几何形状因素影响的钛铝合金定向片层结构演变与机理

Due to its excellent properties of low density, excellent burning and oxidizing resistance at elevated temperature from 750℃ to 900℃,TiAl-based alloy has been received considerable attention as one of the promising structural materials for application in aviation, aerospace, automobile and power generation industries. The efficient of aerospace and transportation are promoted through weight lossing and operation temperture enhancing. The intrinsic brittleness of TiAl alloys can be improved by directionally solidified microstructures with regularly arranged lamellar microstructures. However, during the directional solidification of complex shaped components (e.g. engine turbine blade), changes of temperature and solute field occurred that they were induced by changes in the areas with shaped cross-section. Nucleation induced by these changes would make the control of directionally solidified microstructure difficult, such as morphology, phase content and lamellar orentation. Then the mechanical properties of the components become unstable, especially decreasing in ductility and toughness. Therefore, the influence of different cross-section shapes, namely geometrical factors, on the solidification process can be obtained by analyzing the microstructure of TiAl alloys with different shape prepared by directional solidification. In the present research, attention is paid on the solidification behavior and lamellar microstructure evolution. Effects of withdraw rate and temperature gradient on the crystal growth of variable cross-section will be clarified during directional solidification of TiAl alloys, and the growth mechanism of stray grains and directional lamellar structure will be analyzed. The theoretical basis will be developed for the materials optimization of TiAl alloy, performance improvement and the prospect engineering application.

TiAl具有轻质耐热的特点，是目前公认的新型航空、航天和车辆等结构选材。通过减重和提高工作温度，可促进航空航天等动力系统的高效和低排放。TiAl合金的规则片层取向有利于其本征脆性的改善。但定向凝固复杂形状构件（如涡轮叶片）时，构件截面形状的改变引起温度场、溶质场等能量失稳，会使TiAl原本复杂的包晶凝固（如凝固路径和相选择等）更加趋于复杂化，不但不利于对定向片层取向、形态和含量等的控制，更不利于定向凝固铸件性能的改善，尤其是韧性。因此，如何通过定向凝固来制备变截面形状的TiAl构件，揭示几何形状因素对其定向组织的影响规律，消除变截面区的杂晶，建立控制定向片层结构转变的方法，将具有重要研究价值。本项目拟采用冷坩埚结合惰性陶瓷模壳的复合定向凝固法，研究变结构形状TiAl构件的凝固行为和片层结构演变行为，阐明定向凝固中几何形状因素的影响机理，为优化TiAl基构件组织和性能及工程应用奠定理论基础。

TiAl合金具有低的密度、高比强度、优异的高温抗氧化性能和抗蠕变性能，在航空、航天和汽车等对材料高温性能和轻量化要求较高的领域有极大的应用潜力。但TiAl合金的本征脆性问题限制了其应用，对此研究者提出采用定向凝固强韧化TiAl合金的方法，但目前TiAl合金的定向凝固的研究中只涉及形状简单的棒状和不变截面试样。本研究通过系统的实验研究、数值模拟和理论分析，采用复合冷坩埚定向凝固法对不同截面形状以及变截面试样的TiAl合金进行了定向凝固，揭示了截面形状对定向凝固过程中各项物理场的影响以及对定向凝固TiAl合金组织和力学性能的影响，明确了变截面试样定向凝固过程中温度场分布和杂晶形成机理，在此基础上系统研究并掌握了变截面试样定向凝固工艺方法，为TiAl合金进入实际应用阶段奠定了理论和试验基础。.具体成果：（1）相对感应-石墨加热法，采用复合冷坩埚定向凝固法制备TiAl合金试样能够降低熔体和铸型的界面反应，减少组织中的B2相，并提高试样的力学性能。（2）铸型中的Y2O3颗粒进入TiAl合金熔体的机制有两种： Y2O3颗粒直接被熔体冲刷脱落进入合金；Y2O3在高温下分解后以原子的形式进入熔体并在降温过程中再次反应生成Y2O3颗粒。（3）抽拉速率的增大会细化柱状晶和片层组织，降低Y2O3颗粒含量，提高B2相体积分数。（4）功率的增大会细化柱状晶组织，降低小角度片层比例，提高Y2O3颗粒含量，降低B2相体积分数。（5）在两种定向凝固方法中，增大抽拉速率提高TiAl合金的力学性能的机理为细化组织、降低Y2O3颗粒含量。（6）在定向凝固TiAl合金试样中，流场强度关系满足圆柱形略大于椭圆柱形大于板形，轴线对称性关系满足圆柱形大于椭圆柱形大于板形。（7）反向热流导致的异质形核以及铸型阻挡倾斜柱状晶生长是变截面试样中产生杂晶的原因，故变截面试样的肩部变截面处是最易产生杂晶的位置。（8）结合石墨环补热法和非等厚铸型的使用，可以对变截面定向凝固试样的温度场进行补偿，显著改善变截面试样的杂晶形成问题。（9）采用温度场补偿法制备变截面定向凝固试样，能够消除杂晶，使其组织、性能和不变截面试样无显著差异。（10）采用Cr当量对TiAl合金的合金元素种类和含量进行设计，能够控制相含量优化组织提高性能。