Control of transport phenomena to enable the production of TiAl single crystals

控制传输现象，使TiAl单晶的生产成为可能。普渡大学材料工程学院Matthew J.M.Krane和David R.Johnson材料工程学院摘要：这项研究计划将包括建模和实验，以研究在燃气轮机发动机旋转部件高温合金铸造过程中控制质量和热量传递的方法。这项研究将探讨熔模铸造TiAl(TiAl)合金与TiAl/Ti3Al片层组织定向排列的方法，这种合金在蠕变抗力方面比传统铸造的相同合金有显著的改善。目前，还没有一种TiAl的铸造工艺能够以合理的生长速度产生这些取向的片层组织，因此这项工作的顺利完成应该会引起燃气轮机制造商的兴趣。智力优势：为了发展所需的加工科学，将对几个概念进行评估。采用成分固定、成分有限的晶种对高温hcp相进行取向，使TiAl/Ti3Al在后续热处理过程中得到层状排列的显微组织。然而，块体合金的成分窗口是有限的。通过在晶种和大块合金之间使用预制棒，可以最大限度地减少晶种和合金成分的混合；从而扩大了成功生长晶体的成分窗口。预制件的设计必须允许液态金属渗入，但要有足够小的气孔来限制凝固过程中质量的扩散，抑制熔体中不希望看到的相的形核。用元胞自动机-有限体积(CA-FV)方法模拟了预制棒生长的物理机制，以及不同截面和热抽出方向对单晶组织发展的影响。实验将侧重于了解陶瓷插件在增加多元TiAl合金单晶生长的成分窗口方面所起的作用。CA模拟的结果将用于初始预制件设计。虽然凝固金属在预制件中和退出时的行为受局部传热和传质的控制，但一旦进入模具型腔，微观组织的稳定性也可能受到热溶解对流的影响，这可能会充分扰乱枝晶尖端附近的溶质传输。还将通过数值模拟来研究这些流动的结构及其对微观结构的影响。更广泛的影响：这里描述的工作有可能在发动机运行温度更高的情况下显著提高用于轻型结构应用的部件的机械性能。建模工作扩展了预测多组分/多相合金在实际工艺中的微观组织演变行为的能力。这项研究将使研究生和本科生接触到尖端材料技术，该行业对美国的军事和经济非常重要。国际合作包括由日本京都大学普渡团队和英国伯明翰大学高温材料跨学科研究中心开展的实验工作。

Control of transport phenomena to enable the production of TiAl single crystalsProfs. Matthew J. M. Krane and David R. JohnsonSchool of Materials EngineeringPurdue UniversityAbstract: This research program will consist of modeling and experiments that will examine ways to control the mass and heat transfer during the casting of high temperature alloys for rotating parts in gas turbine engines. The research will address the approaches to investment cast titanium aluminide (TiAl) alloys with aligned TiAl/Ti3Al lamellar microstructures, which exhibit significant improvements in creep resistances over the same alloys conventionally cast. Currently, there is no casting process for TiAl that can produce these aligned lamellar microstructures at reasonable growth rates, so successful completion of this work should arouse interest among the gas turbine manufacturers. Intellectual merit: To develop the needed processing science, several concepts will be evaluated. A seed crystal of fixed and limited composition will be used to orient the high temperature hcp -phase so that an aligned lamellar TiAl/Ti3Al microstructure results during subsequent heat treatment. However, the compositional window for the bulk alloy is limited. By using a preform between the seed and bulk alloy, mixing of the seed and the alloy compositions can be minimized; thus, enlarging the window of compositions for successful crystal growth. The design of performs must allow liquid metal to infiltrate, but have small enough pores to restrict diffusion of mass during solidification, suppressing the nucleation of undesirable phases from the melt. The physical mechanisms underlying the growth through the preform and the effect of varying cross-section and heat extraction direction on the development of the single crystal microstructure will be modeled using a cellular automaton-finite volume (CA-FV) approach. Experiments will focus on understanding the role that the ceramic insert has on increasing the compositional window for single crystal growth of multicomponent TiAl alloys. Results from the CA simulations will be used in the initial preform design. While the behavior of the solidifying metal in the preform and as it exits is governed by local heat and mass transfer, once out into the mold cavity, the stability of the microstructure may also be affected by thermosolutal convection, which may sufficiently disrupt the transport of solute near the dendrite tips. The structure of these flows and their and effect on the microstructure will also be investigated through numerical modeling. Broader impacts: The work described here has the potential to enable a significant advance in the mechanical properties of parts used for lightweight structural applications at higher engine operating temperatures. The modeling effort expand the ability to predict the behavior of microstructural evolution of multicomponent/multiphase alloys in real processes. This research will expose both graduate and undergraduate students to cutting edge materials technology in an industry important to this country's military and economy. International collaborations include experimental work performed by the Purdue team at Kyoto University (Japan) and the Interdisciplinary Research Centre in High Temperature Materials at the University of Birmingham (UK).