[en] Highlights: • Y₂O₃ matrix mould shells with yttria face coats doped with different oxides were prepared via lost wax process. • Discontinuous Ti solid solution layer existed along the boundary of alloys re-melted in Y₂O₃ mould shells doped with oxides. • Two reaction layers could be found between the alloys and Y₂O₃ mould shells doped with oxides. • Mould shells of pure yttria and yttria doped with La₂O₃ + ZrO₂ are fit for the casting of Nb-Si based alloys. To find the best mould shells for investment casting of Nb-Si based ultrahigh temperature alloy, Y₂O₃ mould shells with zirconia face coat and yttria face coats doped without and with CaO + ZrO₂, MgO + ZrO₂, La₂O₃ + ZrO₂ and CeO₂ + ZrO₂ were prepared and their interactions with the melt of the alloy at 1850 °C for 30 min were investigated. The results show that the interfaces between the alloy and the mould shells with yttria face coats doped with CaO + ZrO₂, MgO + ZrO₂ and La₂O₃ + ZrO₂ during re-melting process all comprise two types of reaction layers, which are mainly composed of HfO₂ + Y₂O₃ and TiO + HfO₂. The interface between the alloy and the mould shell with yttria face coats doped with CeO₂ + ZrO₂ is also composed of two types of reaction layers, which consist of HfO₂ + Y₂O₃ and cubic HfO₂ respectively. A single reaction layer consisting of HfO₂ + Y₂O₃ constitutes the interface between the alloy and the pure yttria mould shell, and the interface between the alloy and the mould shell with zirconia face coat is also composed of a single reaction layer but consisting of cubic HfO₂. Both pure yttria and yttria doped with La₂O₃ + ZrO₂ mould shells are more suitable for the investment casting of the alloy.