定向凝固单晶TiAl合金非择优取向调控及生长稳定性研究

The mechanical property of TiAl alloys can be improved by controlling the growth direction of primary α phase and obtaining lamellar structures parallel to the growth direction. However, for the crystal growth direction of the α phase, the evolution processing is not well understood at present. Especially, the mechanisms of the orientation control and growth stability of non-preferred growth direction are lacking for the α phase in TiAl alloys. Therefore, the aim of this study is to investigate the variation of crystal growth orientation and the growth stability of α phase with non-preferred orientation in directionally solidified single crystal TiAl alloys. According to the theory of solidification thermodynamics and kinetics, the growth orientation evolution of directionally solidified sing crystal TiAl alloy is studied by controlling the solidification conditions, such as growth rate, temperature gradient and convection intensity. The critical conditions of non-preferred growth direction growth of α phase are analyzed under the theory of interface stability during solidification. For non-preferred growth direction, the mechanism of stable growth is researched by discussion the variation of the interface energy with solidification parameters. The research results will uncover mechanisms of crystal growth orientation control and the stable growth for the α phase with non-preferred direction in α solidified TiAl alloy. The results can also enhance the solidification theory of intermetallics and provide a theoretical basis and guidance for the precise lamellae orientation control of TiAl alloys.

通过控制TiAl合金中α相沿非择优取向生长，获得平行于生长方向的片层组织，可显著提高其综合力学性能。但是，目前对于TiAl合金晶体生长取向演化规律认识不足，尤其是对α相非择优取向的调控机理及生长稳定性尚不明确。因此，本项目以初生相为α相的单晶TiAl合金为研究对象，通过调控生长速度、温度梯度和对流强度等凝固条件，结合凝固热力学和动力学，研究定向凝固条件下单晶TiAl合金中α相晶体取向演化规律；依据凝固界面稳定性理论，讨论α相非择优取向强制生长的稳定性条件；通过分析界面能各向异性与凝固条件的相互关系及变化规律，探索α相非择优取向的强制稳定生长机制。本项目的成果将揭示TiAl合金中α相非择优取向调控和稳定生长机制，可丰富金属间化合物的晶体生长理论，为TiAl合金片层取向的精确控制提供理论基础，具有重要意义。

为了充分认识TiAl合金晶体生长取向演化规律，尤其是α相非择优取向的调控机理及生长稳定性问题，本项目开展了初生相为α相的TiAl合金非择优取向调控及生长稳定性研究工作。获得了定向凝固条件下生长速度、温度梯度等凝固条件对α相非择优取向生长的影响规律。初生相α相的晶体生长取向随着生长速度的和温度梯度的变化而发生改变。α相择优取向与热力方向的夹角Θ以及α相生长取向与热流方向的夹角Ψ均随着生长速度的和温度梯度的变化而发生改变。通过控制凝固参数的精确控制，可以使TiAl合金中α相晶粒沿着<11-20>晶向或与之接近的方向生长，实现TiAl合金的片层取向控制。讨论了凝固速度对层片取向传递的影响，发现凝固速度只影响晶粒的竞争生长，并不影响定向凝固过程中的层片取向传递。依据凝固界面稳定性理论，获得了α相非择优取向强制生长的稳定性凝固条件范围。在当前实验条件下，TiAl合金籽晶法片取向控制的凝固条件范围为：生长速度V=10μm/s-30μm/s，温度梯度G=10K/mm-25K/mm。在预测的凝固条件范围内，成功的制备了定向凝固单晶TiAl合金试样。本项目的研究成果丰富了金属间化合物的晶体生长理论，为TiAl合金片层取向的精确控制提供理论基础，对TiAl合金定向凝固技术的发展具有重要意义。