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Type Gerstl, Stephan S. A.; Seidman, David N.
  Publication Chemical and Structural Investigation of Internal Domains of Needle-Like Ti3AlC Carbide Precipitates in γ-TiAl with 3-D Atom-Probe Tomography Volume Journal Article
Pages 2006
  Abstract Microscopy & Microanalysis  
  Corporate Author Microsc. Microanal.  
Publisher 12  
Editor S2
  Summary Language 1570-1571 Series Editor  
Abbreviated Series Title Titanium aluminide (TiAl) alloys have shown improvements in high temperature creep resistance, due in large part to the addition of carbides in the microstructure. Two types of carbide precipitates increase the creep resistance: Ti2AlC [1] and Ti3AlC [2]. There is evidence, however, that hexagonal Ti2AlC carbides (Htype) are formed from the metastable perovskite Ti3AlC carbides (P type). Therefore, the nucleation and growth of Ti3AlC precipitates is of interest due to their subsequent transformation into the Ti2AlC phase, which is a stable phase above 800C. Domains within the needle-like carbide precipitates have been observed via strain-field contrast in transmission electron microscopy (TEM), Fig. 1, and high-resolution TEM, Fig. 2. The latter provides evidence that structurally coherent atomic columns traverse the domain boundaries. The chemical makeup of these domains has not been previously investigated. In the current work, three dimensional atom probe (3DAP) microscopy is utilized for resolving the chemical nature of these boundaries [3]. Applying one-dimensional analysis cylinders along the primary axis of Ptype carbides in 3D reconstructions, Fig. 3, makes possible the measurement of the chemical composition variations inside the carbide precipitates. The nominal composition of the γ-TiAl alloy is Ti50Al48CMo, with C added for carbide formation and Mo providing increased ductility [4]. After 24 hrs. at 800C (Fig. 4a)the domains have a mean composition of Ti67Al21C11O1Mo0.4. At the domain boundaries, the Ti, Al, and Mo concentrations are ca. 57, 30, and 2.5 at.%, respectively. After 48 hrs. at 800C (Fig. 4b), the domains have a mean composition of Ti67Al20C11O1.3Mo0.7, and the composition fluctuations at the domain boundaries are less well defined, particularly in the case of the Mo concentration profile, and are indistinguishable from the average domain concentration. The alteration of domain boundary chemical composition reveals the initial steps in the transformation from a P-type to an H-type carbide. [1] P.I. Gouma, K. Subramanian, Y-M Kim, and M.J. Mills, edited by M.J. Mills E.P. George, M. Yamaguchi (MRS, 1999), Vol. 552. [2] J. Muellauer and F. Appel., Mat. Res. Soc. Symp. Proc. 5.1.1 (2003) 753. [3] S. S. A. Gerstl. PhD Thesis, Northwestern University (2006). [4] S. Kim, G. D. W. Smith, S. G. Roberts, and A. Cerezo., Mater. Sci. Eng. A 250(1), (1998) 78. [5] This research was supported by the DOE Basic Energy Sciences.
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