The solidification velocity of pure nickel

The solidification velocity of pure nickel processed in three different gas environments was measured using an ultra-high speed imaging technique (1 million frames per s). The results indicate that present solidification theories do not correctly predict the solidification behavior of undercooled metals. The solidification velocity as a function of undercooling was measured for pure nickel processed in 99.999% pure helium, processed in helium/20 wt.% hydrogen, and processed in a contaminated gas environment. The results indicate a transition in the solidification velocity/undercooling relationship in the first two data sets and a continually increasing, but significantly depressed, relationship for the last data set. A solidification model is proposed to explain the correlation and the deviation of the experimental results to solidification theory. The model asserts that at low undercoolings the solid–liquid interface is comprised of dendrites spaced widely enough apart that adjacent dendrites do not thermally interact. As the undercooling increases, the dendrites become more closely spaced and the thermal fields surrounding each dendrite begin to overlap. This occurrence causes the transition in the solidification velocity/undercooling relationship and the reduced dependence of the solidification velocity on undercooling at large undercoolings. Based on the experimental results, careful examination of the Boettinger, Corriell, and Trivedi (BCT) theory, and the proposed solidification model it is concluded that collision limited growth is an incorrect assumption for nickel solidifying at the undercoolings attained by electromagnetic levitation.