Study on the composition optimization method for improving the fluidity of cast Ti$_2$AlNb alloy and its mechanism

In this paper, the effects of Al, Nb main elements, Fe, Mo, W, Co, B, Si and their contents on the fluidity of Ti-22Al-25Nb alloy were investigated. The composition that was beneficial to improve the fluidity was screened through the thermodynamic software calculating thermophysical parameters affecting the fluidity of Ti$_2$AlNb alloy, the numerical simulation test of its fluidity and the verification test of the fluidity of optimized alloys. Finally, the improvement mechanism of the alloy fluidity was discussed. Results showed that the appropriate reduction of Nb element was better than Al element for the improvement of fluidity. The addition of trace Fe, B and Si elements were beneficial to the improvement of fluidity, the improvement effect of B element was best, while the addition of trace Mo, W, Co were not conducive to the improvement of fluidity. The cessation mechanism of Ti$_2$AlNb alloy is the cessation mechanism of the alloy with a wide crystallization temperature range. The composition which was most beneficial to improve the fluidity was Ti-22Al-24Nb-0.1B. The main reasons for the improvement of the fluidity had two sides: on the one hand, the reduction of 1at% Nb and the addition of 0.1at% B not only increased the superheat and crystallization latent heat of the alloy, but also reduced the melt viscosity and thermal conductivity, thus improving the fluidity. On the other hand, the TiB phase refined the grains, the fine grains prevented the dendrite from growing into developed dendrite networks, inhibited the adverse effect of the increase in the width of the solidification zone on the fluidity, reduced the flow resistance of the molten metal, and further improved the fluidity of the alloy.