Selection Of Oxide Crucibles Used For Melting Nb-Based Ultrahigh Temperature Alloys And The Preparation Of Inert Coatings On Graphite Crucibles

At present, the directional solidification (DS) of the Nb-based ultrahigh temperature alloy was mainly conducted by either Czochralski method in a water-cooled copper crucible or the methods of electron beam floating zone melting, optical floating zone melting and induction floating zone melting without crucibles. In order to find out the crucible suitable for the whole directional solidification of this kind of alloy, thermodynamic calculation has been firstly employed to analyze the reactivity of several oxides with the Nb-based ultrahigh temperature alloy at high temperatures. Then the Nb-based ultrahigh temperature alloy was remelted respectively in pure ZrO₂, Y₂O₃ or Al₂O₃ crucibles. Furthermore, a compound coating composed of inner CVD SiC layer and outer Y₂O₃ soak and painted layer was successfully prepared on graphite crucibles and then the Nb-based ultrahigh temperature alloy was remelted in this kind of crucibles. The microstructure of the reaction zones was examined using an optical microscope(OM), a scanning electron microscope(SEM) equipped with an energy dispersive X-ray spectrogram analyzer(EDX).Through thermodynamic calculation, it has been found that both yttria and beryllia have good stabilities to liquid Nb-based ultrahigh temperature alloy at 1027 °C～2027 °C.After Nb based ultrahigh temperature alloy was remeled in ZrO₂ crucibles at 1780°C and 1810°C for 30 minutes respectively, big blocks of (Nb,Ti)₅Si₃ was found in the edge of the alloy. While in the middle of the alloy, the microstructure was similar to that of the arc-melted ingot. In the alloy, the element of Zr was mainly distributed in the phase of HfO₂. The melt contaminated the ZrO₂ crucibles mainly through the reaction of Ti and Hf to them, and the contamination at 1810°C was more serious than that at 1780°C.The Nb-based ultrahigh temperature alloy was remeled in Y₂O₃ crucibles at 1780°C for 10 minutes, 1810°C for 30 minutes and 1830°C for 30 minutes respectively. It has been found that with the increasing of the remelting temperature, the alloy melt contaminated the crucibles through the reaction of Ti and Hf more seriously. A layer of Y enriched oxides existed at the edge of the alloy, and its thickness increased from 60μm at 1780°C to 120μm at 1830°C. In the alloy, the...