Characterizations of Lamellar Interfaces and Segregations in a PST-TiAl Intermetallic Alloy by an Analytical Scanning Tr
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Characterizations of Lamellar Interfaces and Segregations in a PST-TiAl Intermetallic Alloy by an Analytical Scanning Transmission Electron Microscope Wei (Wayne) Zhao and David E. Luzzi Department of Materials Sciences and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104-6272
ABSTRACT Polysynthetically-twinned titanium aluminide (PST-TiAl), a fully lamellar γ−TiAl + α2Ti3Al dual-phase alloy, is under evaluation for applications in rotary components in aircraft and automobile industries due to its high specific strength, and a high strength-retention capability at elevated-temperatures. However, the low ductility at room- to mid-high temperatures of the material hinders its application. Additions of certain tertiary elements to the binary TiAl system appear to improve the ductility at room- to mid-high temperatures, thus a balance among strength, ductility, and fracture toughness can be expected. In this article, segregation of tertiary elements to the lamellar interfaces is investigated. Single crystals of a TiAl with 0.6% atomic percentage tertiary additions are grown by an optical float-zone method. Segregation to the lamellar interfaces and the microstructure of the interfaces are investigated. Structures of the lamellar interfaces are characterized, and microchemistry and distribution habits of these elements along the γ+α2 lamellar boundaries as well as the γ-γ lamellar and domain boundaries are analyzed.
INTRODUCTION Polysynthetically-twinned titanium aluminide (PST-TiAl) single crystals comprised of lamellar TiAl and Ti3Al provide the opportunity to distinguish the behavior of the two phases and the different lamellar interfaces. The near coherency of the lamellar interfaces allows direct imaging of the interface atomic structure with the atomic-level flatness enabling high-resolution studies of the segregation of tertiary elements. With precise crystal lattice relationships maintained over macroscopic distances, crystal orientation effects on properties can be studied. At the present time, little is known about the segregation properties of the different lamellar interfaces in TiAl alloys, and almost nothing is known about the effects of the segregation on the strength and deformation behavior of these lamellar interfaces. This article reports the partitioning habit of tertiary elements in the PST TiAl alloy with lamellar structures. The results presented here are important to understand the relationships between atomic structure, segregation and strength at γ-γ and α2-γ interfaces in polysynthetically twinned (PST) TiAl containing ternary alloying additions.
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EXPERIMENTS Both binary and ternary alloys were prepared by arc-melting using a non-consumable tungsten electrode. One of the elements W, Ta, Cu, or Mo, was added to each ternary alloy. The nominal composition of the binary alloy was Ti-49.3 at.%-Al. Tertiary additions of 0.6 at.% were used to facilitate the growth of the PST single crystals. This addition replaced the Ti and Al in equal measure producing
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