Additivity of Hardening by Nanolamellar Structure and Antiphase Domain in Ti-39at%Al Single Crystals
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Additivity of Hardening by Nanolamellar Structure and Antiphase Domain in Ti-39at%Al Single Crystals Yuichiro Koizumi, Yoritoshi Minamino, Takayuki Tanaka, and Kazuki Iwamoto Department of Adaptive Machine Systems, Osaka University, 2-1 Yamadaoka, Suita, 565-0871, Japan ABSTRACT A mixed microstructure of antiphase domains (APD) and fine lamellar structure were introduced in a Ti-39at%Al single crystal and it was examined whether the APD hardening works even in nano-scaled lamellar structures. The hardness increases with decreasing APD size even where the L is smaller than 100 nm below which the hardening by lamellar refining saturates. The mechanism of the additivity of strengthening by APD and lamellar structure is discussed in the context of the geometries of slip direction, lamellar boundaries and APD boundaries (APDBs). For {1 1 00} prism slip (the easiest slip system of α2-Ti3Al), the lamellar boundaries are parallel to the slip direction, and therefore they interrupt the motion of screw dislocations effectively. On the other hand, APDBs inclined from lamellar boundaries can effectively obstruct the dislocation motion regardless of the dislocation character because the shear of such APDBs results in the formation of step-like APDBs on the slip-plane and requires additional stress for dislocation motion whereas APDBs parallel to the slip direction can be sheared without forming such a step-like APDB. Accordingly, APDs and lamellar structure can contribute to the strengthening complementarily. INTRODUCTION Additivity of strengthening of alloys by different mechanisms, for instance, grain refining, dispersion hardening and solution hardening, has been of great interest from both scientific and industrial points of view [1-3] because the superposition of different resistances is a challenge for extending dislocation theory. Also, the combinations of multiple strengthening mechanism is expected to give rise to excellent mechanical properties which can not be achieved by only one type of strengthening mechanism. However, no general interpretation of additivity or superposition of strengthening has been established although an empirical rule called linear additivity is sometimes applied [1-3]. In order to obtain general understanding of the additivity of strengthening, more experimental data and theoretical considerations are required. Ti-Al alloys are getting popular as light-weight, high temperature structural materials for engines of the newest automobiles and aircraft [4]. The high strength of Ti-Al alloys is mainly due to the lamellar structure composed of γ-TiAl and α2-Ti3Al phases. The yield stress of lamellar Ti-Al alloy deformed parallel to the lamellar structure increases with decreasing lamellar spacing (L) in the range over 100 nm [5-7]. However, in the smaller range, it saturates at approximately 1 GPa [5]. On the other hand, when α 2-Ti3Al crystal is deformed parallel to its (0001) plane to which the lamellar interface of the Ti-Al alloy is parallel, the yield stress increases with decreasin
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