Constrained Deformation of TiAl PST Crystals
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1225-HH03-07
Constrained Deformation of TiAl PST Crystals Kengo Goto, Kyosuke Kishida and Haruyuki Inui Department of Materials Science and Engineering, Kyoto University Sakyo-ku, Kyoto 606-8501, JAPAN ABSTRACT TiAl polysynthetically twinned (PST) crystals were deformed under plane strain condition, in which the anisotropic macroscopic deformation of PST crystals is restricted with a channel die, in order to clarify the deformation behavior of TiAl/Ti3Al lamellar structure under constraint conditions. TEM analysis of deformation modes together with the Taylor analysis reveals that all TiAl orientation variants deform to yield the relaxed-constraint-type plastic strain, where three shear strain components are not zero for each TiAl variant but are macroscopically compensated to zero by the existence of twin-related TiAl lamellae at the early stage of deformation. The Taylor analysis assuming the relaxed constraint conditions is found to be adaptable for predicting the operative deformation modes in TiAl at the early stage of deformation and also for correlating quantitatively the stress-strain behavior of PST crystals under external constraint with those under the unconstraint condition. INTRODUCTION TiAl/Ti3Al two-phase alloys with fully lamellar structure are of special interest because of their superior tensile properties, fracture toughness and creep resistance compared to those with the other microstructure types [1]. We have been conducting a systematic study of the inherent deformation mechanisms of the TiAl/Ti3Al lamellar structure using our polysynthetically twinned (PST) crystals composed of the lamellar structure with a single lamellar orientation over the entire crystal [2]. We have revealed that the macroscopic deformation behavior of PST crystals is highly anisotropic in the plane perpendicular to the loading axis, when the specimens are deformed in uniaxial compression or uniaxial tension. For example, when the loading axis is parallel to the lamellar boundaries, the normal strain in the direction perpendicular to the lamellar boundaries is essentially zero, while that in the parallel direction increases. Such anisotropic macroscopic deformation has been confirmed to be in good agreement with that deduced from the operative deformation modes determined by TEM analysis for six orientation variants in TiAl lamellae, which are three matrix variants IM, IIM and IIIM and three twin variants IT, IIT and IIIT [2]. In the case of polycrystalline TiAl based alloys, the anisotropy of macroscopic deformation behavior of PST crystals are considered to be restricted by surrounding crystals. It is thus important to understand the deformation mechanism of PST crystals under constraint conditions. In the present study, PST crystals with the loading axis parallel to the lamellar boundaries (corresponding to orientation A1 and A2 in our previous studies [2]) were deformed under plane strain condition, in which the inherent anisotropic macroscopic deformation of PST crystals described above is restricted with a channel
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