Microstructural Evolution during Superplastic Deformation in Large-grained Iron Aluminides
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reductions at 950°C-1050°C. Tensile specimens were cut from the bars using a spark machine. The gauge section of tensile specimens for deformation at 900'C was 3.2xl.Oxl2mm while that for deformation at 10000C was 4.Oxl.2x8mm. Tensile tests were conducted on a SHIMADZU AG - 100 kNA test machine at 900'C under a strain rate of 2.08x10-2 s-1for the specimens with gauge section of 3.2xlxl2mm and at 1000'C under a strain rate of 1.39x10 2s-1 for the specimens with gauge section of 4.Oxl.2x8mm. Thin foils were sliced from the specimens whose elongation was 80% (for the specimen deformed at 900'C), 150% and 290% (for the specimens deformed at 1000'C). TEM foils were prepared by conventional twin jet polishing technique using an electrolyte of 10% perchloric acid in methanol at -30'C. The foils were examined in JEM-200CX and JEM-2000FXII electron microscopes operating at 200kV. Two-beam conditions and 1g/3g weak-beam conditions were obtained using a double-tilt specimen holder. EXPERIMENTAL RESULTS Metallographic examination showed that the average grain size of large-grained iron aluminides decreased during superplastic deformation. Figure 1 shows an example, in which the average grain size of Fe-36.5A1-2Ti alloy changed from 500Mrm before deformation to about 50.m after deformed to 290% at 1000°C.
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Figure 1. Optical micrographs of the Fe-36.5AI-2Ti alloy. (a) before deformation, (b) deformed by 290% at 10000C.
The results of TEM observation on the dislocation configurations showed that there exist a great number of subgrains in the specimens, indicating that a recovery process occurred. The misorientations between subgrains in the sample deformed at 900'C with a strain of 80% were measured from the deviation of Kikuchi lines in the diffraction patterns of these subgrains. They range from 0.40 to 3.23'.
Figure 2a shows the grains in the sample deformed at 1000°C with a strain of 290%. The misorientations between grains range from 0.25' to 19.44' as shown in Figure 2b. Relatively large misorientations have been found. The boundaries between grains labeled as 11 and 15, 8 and 15, 8 and 14, 1 and 14, 1 and 13, 12 and 13, 12 and 19 have higher misorientations from 5.560 to 10.30. Although these grains have high misorientations, they have the same diffraction patterns if the misorientations are ignored. However, the diffraction pattern of grain labeled as 22 is different from those of the other grains. The misorientations between 22 and other grains are near 190. As the diameter of grain 22 is only about 25 gm, which is much smaller than that of the initial grain size of 3 50um, this grain is not likely to be the initial grain. KK8.21.2
Figure 2. (a) Grains in the sample deformed at 10000C with a strain of 290%, (b) Schematically description on the misorientations of the grains.
Since there exist a large number of subgrains, it is necessary to investigate the dislocation structures of the subgrain boundaries. Figure 3 and 4 show the subgrain boundaries and low angle grain boundaries in the sample deformed at
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