定向凝固过程中的晶粒竞争生长机制研究

Directional solidification has been widely used to produce aero-engine and gas turbine blades of Ni-base superalloys. A preferred crystallographic orientation is produced in the directional solidification process due to competitive grain growth. Numerous beneficial effects were demonstrated because of the introduction of a preferred grain orientation with a low modulus parallel to the main stress axis and the elimination of grain boundaries perpendicular to the same stress axis. It is therefore important to study the issue of competitive grain growth from the both aspects of science and engineering. The generally accepted model for competitive grain growth was proposed by Walton and Chalmers. However, the recent experimental results in our works were not in accordance with Walton and Chalmers model. Since the present results from our works and literatures are not able to tell the fundamental mechanism dominating the competitive grain growth, a reasonable model has not been built in our works. In order to find the key factor dominating the competitive grain growth, the present project explores the effects of thermal field, fluid field and solutal field on grain selection by means of changing the solidification parameters, sample geometries and alloy contents in the casting experiments, respectively. Based on the studies, a reasonable model can be built to explain the mechanism of grain selection and directional solidification will become a more effective technology to produce high quality turbine blades of Ni-base superalloys because of the better insight into the mechanism.

定向凝固技术广泛应用于航空发动机和燃气轮机的高温合金叶片制造，凝固过程中的晶粒竞争生长是决定能否获得合格单晶以及定向叶片的一个关键因素，同时也是金属凝固领域的一个重要科学问题。申请人的前期研究发现目前用于描述晶粒竞争生长机制的Walton-Chalmers模型与实验结果存在矛盾。但是，已有的研究结果还不能揭示控制晶粒竞争生长的物理机制，因此不能提出完善的模型。本项目通过定向凝固工艺参数、试样尺度、合金组元与含量的变化性实验以及模拟计算，分别揭示温度场、流场、溶质场在晶粒选择与淘汰过程的作用，澄清控制晶粒竞争生长的主导因素，建立晶粒竞争生长机制模型，拓展人们对定向凝固过程的认识，从而更有效地利用定向凝固技术制备高温合金部件。

面心立方结构合金定向凝固时[001]晶向与热流方向平行的取向择优晶粒与枝晶会淘汰[001]晶向与热流方向不平行的取向偏离晶粒与枝晶，形成具有[001]织构特征的柱状晶组织，描述这一过程的模型称为Walton-Chalmers模型。前期工作中，我们发现这一晶粒与枝晶竞争生长模型与实验现象存在不符。为了揭示控制晶粒与枝晶竞争生长的物理机制，本项目研究温度场、流场与溶质场对晶粒与枝晶竞争生长与淘汰的影响。目标是澄清控制晶粒与枝晶竞争生长与淘汰的主导因素，建立晶粒与枝晶竞争生长机制模型，完善定向凝固理论。.研究结果表明，抽拉速度显著影响汇聚晶粒与枝晶竞争淘汰速度，对发散晶粒与枝晶竞争淘汰速度影响很小。温度梯度几乎不影响汇聚及发散晶粒与枝晶竞争淘汰速度。无论是平面还是下凹固液界面，都不会改变晶粒与枝晶竞争淘汰基本规律。.块体试样定向凝固时，合金熔体中存在较大的熔体流动。薄膜试样由于受表面张力作用，熔体流动可以忽略。当试样厚度从0.2mm变化到10mm时，熔体流动强度为数量级变化，但是晶粒与枝晶竞争淘汰规律保持一致。.合金成分显著影响汇聚晶粒与枝晶竞争淘汰速度，但是对发散晶粒与枝晶竞争淘汰影响不明显。溶质场越强，汇聚晶粒与枝晶竞争淘汰速度越大。低抽拉速度下，两个汇聚枝晶在尖端区域会出现溶质场重叠，使得取向偏离枝晶可以淘汰取向择优枝晶；高抽拉速度下，两个汇聚枝晶在尖端区域不容易出现溶质场重叠，造成取向偏离枝晶很难淘汰取向择优枝晶。.上述结果表明，控制晶粒与枝晶竞争淘汰的主要因素不是温度场与流场，而是溶质场。因此，本项目提出的晶粒与枝晶竞争生长与淘汰新机制模型主要考虑枝晶尖端溶质场的相互作用。汇聚生长枝晶在尖端区域出现溶质场重叠时，就会出现与Walton-Chalmers模型不同的竞争淘汰行为。这种行为使得抽拉速度和合金成分会显著影响晶粒与枝晶的竞争生长与淘汰。