First-principles study of deformation-induced phase transformations in Ti-Al intermetallics
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H.Q. Ye Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China; and Electron Microscope Laboratory, Peking University, Beijing 100871, China (Received 4 August 2008; accepted 15 September 2008)
The structural phase stability and electronic properties of the Ti–Al intermetallic compounds were investigated by means of density-functional theory (DFT) calculations in a generalized gradient approximation. Through comparison of the calculated formation energies of the parent and product phases, an in-depth theoretical understanding of the deformation-induced g $ a2 phase transitions observed previously in TiAl alloys was achieved. The formation energy plays an important role in evaluating the feasibility of these phase transformations during plastic deformation of TiAl alloys. In addition, the density of states (DOS) was also calculated and used to analyze the stability of Ti–Al intermetallic compounds. The reasons for the absence of the deformation-induced (DI)-a2 and DI-g (L12) phases in underformed TiAl alloys were analyzed. I. INTRODUCTION
Titanium aluminides are one of the promising candidates for the next generation of high-temperature structural materials due to low density and good high-temperature properties.1–5 Comparing with the single-phase alloys, the two-phase alloys consisting of a2-Ti3Al and g-TiAl exhibit considerably improved ductility and toughness.6,7 Thus, during recent years extensive research was carried out to develop the two-phase a2 + g alloys for structural applications.8,9 Because their potential service conditions are usually under high temperature and high stress, the microstructural stability of TiAl alloys is extremely important for their safe applications.10–12 In recent years, deformation-induced (DI)-g $ a2 phase transformations have been reported many times during plastic deformation of TiAl alloys, some even in a room-temperature deformation process.13–18 Thanks to extensive high-resolution electron microscopy (HREM) investigations, the mechanisms of structural transition during the g $ HCP (a and a2) phase transformation become obvious: a=6h112 i Shockley partial dislocations sweep on alternate f111gg planes to fulfill the conversion of the stacking sequence from ABCABC. . . to ABAB. . . in the process of g ! HCP (a and a2) phase transformation. In the reverse transformation, a=3h1100i Shockley dislocations sweep on the interval basal planes a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0211
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 14 Mar 2015
ð0001Þa2 , which change the stacking sequence from ABAB. . . to ABCABC. . . . However, there are still controversies whether there is compositional diffusion or not during the deformation-induced g $ HCP (a and a2) phase transformations. In addition, because the g $ HCP phase transformations normally occur in the context of thermal effects, some may doubt whether such a
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