Microstructure stability during creep deformation of hard-oriented polysynthetically twinned crystal of TiAl alloy
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9/3/03
9:54 AM
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Microstructure Stability during Creep Deformation of Hard-Oriented Polysynthetically Twinned Crystal of TiAl Alloy HEE Y. KIM and K. MARUYAMA The hard-orientated polysynthetically twinned (PST) crystal with the lamellar plates oriented parallel to the compression axis was deformed at 1150 K under the applied stress of 158 to 316 MPa. Microstructural changes were examined quantitatively for the PST crystal during creep deformation. In the as-grown PST crystal of the present study, proportions of 2/, true twin, pseudotwin, and 120 deg rotational fault interfaces were 12, 59, 12, and 17 pct, respectively. After creep deformation, lamellar coarsening by dissolution of 2 lamellae and migration of / interfaces were observed. The acceleration of creep rate after the minimum strain rate in the creep curve was attributed to the lamellar coarsening and destruction of lamellar structure during the creep deformation. Thirty-two percent of 2/ interfaces, 51 pct of true twin interfaces, 74 pct of pseudotwin interfaces, and 80 pct of 120 deg rotational faults disappeared after 4 pct creep strain at 1150 K. The 2/ interface was more stable than / interfaces during the creep deformation. The pseudotwin interface and 120 deg rotational fault were less thermally stable than the true twin interface for / interfaces.
I. INTRODUCTION
THE TiAl-based alloys have been intensively investigated for the applications in aerospace and automotive engine components due to their attractive properties such as high strength, low density, and excellent creep resistance.[1,2,3] Various TiAl alloys have been developed so far, but their creep resistance has not yet been fully improved.[4] Creep resistance of materials is sensitive to their microstructure, and can be improved by appropriate microstructure design. The fully lamellar structure of TiAl()/Ti3Al(2) two-phase alloy is considered as the most favorable for forthcoming applications owing to its superior strength and creep resistance compared to those of any other microstructure type.[5–10] Consequently, substantial research has been carried out on the lamellar-type microstructure because creep resistance will be a designlimiting factor in many potential applications. Microstructural design requires a detailed knowledge of the correlation between microstructure and creep resistance. The fully lamellar structures consist of a stacking of and 2 lamellae. The phase forms with the particular crystallographic orientation relationship during cooling from the single-phase field: {111}g // (0001)a2 and 110g 112 0a2 known as the Blackburn relationship.[11] Due to the tetragonality of the L10 structure of TiAl, the 110 directions are not equivalent and six different domain orientations are distinguished. Because of the presence of different orientation variants, three types of interfaces are observed between phases. Thus, four different interfaces exist in TiAl, 2/ HEE Y. KIM, formerly Research Associate, Department of Material Science, Tohok
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