Deformation of Bi-PST Crystals of TiAl Produced by Diffusion Bonding
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Component PST crystals measuring 20x5x2.5 mm 3 were cut from as-grown PST ingots with a composition of 48 at.% Al, and their mating surfaces (20x5 mm 2) were polished using silicon wafer KK3.9.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society
polishing technology. Diffusion bonding was carried out at 800'C for 24 hours in a vacuum of 8 x 105 Pa applying a bonding pressure which was much smaller than the yield stress of PST crystals with orientation B1 at 800°C [6, 7,11]. Orientations of component PST crystals are given using the notation used in our previous studies (for example, ref.[10]) and 0 indicating the angle by which one of the two component crystals is rotated about the loading axis with respect to the other. In this study, bi-PST crystals with orientations of B 1 (0) /B 1 (00), A1 (0)/A
1
(0°) and A2 (0)/A
2
(00) (Fig.l) were prepared and investigated.
After bonding, all bi-PST crystals were annealed at 1350 0 C for lh and their bond interface was examined by an optical microscope to check if recrystallization occurs. As was found in diffusion bonding studies of TiAl-base alloys[17, 18], recrystallized 7 grains tend to appear at the bond interface and grow into both sides of interface. Indeed, in the previous paper [16], we could not prepare biPST crystals consisting of component crystals with some asymmetric orientations. However, we found that the difficulty can be overcome by choosing bonding conditions properly, in particular, by giving great care to the flatness and smoothness of mating surface and to making bonding pressure as small as possible. Deformation of Bi-PST Crystals in Tension 3 Specimens with the gauge section size measuring 2.0 x 2.0 x 5.0 mm (Fig.2) were cut from biPST crystals and they were tested at a strain rate of 2 x 104s-1 at room temperature in air. Macroscopic deformation of deformed bi-PST crystals and deformation structures on their surface were examined using scanning electron and optical microscopy, respectively.
Z: loading axis
WX
A1(0) A1(00)
B1(0) B1(00 )
A2(0) A2(00)
Figure 1. Geometry of bi-PST crystals.
2mm Figure 2. Geometry and size of bi-PST tensile specimen.
Z: loading E E
axis
2mm
to
F,0
KK3.9.2
RESULTS AND DISCUSSION Bond Interface
Figure 3 shows a microstructure of the bonding interface in a bi-PST crystal with a orientation of BI(90°)/B 1(0°). The component crystals are seen to be bonded without forming recrystallized y grains at the bond interface even though they are highly asymmetrical with respect to the interface. Diffusion bonding and subsequent annealing conditions were selected so that the bond interface remains optical-microscopically flat. However, bi-PST crystals with such an interface as that observed in polycrystalline TiAl-base alloys with the lamellar microstructure where y and oC 2 lamellae of the two adjacent lamellar grains are interlocked can be produced by increasing annealing time after diffusion bonding. The deformation behavior of bi-PST crystals with such an interlocked type interface would be
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