High-Temperature Deformation of Uniaxially Aligned Lamellar TiAl/Ti3Al
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The microstructure of samples deformed below 900 K indicates that deformation twinning is a very important mechanism contributing to plastic deformation [10]. In micromechanical simulations Schlogl and Fischer [11] determined contributions of 20% to 50% of deformation twinning to the total plastic deformation of uniaxially aligned lamellar 7l% crystals. In this work the evolution of the strain rate sensitivity and of the flow stress during uniaxial compression tests of uniaxially aligned lamellar two-phase Ya crystals is determined. The influence of deformation twinning and of the motion of ordinary dislocations on the microstructure after hightemperature deformation is also investigated. EXPERIMENTAL Cylindrical bars (8 mm in diameter and 18 cm in length) of a Ti-48 at.% Al alloy were produced by arc melting and casting in an Ar atmosphere. An induction furnace was used for the subsequent zone melting process. In an Ar atmosphere, the material was moved through the induction zone at a speed of 2 mm/h. The zone melting process resulted in a single grain over most of the length of the bar. In this grain the % 2 and the yphases are arranged in lamellae with parallel interfaces. The samples were oriented by Laue back reflection X-ray patterns and cut by spark erosion to samples of dimensions 3 mm x 3 mm x 6 mm. The samples were deformed in uniaxial compression in a Zwick 1474 testing machine with different orientations of the lamellar interfaces to the deformation axis. The lamellae were oriented parallel to the deformation axis (00), perpendicular (900) or inclined (30', 450 or 600) on the zone circle. The samples were deformed at a strain rate of 2x10-4 s-1 with repeated relax-
ation tests in vacuum. The activation volume was also determined from deformation tests with repeated changes of the strain rate between 10-' s-' and 10-4 s-. Electrolytically thinned parts of the samples were investigated with transmission electron microscopy in a JEOL 200CX and a Philips CM30. RESULTS The yield stress of samples with lamellae oriented parallel or perpendicular to the compression axis is much higher than for samples with intermediate lamellar orientations. The temperature dependence of the yield stress is shown in Figure 1.The present yield stresses for Ti-48 at.% Al are higher than the yield stresses obtained by Inui et al. [6] as the Al concentration of their samples is higher (49.3 at.%). The deformation curves of samples with lamellae oriented perpendicular to the deformation axis consist of three parts (Figure 2). After the elastic range a region of about Ae2% with high strain hardening (7 GPa) occurs. In the third part of the deformation curves the strain hardening rate is low. Above 900 K the samples with lamellae oriented perpendicular to the deformation axis show a decrease of the flow stress with increasing plastic strain (Figure 2). Especially at high temperatures the flow stress strongly decreases upon repeated stress relaxations of 100 s duration. For the sample deformed at 573 K the strain rate sensitivity is
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