Strain-Induced Recrystallization of Tial
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STRAIN-INDUCED RECRYSTALLIZATION OF TiAl C.R. FENG, D.J. MICHEL AND C.R. Naval Research Laboratory, Washington, DC 20375-5000
CROWE
ABSTRACT Transmission electron microscope investigation of extruded and rolled XDTMi titanium aluminide sheet has shown the development of recrystallized TiAl regions adjacent to grains which exhibit submicron scale TiAl twins. Detailed diffraction analysis confirmed that the TiAl twins were separated by a layer of Ti 3 Al. This is consistent with previous observations of the microstructure of extruded XD'm titanium aluminide. In the extruded material, however, the scale of the twins was larger, but the extent of the recrystallized region was smaller. The evidence suggests that the recrystallization of the TiAl regions may result from the impingement of twins from adjacent grains. INTRODUCTION A previous investigation[l] has shown that twin-related TiAl regions, TiAlT, were frequently separated from the TiAl matrix, TiAlM, by a Ti 3Al layer(region) in XD'r processed[2] and cast/extruded Ti-45at%Al with 7.5v%TiB 2 (Ti-45A1/TiB2 ) material. A mechanism involving the formation of TiAl and Ti 3 Al stacking faults was employed to explain how a intermixed TiAIM, TiAlT and Ti 3 AI lath structure can be formed during the deformation of this material[3]. The purpose of this paper is to present the results from an investigation of the spontaneous recrystallization of asrolled Ti-45AI/TiB2 . Previous study[4] has shown that refinement and recrystallization was the major microstructure developement after rolling, including equiaxed TiAl regions. EXPERIMENTAL PROCEDURE The materials used in the investigation were Ti45A1/TiB2, initially produced by Martin Marietta Laboratories through XDTh processing[2], followed by casting, extrusion and rolling. The extrusion temperature was 12000 C and the extrusion ratio was 3.5:1. After extrusion, the material was sliced and canned in a titanium frame pack. Subsequent to heating at 1300 0 C for thirty(30) minutes, the pack was rolled 0 at 800 C. Rolling was conducted in three passes, with reheating at 800°C for 10 minutes between each pass. The total thickness reduction was 0.9[4]. Transmission electron microscopy(TEM) foils were prepared as previously described(l]. Briefly, 3mm discs were cut from the rolled sheet by electrodischarge machining, and the thickness of the discs was reduced to 100pm by hand grinding. Final thinning was conducted by twin-jets technique in a 7.5v% sulfuric acid: methanol mixture at 25V at 230K. All thin foils were examined in a JEOL 200CX STEM operated at 200kV. Mat. Res. Soc. Symp. Proc. Vol. 133. '1989 Materials Research Society
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RESULTS AND DISCUSSION The microstructur.e of as-rolled Ti-45AI/TiB2 material is shown in Fig. 1. The intermixed TiAlM, TiAlT and Ti 3AI lath stucture is simliar to the as-cast and as-extruded materials[1,3], but the scale of as-rolled material is much finer. Both {lll} and the forbidden {l1l}/ 111) twins were observed, Fig. 2. This is in agreement with the previous results[1,3]. A similar structu
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