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。摘要：该研究项目将包括建模和实验，以研究控制燃气涡轮发动机旋转部件高温合金铸造过程中的质量和传热的方法。该研究将解决具有排列TiAl/Ti3Al层状显微组织的熔模铸造钛铝合金（TiAl）的方法，与传统铸造的相同合金相比，其抗蠕变性能显着提高。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排列的层状微观结构，因此这项工作的成功完成应该引起燃气轮机制造商的兴趣。智力优势：为了发展所需的处理科学，将评估几个概念。一种成分固定且有限的种子晶体将用于定向高温hcp相，以便在随后的热处理中产生对齐的层状TiAl/Ti3Al微观结构。然而，块状合金的成分窗口是有限的。通过在种子和大块合金之间使用预成型，种子和合金成分的混合可以最小化；因此，扩大了成功晶体生长的组分窗口。熔体的设计必须允许液态金属渗透，但必须有足够小的孔来限制凝固过程中质量的扩散，抑制熔体中不需要的相的成核。利用元胞自动机-有限体积（CA-FV）方法，对预制体生长的物理机制以及不同截面和热提取方向对单晶微观结构发展的影响进行建模。实验将着重于了解陶瓷镶片对增加多组分TiAl合金单晶生长的成分窗口的作用。CA模拟的结果将用于初始预成形设计。虽然金属在预制件中的凝固行为及其出口受局部传热和传质控制，但一旦进入模腔，微观结构的稳定性也可能受到热溶质对流的影响，热溶质对流可能会充分破坏枝晶尖端附近溶质的传输。这些流动的结构及其对微观结构的影响也将通过数值模拟进行研究。更广泛的影响：这里所描述的工作有可能在更高的发动机工作温度下显著提高用于轻型结构应用的部件的机械性能。建模工作扩展了在实际过程中预测多组分/多相合金微观组织演变行为的能力。这项研究将使研究生和本科生都能接触到对这个国家的军事和经济至关重要的行业中的尖端材料技术。国际合作包括由京都大学（日本）普渡大学团队和伯明翰大学（英国）高温材料跨学科研究中心进行的实验工作。

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).