Keywords:Ti-6Ale2Sne4Zre2Mo-0.1Si alloy Superplasticity Ultrafine-grained microstructure Metastable microstructure Grain-boundary-sliding Accommodation mechanism A B S T R A C T Herein we quantitatively clarify the effects of grain size, β fraction, and morphology on the high-temperature deformation behavior of the Tie6Ale2Sne4Zre2Mo-0.1Si alloy. For this purpose, five materials were subjected to high-temperature tensile deformation: the UFG1 specimen (having an equiaxed morphology with d α = 0.78 μm and V β = 2.8%), the UFG2 specimen (having an equiaxed morphology with d α = 0.99 μm and V β = 24.2%), the FG1 specimen (having an equiaxed morphology with d α = 2.65 μm and V β = 11.2%), the FG2 specimen (having an equiaxed morphology with d α = 4.12 μm and V β = 11.0%), and the STQ specimen (with an acicular α′ martensite morphology). The UFG1 specimen is produced by hot-rolling of the STQ specimen having an acicular α′ martensite microstructure at 750°C. The UFG2 specimen is prepared by heat treatment of the UFG1 specimen at 400°C. The FG1 specimen is as-received Tie6242S alloy plate, and the FG2 specimen was prepared by heat treatment of the FG1 specimen at 900°C. The UFG specimens exhibited higher ductility associated with frequent activation of superplasticity than the FG specimens owing to the effect of decreasing grain size. The STQ specimen exhibited higher ductility at 700°C than the FG specimens. Quantitative analysis of the deformation mode according to internal-variable theory revealed much more grain boundary sliding in the UFG specimens. A comparison of the deformation behavior of the UFG1 and UFG2 specimens revealed excellent superplastic ductility in the UFG2 specimen at higher strain rates (10 −3 and 10 −2 s −1 ) and in the UFG1 specimen at lower strain rates (5 × 10 −4 and 10 −4 s −1 ). This behavior is ascribed mainly to different accommodation mechanisms during deformation of these specimens; dynamic β precipitation from supersaturated α microstructure occurred in the UFG1 specimen, whereas a decomposition process in which supersaturated β precipitates dissolve into the α phase was enhanced in the UFG2 specimen. In addition, the excess β precipitation observed in the UFG2 specimen led to enhanced α/β grain boundary sliding, resulting in further enhancement of the superplasticity. (H. Matsumoto). Fig. 13. Fraction of GMD and GBS relative to the overall deformation for the UFG1, UFG2, FG1, and FG2 specimens at (a), (b) 700°C, and (c), (d) 800°C and at strain rates of (a), (c) 10 −3 s −1 and (b), (d) 10 −4 s −1 .

Titanium alloys are widely used in the aircraft industry. Under sheets form, they can be employed to the manufacturing of pylon or engine parts. With the aim of a cost reduction, this study proposes to act on the starting microstructure so as to improve the mechanical properties during the forming stages. In the present study, investigations are focused on Ti-6Al-2Sn-4Zr-2Mo (Ti6242) alloy specially used for the hot areas (e.g. parts close to the engine or the combustion chambe...). Presently, an important mechanical test campaign was performed on Ti6242 alloy, it examines, on the one hand, the microstructure qualified by the aircraft industry and, on the other hand, a new range of refined microstructures obtained by hot straining process. For each test, microstructural observations exhibited complex phenomena including simultaneously both grain growth and dynamic recrystallization. The occurrence, sequencing and coupling of the mechanisms, strongly depend on the starting microstructure and the test conditions (time-temperature and strain rate) investigated. They are not easy to understand and require further tests and observations. In such a framework, the implementation of mechanical models are efficient and relevant to promote a better knowledge of the microstructural evolution observed and their influence on the mechanical behavior.

To clarify the hot deformation characteristics of Ti alloys, flow behaviour, micro structural evolution and deformation mechanisms were investigated in a Ti-6Al-2Sn-4Zr-2Mo alloy with two initial micro structure: an ultra-fine grained (UFG) and a fine-grained (FG) microstructure (dα=0,8 µm and dα=3 µm respectively) by isothermal interrupted tensile tests, SEM observations and through electron back scatter diffraction experiments. Depending on the test conditions and on the initial α grain size, the flow behaviour can exhibit steady state flow and/or hardening and/or softening. The microstructure and texture evolutions have been studied mainly by using electron backscatter diffraction (EBSD) technique and SEM observations. They evidenced in particular the occurrence of α grains growth as well as dynamic recrystallization (DRX). The different flow behaviour associated to the microstructure evolution is shown and discussed to clarify the main deformation mode that could be assume to occur depending on the microstructure, the temperature and the strain rate.