Interfacial desulfurization reaction between binary Ni-Al/Ti/Ta alloy melt and CaO ceramic refractory

Reducing the sulfur content to be lower than 1 ppm is widely acknowledged to be an effective way to improve the oxidation properties of superalloys. CaO refractories are usually used to reduce the sulfur content during smelting process. Since the compositions of superalloys are quite complex, the role of the reactive elements, such as Al, Ti and Ta, on the desulfurization process remains unclear. In this work, the interfacial reactions of binary Ni-5Al, Ni-5Ti, Ni-5Ta melts with the CaO refractory are investigated. The results show that desulfurization at the interface between the melt/refractory will occur in all the three binary alloy systems when smelting at 1500 degrees C. The interfacial reaction layer formed by the desulfurization reaction between CaO and Ni-Al is about 250 mu m, which is much larger than the tens of microns formed by the reaction between CaO and Ni-5Ti/Ni-5Ta. A model is established to describe the interfacial desulfurization reaction: xCaO(s) + y[S]Ni(l) + z[M]Ni(l) = yCaS(s) + Ca(x_y)MzOx(l/s). During the reaction process, CaS is found to be the desulfurization product, while Ca(x_y)MzOx is the transfer medium which connects the melt and CaO refractory, transferring the sulfur and Al/Ti/Ta in the melt so as to promote the desulfurization reaction.