Controlled undercooling of liquid iron in contact with Al2O3 substrates under varying oxygen partial pressures

The objective of this study was to determine the conditions under which alumina can act as a heterogeneous nucleant to initiate the solidification of undercooled liquid iron. The undercooling of a pure iron sessile droplet in contact with Al₂O₃ substrates was measured under controlled oxygen partial pressures by observing droplet recalescence. The experimental results indicated that the undercooling of liquid iron, in contact with an Al₂O₃ substrate, did not have a unique value, varied from 0 °C to 290 °C, and was significantly affected by the oxygen content of the gas phase and the degree of interaction between the oxide and the metal. Deep undercoolings are possible at low oxygen potentials, provided the oxygen potential is such that substantial substrate decomposition does not occur. The measured undercooling was a strong function of gas phase oxygen content and a maximum in undercooling of 290 °C was measured at PO₂=10⁻¹⁹ atm. The variation in undercooling was related to the wetting of the substrate by the liquid metal, where the deepest undercoolings occurred when the highest contact angle between the substrate and the liquid droplet was achieved.