Zirconia Refractory For Melting Nickel Based Superalloy

Nickel-based superalloy has been widely used in aircraft and power-generation turbines for its excellent mechanical properties.Vacuum induction melting（VIM）processing has been the primary melting method for nickel-based superalloy.But VIM is the only vacuum melting method which uses a refractory crucible made of oxides such as Al₂O₃ and MgO.Unfortunately,the refractory is a contamination source of oxygen because of metal/refractory reactions leading to crucible disintegration.Oxygen is a harmful trace element that exists both in solid solution and oxide inclusions in the superalloy.Oxide inclusions can act as crack initiation sites and propagation paths,so it can dramatically affect the properties of the nickel-based superalloy.Therefore,it is important to design an appropriate refractory to melt the superalloy.The MgO-PSZ refractory has already become one of the most important materials because of its corrosion resistant,excellent chemical and thermomechanical properties.In this work,the optimized sintering conditions for the MgO-PSZ refractory and effect of MgO content on the properties of MgO-PSZ were studied.Interfacial behaviors and corrosion mechanism of MgO-PSZ refractory by nickel-based superalloy were investigated according to simulated corrosion experiment.Moreover,the corrosion resistant of MgO-PSZ was improved by doping Al₂O₃ in the materials.Finally,the effect of refractory on oxygen content in nickel based superalloy was confirmed by VIM processingThe optimized sintering conditions of MgO-PSZ were determined using MgOZr O₂ phase diagram,high temperature dilatometry and differential scanning calorimetry analysis.The effect of MgO content on properties of MgO-PSZ was studied.It shows that 3.5 wt% MgO-PSZ had the highest ratio of bend strength to Young’s modulus and the lower value of CTE.Interfacial behaviors of 3.5 wt% MgO-PSZ substrate by nickel-based superalloy were investigated by simulated corrosion experiment.Interfacial reaction between MgO-PSZ substrate and nickel-based superalloy occurred.The Al₂O₃ was formed because of the reaction between the aluminum from nickel-based superalloy and the oxygen from substrate.Subsequently,the Al₂O₃ reacted with MgO from 3.5 wt% MgOPSZ substrate,leading to the formation of MgAl2O4.Hence,the destabilization of 3.5 wt% MgO-PSZ ceramic was caused by the presence of Al₂O₃.With increasing the number of simulated corrosion experiment,the spallation of 3.5 wt% MgO-PSZ substrate occurred.Chemical process and thermal cycling lead to the corrosion of 3.5 wt% MgO-PSZ.Chemical process is the main factor for 3.5 wt% MgO-PSZ corrosion.At the same time,thermal cycling accelerated corrosion of 3.5 wt% MgO-PSZ.In order to improve the corrosion resistant of 3.5 wt% MgO-PSZ,（Al₂O₃,MgO）-PSZ were synthesized via conventional solid-phase reaction.When the Al₂O₃ content was 0.8 wt%,the cyclic corrosion resistant of 3.5 wt% MgO-PSZ improved obviously.The Al₂O₃ as dopant reacted with stabilizer MgO in 3.5 wt% MgO-PSZ substrate to form MgAl2O4.Further,the stability of MgO in matrix improved.Hence,diffusion rate of MgO from matrix to interface reduced and the chemical reaction rate between MgO and interface product Al₂O₃ decreased.Furthermore,the effect of chemical reaction on 3.5 wt% MgO-PSZ reduced.Besides,the thermal cycling stability of 3.5 wt% MgOPSZ doping with Al₂O₃ improved.The effect of refractory on oxygen content in nickel based superalloy was investigated by VIM using 3.5 wt% MgO-PSZ doping with 0.8 wt% Al₂O₃ crucible,respectively.There is a little influence of 3.5 wt% MgO-PSZ doping with 0.8 wt% Al₂O₃ crucible on oxygen content during VIM processing.In summary,3.5 wt% MgO-PSZ doping with 0.8 wt% Al₂O₃ has good chemical stability and thermal cycling stability.It shows a potential applications for melting nickel based superalloy.