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首页> 外文学位 >Heat extraction and dendritic growth during directional solidification of single-crystal nickel-base superalloys.
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Heat extraction and dendritic growth during directional solidification of single-crystal nickel-base superalloys.

机译：单晶镍基高温合金定向凝固过程中的热量提取和枝晶生长。

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摘要

Single crystal (SX) growth requires directional solidification (DS) with a sustained thermal gradient in the withdrawal direction. By increasing the thermal gradient, the dendrite structure in nickel-base superalloys is refined resulting in a mechanically-superior SX with reduced defect occurrence. One method to achieve higher thermal gradients is the utilization of the Liquid Metal Cooling (LMC) process. Optimization of the process has been limited by a lack of the fundamental understanding of the various heat transfer processes associated with the technique.;Solidification experiments of nickel-base superalloys have been conducted in a DS furnace capable of Bridgman or LMC modes. The degree of structure refinement has been investigated in castings with varying cross-sectional areas. The feasibility of a mono-crystalline ring comprised of multiply-seeded, SX segments has been investigated.;Three-dimensional, finite-element (FE) solidification modeling has been used to investigate the thermal characteristics of the Bridgman and LMC DS processes. The dominant heat-transfer step during solidification is radiation from the mold for the Bridgman process and conduction through the mold for the LMC process. In the LMC process, the primary role of the floating baffle is to provide a thermal barrier between the furnace environment and the coolant. Relationships between dendritic structure and the local thermal field are presented. The preferred process conditions occur when the solidification-front position is just above the baffle for both the Bridgman and LMC processes.;The stability of unidirectional dendritic growth is dependent on the curvature of the solid-liquid interface. Some processing conditions cause substantial lateral heat extraction that leads to a change in dendrite morphology, resulting in grain nucleation or lateral growth -- the formation of long secondary dendrite arms overgrowing favorably aligned primary dendrites. The conditions under which lateral growth occurs have been studied experimentally and via solidification modeling using FE and diffusional dendrite growth modeling. Lateral growth of dendrites occurs at interface inclination angles as low as 25 deg., and grain nucleation and breakdown of the solidification front occur when the interface inclination reaches 45 deg. The misorientation of the [001] SX orientation from the withdrawal axis significantly contributed to the onset of lateral growth.
机译：单晶（SX）的生长需要定向凝固（DS），并且在撤出方向具有持续的热梯度。通过增加热梯度，可以改善镍基高温合金中的枝晶结构，从而获得具有优异缺陷率的机械性能优越的SX。实现较高热梯度的一种方法是利用液态金属冷却（LMC）工艺。缺乏对与该技术相关的各种传热过程的基本了解，从而限制了工艺的优化。镍基高温合金的固溶实验已在具有Bridgman或LMC模式的DS炉中进行。已经研究了具有不同横截面面积的铸件的组织细化程度。研究了由多种子SX段组成的单晶环的可行性。三维有限元（FE）凝固模型已用于研究Bridgman和LMC DS工艺的热特性。固化过程中主要的传热步骤是Bridgman工艺从模具发出的辐射，而LMC工艺则通过模具进行传导。在LMC工艺中，浮动挡板的主要作用是在炉子环境和冷却剂之间提供热障。提出了树枝状结构与局部热场之间的关系。对于Bridgman和LMC工艺，当凝固前沿位置刚好位于挡板上方时，便会出现优选的工艺条件。单向树枝状生长的稳定性取决于固液界面的曲率。某些加工条件会导致大量的侧向热量提取，从而导致枝晶形态发生变化，从而导致晶粒成核或横向生长-长的次级枝晶臂的形成，过度生长的初级枝晶会很好地排列。已经通过实验以及通过使用有限元的凝固模型和扩散枝晶生长模型研究了发生横向生长的条件。树枝状晶体的横向生长发生在界面倾斜角低至25度时，当界面倾斜达到45度时，晶粒成核和凝固前沿破裂。 [001] SX方向相对于退出轴的方向不正确，显着促成了侧向生长的开始。

著录项

  • 作者

    Miller, Jonathan D.;

  • 作者单位

    University of Michigan.;

  • 授予单位 University of Michigan.;
  • 学科 Chemistry Inorganic.;Engineering Chemical.;Engineering Materials Science.
  • 学位 Ph.D.
  • 年度 2011
  • 页码 305 p.
  • 总页数 305
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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