Evidence for a transition from diffusion-controlled to thermally controlled solidification in metallic alloys

Dendrite growth velocities have been measured as a function of undercooling in droplets of dilute alloys of B in Ni undercooled by up to 306 K using an electromagnetic levitation technique. At a critical undercooling, depending strongly on the B concentration, a steep rise of the dendrite growth velocity is observed. The analysis of the results within current theories of dendrite growth gives evidence for a transition from diffusion-limited to purely thermally controlled growth with partitionless solidification as a consequence.