The purity of the Ni-based superalloy is directly related to the alloy-crucible interaction during induction melting. It is thus of great significance to clarify the interaction mechanism for the achievement of preparing the high-purity superalloys. In this study, pure Ni and Ni-based superalloys were melted in MgO crucibles, respectively. The phase composition, microstructure of the crucible and the oxygen concentration of the metals were analyzed by scanning electron microscopy, X-ray diffractometry and O/N analyzer. The interaction between the metals and the crucibles were investigated, and the interaction mechanism was elucidated. The results indicate that after melting pure Ni in MgO crucible, pure Ni melt exhibits a good wettability to the crucible, but little interaction occurs. However, a significant interaction between the superalloy melt and MgO crucible occurs. This reaction primarily involves the dissolution and decomposition of MgO in the alloy melt. The decomposed element O reacts with element Al to form Al₂O₃ products, which can float to the surface of the superalloy. Also, some Al₂O₃ can further react with MgO crucible matrix to generate MgAl₂O₄ product, which can attach to the inner surface of the crucible, and float into the slag, respectively. A slag layer with the thickness of approximately 80 μm composed of Al₂O₃, MgAl₂O₄, and a portion of the alloy can be formed on the surface of the alloy.

In this study, an industrial grade MgO (purity>99.5%) was used as a raw material. The MgO powder was mixed and ground in ethanol in a concrete mixer with yttria stabilized zirconia (YSZ) balls at a speed of 300 r/min. The mass ratio of powders, YSZ balls and ethanol were 3.0 ∶ 5.0 ∶ 0.8. The ground powder was then dried in an oven for 12 h. The MgO crucible green body was fabricated via cold isostatic pressing at 150 MPa for 3 min. Subsequently, the green body was sintered in a high-temperature silicon molybdenum rod sintering furnace at 1 750 ℃ for 6 h. The outer diameter, inner diameter, and height of the sintered crucible are 60, 50 and 70 mm, respectively.

An electrolytic pure Ni and DD419 alloy was used as an experimental metal. Pure Ni of 100 g was ground and acid pickled. The DD419 alloy was cut into 2 cm×2 cm×4 cm block caret, and its weight was 120 g. After grinding and acid pickling, the alloy was used as an experimental metal. The sintered MgO crucible was placed in a coil of a vacuum induction melting furnace, and filled with fused MgO sand around the crucible. It could prevent the short circuit of the induction coil due to the leakage of the molten metal. The experimental metal was inserted into the crucible, and the furnace was evacuated with a mechanical pump and a molecular pump to approximately 1×10^-2 Pa. The furnace was backfilled with high-purity argon to 0.06 MPa, and repeated for three times in order to eliminate the influence of the residual oxygen. The melting experiment was conducted in high vacuum. The metal temperature was raised slowly until the appearance of the liquid at the bottom of the metal. The temperature of the molten metal rapidly increased to 1 550 ℃, and hold at this temperature for 20 min. Finally, the molten melts were cooled in the crucibles to obtain the corresponding samples. After melting, the samples were cut via wire-electrode cutting and polished for the coming use.

The surface of pure Ni after melting in the MgO crucible is smooth without the appearance of the impurities. However, the surface of the superalloy has a black slag layer with a thickness of 80 μm, indicating that the slag is formed during the interaction between the alloy and the crucible. The surface of the slag can form a wrinkled shape due to the difference in the cooling rate between the slag and the alloy melt. No corrosion phenomenon occurs on the surface of the crucible after melting pure Ni. However, some Ni melt is residual on the surface of the crucible due to the wettability. In contrast, after melting the alloy in the crucible, the grain morphology and grain boundaries on the surface of the crucible disappear, indicating that the surface of the crucible is corroded by the alloy melt, thus leading to the generation of MgAl₂O₄ products. The slag primarily consists of Al₂O₃ and MgAl₂O₄ as well as some residual alloy melt, respectively. The generation of Al₂O₃ and MgAl₂O₄ products is attributed to the reaction at the melt-crucible interface. These products can float into the surface of the alloy melts under the electromagnetic stirring because Al₂O₃ and MgAl₂O₄ products have a lower density than that of the alloy melts. The thermodynamic stability for Al₂O₃, MgO, Cr₂O₃ and NiO indicates that element Al in the alloy has an intense affinity for element O, leading to the dissolution of MgO refractory and the oxygen contamination of the alloy melt. The uneven distribution of MgAl₂O₄ in both the inner surface of the crucible and the slag exhibits that the interaction and electromagnetic stirring significantly affect the formation of the slag.

Pure Ni did not react with MgO crucible. However, DD419 alloy could exhibit a significant interaction with the crucible, resulting in the alloy contamination and the formation of a slag layer. The thickness of the slag layer was 80 μm, and it consisted of Al₂O₃, MgAl₂O₄ and some alloy. In addition, Al₂O₃ and MgAl₂O₄ were the interaction products between the alloy melt and the crucible. The dissolution of MgO refractory could result in the release of elements Mg and O, respectively. Element Al reacted with element O to generate Al₂O₃ products. A portion of the products could float, and another part could react with MgO matrix on the inner wall of the crucible to form MgAl₂O₄. Meanwhile, some of the generated MgAl₂O₄ was adhered to the inner wall of the crucible, and another portion was floated into the slag layer.