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Discontinuous coarsening of the lamellar structure of γ-TiAl-based intermetallic alloys and its control

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Abstract

Discontinuous coarsening (DC) of the primary lamellar structure (PLS) occurring at lamellar colony boundaries (LCBs) and in surface layers of various Ti-(40 to 45) at. pct Al binary and Ti-46 at. pct Al-X (X=Si and C) ternary alloys was systematically investigated by using optical microscopy and scanning and transmission electron microscopy. The compositions of the α 2 and γ phases in the primary lamellar structure were estimated based on the weight fractions of the two phases, determined by X-ray diffraction. When the solution-treated Ti-(40 to 45) at. pct Al binary alloys were subsequently soaked at 1000 °C, the primary lamellae in the Ti-40 at. pct Al alloy were the most stable, while those in the Ti-44 at. pct Al were the most unstable. Both the thermodynamic analysis and experimental results confirm that the driving force of the coarsening is mainly derived from the reduction of the chemical free energy (i.e., out-of-equilibrium chemical composition) and the interfacial energy of primary lamellae, whereas the coarsening resistance is mainly from the increase of the elastic strain energy of lamellar interfaces and the surrounding during coarsening. It is found that Si has an exceptional ability to hinder the coarsening of the primary lamellar structure at high temperatures, but the precise mechanism for this improvement is uncertain now. Based on this study, a proposal is finally addressed to improve the thermal stability of the primary lamellar structure of titanium aluminides.

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Abbreviations

DC:

discontinuous coarsening

EM:

eutectic microstructure containing the Ti5(Al,Si)3 phase in the Ti-Al-Si system

PLS:

primary lamellar structure

L:

lamellae

LCB:

lamellar colony boundary

SL:

surface layer of one specimen

ΔF :

driving force of discontinuous coarsening

ΔG I :

reduction of interfacial energy after the coarsening

ΔG C :

chemical free energy loss after the coarsening, i.e., difference of ΔG /Total m and ΔG /PLS m at a given temperature

ΔG /Total m :

chemical free energy loss from a superstaturated α 2 matrix to the equilibrium α 2 and γ phases at a given temperature

ΔG /PLS m :

chemical free energy loss from a supersaturated α 2 matrix to the metastable α 2 and γ phases (primary lamellar structure) at a given temperature

ΔG S :

strain energy stored in surface layer due to specimen preparation

ΔF S :

driving force of the coarsening in surface layer

ΔG R :

discontinuous coarsening resistance

ΔG ES :

increase of elastic strain energy during the coarsening

ΔG B :

increase of grain boundary energy during the coarsening associated with the increase of DC cell boundary area

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Authors and Affiliations

  1. the National Institute of Advanced Industrial Science and Technology, 983-8551, Sendai, Japan

    G. W. Qin (Research Fellow), K. Oikawa, Z. M. Sun, S. Sumi & T. Ikeshoji (Senior Researchers)

  2. the Department of Materials Science, Tohoka University, 980-8579, Sendai, Japan

    J. J. Wang (Doctoral Student)

  3. the Department of Materials Science and Engineering, Northeastern University, 110006, Shenyang, P.R. China

    S. W. Guo (Professor) & S. M. Hao (Professor)

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  1. G. W. Qin
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  2. K. Oikawa
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Qin, G.W., Oikawa, K., Sun, Z.M. et al. Discontinuous coarsening of the lamellar structure of γ-TiAl-based intermetallic alloys and its control. Metall Mater Trans A 32, 1927–1938 (2001). https://doi.org/10.1007/s11661-001-0005-1

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