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

Purdue University, West Lafayette IN

Investigators

Abstract

Control of transport phenomena to enable the production of TiAl single crystals Profs. Matthew J. M. Krane and David R. Johnson School of Materials Engineering Purdue University Abstract: 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).