Strain-assisted Formation of Nano-scaled Lamellar Structure in Ti-39at%Al Single Crystals
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1059-KK04-02
Strain-assisted Formation of Nano-scaled Lamellar Structure in Ti-39at%Al Single Crystals Yuchiro Koizumi1,2, Takayuki Tanaka1, Fujita Takeshi1, and Minamino Yoritoshi1 1 Department of Adaptive Machine Systems, Osaka University, 2-1 Yamadaoka, Suita, 565-0871, Japan 2 Department of Materials Science and Engineering, Masshchusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139 ABSTRACT We have studied the effects of straining prior to annealing for lamellar structure formation in a Ti-39at%Al single crystal intending to accelerate the lamellar structure formation and decrease the resultant lamellar spacing via preferential nucleation of γ-TiAl phase at dislocations in α2-Ti3Al phase. In a crystal cold-rolled to 10% reduction before annealing, a fine lamellar structure with 88 nm average spacing was formed by annealing at 1073 K for 1×104 s whereas no lamella were observed in the crystal subjected to the same annealing after 5%-coldrolling. In addition, it was also demonstrated that when the crystal is locally strained by indentation and annealed at 1073 K for 1×104 s, a lamellar structure with 43 nm average lamellar spacing is formed in areas near the indentation and there were no lamella beyond these areas. INTRODUCTION The lamellar structure of Ti-Al alloys has been extensively studied because it is crucial for their excellent mechanical properties as high-temperature and light-weight structural materials [1]. The lamellar structures are composed of γ-TiAl and α2-Ti3Al laminated parallel to their close-packed planes (i.e. ({111}plane of TiAl with L10 structure and (0001) plane of Ti3Al with D019 structure). In studies focusing on the atomic scale structure of lamellar boundaries and dislocation structure in α2-lamellae, techniques to remove only the γ-TiAl phase from lamellar TiAl alloy have been developed. The remaining structures composed of α2-Ti3Al look like finarrays with micron-sized gaps [2]. Such a structure has potential to be used for various practical purposes which require a high density of surface area such as fins, electrodes and catalyst supports. Since the dimensions of the gap is considered to be determined by that of γ-lamellae in the lamellar Ti-Al alloy, the dimensions of the gap can be controlled by controlling the dimension of γ-lamellae. Formation mechanisms of lamellar structure have been also extensively investigated [5, 6]. It has already been accepted that the lamellar structure is formed by precipitation of γlamellae from the α-phase or the α2-phase rather than by pearlitic transformation from α-phase to (α2+γ) dual phase. The matrix phase (α or α2) from which γ-phase precipitates depends on the chemical composition and cooling rate. The γ-lamellae precipitation is also called a displacive and diffusional transformation. It has been observed that the process is assisted by dislocations [7, 8]. Dislocations whose Burgers vector is 1/6 can act as preferential nucleation sites by dissociating into two Shockley partial dislocations and forming stacking f
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