synchrotron radiation on levitation-processed samples unambiguously shows a transition from ordered to disordered growth at ΔT*. The sharp interface model is extended to describe the transition from ordered to disordered dendrite growth by taking into account the velocity dependence of the order parameter and the kinetic growth coefficient.