Xiaoxia Tang

Interactions between γ-TiAl melt and Y2O3 ceramic material during directional solidification process

Abstract A γ-TiAl alloy with nominal composition of Ti-47%Al (molar fraction) was directionally solidified in an alumina mould with an Y 2 O 3 protective coating. The effects of processing parameters (melting temperature and interaction time) on the metal-coating interface, microstructure and chemical composition of the alloy were evaluated. The result shows that the Y 2 O 3 protective coating exhibits an effective barrier capability to avoid direct contact between the mould base material and the TiAl melt, although the Y 2 O 3 coating is found to suffer some erosion and be slightly dissolved by the molten TiAl due to the coating-metal interactions. The directionally solidified alloys were contaminated with Y and O, and Y 2 O 3 inclusions were dispersed in the metal matrix. The reason for this metal contamination is the Y 2 O 3 coating dissolution by the TiAl melt. One mode of the interaction between Y 2 O 3 and the TiAl melt is dissolution of yttrium and atomic oxygen in the melt by reaction Y 2 O 3 (s)=2Y (in TiAl melt)+3O (in TiAl melt). Both the extent of alloy contamination and the volume fractions of Y 2 O 3 inclusions depend on the melting temperature and the interaction time.

Thermodynamic analysis of interactions between Ti–Al alloys and oxide ceramics

Abstract The thermodynamics of interactions between various oxides (CaO, MgO, Al2O3 and Y2O3) and molten Ti and Ti alloys was investigated. The dissolution mechanism of oxides in molten Ti alloys was provided and the stability of oxides in molten Ti alloys was investigated and predicted by thermodynamic analysis. Interactions between oxides and Ti-Al melts were studied by oxide crucible melting experiments. By quantitative analysis, it is indicated that impurity contents in alloys are proportionally decreased with increasing the Al content in alloys and decreasing the melt temperature, which is in agreement with the results of the predicting thermodynamic stability.

Interactions between TiAl Alloy and Different Oxide Moulds under High-temperature and Long-time Condition

Abstract Interactions between Ti-47Al-2Cr-2Nb (at.%) alloy and alumina, zirconia and yttria moulds were studied under high-temperature and long-time condition, and the reaction mechanisms were explored. The moulds were prepared by traditional precision casting technique, and the experiments were carried out in a carbon resistance furnace at 1550°C for 30 min. The results indicated that the interfacial reactions between TiAl alloys and alumina and zirconia moulds were serious. Obvious reaction layers composed of Ti-rich phases and Al2O3 inclusions were formed on the alloy-mould interfaces, and a large amount of dark Al2O3 inclusions were found in the alloy matrixes. Interaction between TiAl and yttria mould was mild. No obvious reaction layer was formed on the alloy-mould interface, and only a small amount of white inclusions inferred as Al-Y oxide were found in the alloy matrix. Al2O3 and ZrO2 cannot be used as mould materials for the directional solidification of TiAl alloys. Y2O3 mould possesses a higher stability, which can be used in the directional solidification of TiAl alloys by the proper choice of more stable binder.

Microstructure and Mechanical Property Optimization of NiTiAl-based Alloys: Directional Solidification in Novel Ceramic Crucibles

Abstract Directional solidification technique was successfully used to prepare highly reactive NiTiAl-based alloys using novel Y2O3-coated crucibles. Compared with the as-cast alloys fabricated by vacuum arc melting, the microstructure of the directionally solidified alloys was optimized and mechanical properties remarkably improved. After directional solidification, the microstructure of the alloys became much finer and preferentially grew along [001] orientation. The tensile strength and strain of the directionally solidified alloys at 1550°C were promoted to 1240 MPa and 2.1% respectively, which were 91.4% and 75% higher than that of the as-cast alloys. The strength increased up to 1328 MPa and 1419 MPa after homogenizing treating. The Y2O3-coated crucibles assured significant purification effect of the alloys. The maximum oxygen increase in the steady-state region of the ingot obtained at 1750°C was no more than 0.014 wt.%.