MICROSTRUCTURE COOLING RATE CORRELATIONS IN MELT-SPUN ALLOYS

Photocalorimetric techniques have been used to measure top surface temperatures during melt spinning of Ni-Al and 316L stainless steel ribbons, in order to investigate the effect of cooling rate on the melt-spun alloy microstructures. Cooling conditions during melt-spinning are found to be near-Newtonian, with mean cooling rates, heat transfer coefficients and Nusselt numbers in the range 4 x 10(4) to 5 x 10(5) K sec-1, 5 x 10(4) to 3 x 10(5) W m-2K-1 and 0.07 to 0.22, respectively, for wheel speeds in the range 4 to 36 m sec-1. The cooling rate during melt-spinning is directly proportional to the wheel speed and inversely proportional to the square of the ribbon thickness. Melt-spun Ni-Al and 316L stainless steel ribbons exhibit a columnar through-thickness solidification microstructure, with a segregation-free region adjacent to the wheel surface. Solidification takes place by heterogeneous nucleation of the undercooled liquid on the wheel surface, followed by partitionless solidification during recalescence, and finally cellular breakdown and segregated solidification. The columnar grain size decreases and the fractional segregation-free thickness increases with increasing wheel speed and cooling rate, indicating that the nucleation undercooling in the liquid is proportional to the cooling rate.