NONEQUILIBRIUM SOLIDIFICATION IN UNDERCOOLED NI-B ALLOYS

Electromagnetic levitation has been used to melt and undercool Ni(100-x)B(x) alloys (x = 0.7, 1.0). Dendrite growth velocities in the levitated droplets have been measured as a function of undercooling. Owing to the extremely small solubility of boron in solid nickel, the growth velocities in the alloys are much lower than those in pure nickel. At a critical undercooling DELTA-T* a sudden rise in the growth velocities with increasing undercooling is observed, indicating a transition from growth controlled both by solutal and thermal diffusion to a purely thermally controlled and segregation free growth of the supersaturated Ni-B dendrites.