Investigations On Interfacial Reactions Between Molten Alloys And Oxide Refractories

In recent years,the main method for metallurgy of special alloy is vacuum induction melting in the process of which chemical attack of molten alloy on the inner line of crucible cause the dissolution of refractories and its detachment from crucible.with increasingly required quality of special alloy and the need for ultra-pure smelting technology,the influence of refractory materials on alloy quality has been paid more attention.In order to reduce the contamination of refractories and ensure the purity of alloy melts,ordinary refractory materials can no longer reach the demands of smelting special alloy.Therefore,it is of important theoretical and practical significance to study the mechanism of interaction between refractory materials and special alloy melts.Cold isostatic pressing and solid phase sintering technology were applied to prepare high-purity oxide crucibles:aluminum oxide,magnesium oxide,calcium oxide fully stabilized zirconia(CSZ),yttria partially stabilized zirconia(Y-PSZ),calcium oxide and yttrium oxide crucible.The bulk density and apparent porosity of samples were evaluated by an apparent porosity tester.The phases compositions of prepared crucibles were characterized by X-ray diffraction and the surfaces and fracture of morphologies were observed based on field emission scanning electron microscope.Then each kind of oxide crucibles was used to smelt the nickel-based superalloy K417G and Ti-5wt.%Cu alloy.Using FEI tungsten filament scanning electron microscope and energy spectrometer to observe the interaction between the melt and the refractory material.Finally,chemical analysis techniques such as ICP,OES and XRF were employed to determine the impurity content of the alloy after melting.The results are as follows:(a)Under molding pressure of 280Mpa of the cold isostatic pressing and the sintering temperature of 1680℃,the densification sequence of refractory materials is as follows:CaO>Al2O3>MgO>Y-PSZ>Y2O3>CSZ and their apparent porosity is 1.27%,1.23%,1.23%,15%,21.5%and 28%,respectively.As mentioned above,Al2O3,CaO and MgO crucibles are easier to sinter than Y-PSZ,CSZ and Y2O3.(b)At a temperature of 1600℃,six kinds of crucibles,Al2O3,CaO,MgO,Y-PSZ,CSZ and Y2O3,were employed to smelt the nickel-based superalloy K417G respectively.The results indicate that the physical erosion is the primary mechanism of interaction between Al2O3 crucibles and alloys and the readily detached particles of Al2O3 result in its inclusion in the alloys;a liquid phase of Ca3Al2O6 produce at melt-crucible interface promote the wettability between the alloy and CaO,resulting in serious adhesion at the interface;the reaction of MgO crucible to aluminum in alloy cause MgAl2O4 at the melt-crucible interface which enter consequently the alloy to form inclusions;No corrosion of Al2O3 reaction layer to the Y-PSZ crucible indicate its possession with excellently resistance corrosion to slags of Al2O3;the interaction between the CSZ crucible and the alloy generates a liquid phase CaAl2O4,which cause the crucible to become unstable and to dissolve into the alloy;Al2Y4O9 reaction layer mainly formed at the interface between Y2O3 and alloy,and Y2O3 dissolves into the alloy more seriously compared to other oxide refractories.Melts-crucible interaction has also a significant impact on the oxygen contents of K417G.No increase in oxygen concentration of alloy fused by CaO,Y2O3 and CSZ crucible,respectively,while the oxygen content of alloy melted in CSZ,MgO and Al2O3 crucible rise from 7ppm to 11ppm,34ppm and 13 5ppm respectively.(c)The used four refractory materials CaO,Y-PSZ,CSZ and Y2O3 smelted Ti-5wt.%Cu alloy at the temperature of 1680℃ for 15 minutes.The results reveal that the CaO refractory material decomposes violently,which causes a lot of bubbles formed at the interface,the melt only penetrates the crucible physically,and no new phase formation is detected;In addition to the physical penetration of the melt,the interaction between Y-PSZ and Ti-5wt.%Cu alloy also include a diffusion of Y-PSZ into the alloy at the interface and it is found that new phases of Ti2ZrO and Ti3O formed;CSZ crucible is most severely affected by melt penetration and chemical erosion.Y2O3 shows excellent thermodynamic stability and resistance to melt penetration during smelting Ti-5wt.%Cu alloy.The interaction interface between the alloy and the melt is clearly visible.No melt penetration into the crucible is found,but the exfoliation of Y2O3 particles into the alloy results in many Y2O3 inclusions in the alloy.The oxygen content in the Ti-5wt.%Cu alloy smelted by each kind of oxide crucibles,CaO,Y-PSZ,CSZ and Y2O3 is 1.1%,3.6%,5.48%and 0.42%,respectively.According to the experimental phenomenon,the stability order of the four refractory materials tends to be consistent with the thermodynamic calculation results as follow:Y2O3>CaO>Y-PSZ>CSZ.