几种镍基高温合金的过冷能力和单晶可铸性的研究与比较

Investigation on Undercoolability and Single Crystal Castability of Some Ni-Based Superalloys

对6种镍基高温合金熔体在陶瓷型壳中的凝固行为进行了检测,得出了各种合金的液相线温度TL、临界形核温度TN和临界形核过冷度ΔTN=TL-TN,发现它们具有明显不同的过冷能力。在单晶叶片的铸造实验中,具有高过冷能力(高于40℃)的合金在宏观上抗杂晶能力最强,但容易形成枝晶碎臂微观缺陷。具有中等过冷能力(20~30℃)的合金,既能有效防止宏观杂晶缺陷的发生,又能避免微观碎晶的形成,显示出最佳的单晶可铸性。而具有很低过冷能力(低于10℃)的合金,抗杂晶能力非常弱,表现出很差的单晶可铸性。镍基高温合金具有相似的化学成分,却显示出明显不同的过冷能力和单晶可铸性,对其原因还需要做进一步的研究。

The undercoolability of Ni-base superalloys is an important physical property to determine its single crystal (SC) castability, because stray grains will be formed if the local undercooling at the component extremities exceeds the undercoolability of the used alloys. In the present work, both the undercoolability of 6 Ni-based superalloys and their SC castability were investigated during isothermal cooling processes and directional solidification, respectively. The undercooling behavior of the alloys during isothermal heating and cooling processes was investigated under the same process condition as the SC casting experiment. The liquidus temperature TL and the critical nucleation temperature TN of the alloys were measured, respectively. The average critical nucleation undercooling ΔTN=TL-TN was determined for each alloy. Using a Bridgman furnace the directional solidification experiments were carried out to cast SC turbine blades of the used alloys in the homemade shell molds. The alloy CMSX-6 with high undercoolability has the best capacity to avoid stray grains, but revealing also fragment of dendrite arms due to the rapid dendrite growth in deeply undercooled platforms. In the castings of the alloys with medium undercoolability, neither stray grains nor micro-defect were found, revealing a defect free SC structure. As expected, the alloy DD483 with low undercoolability is very difficult to be cast into SC blades without stray grain, exhibiting a very low SC castability. It is hard to understand why the investigated Ni-base superalloys have so different SC solidification behaviors in spite of their similar chemical compositions. Further investigation should be carried out to find a reasonable explanation.