The Influence of Structural Similarity on Phase Transformations Between Bcc and the Laves Compounds
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Mat. Res. Soc. Symp. Proc. Vol. 398 0 1996 Materials Research Society
on a 60 barn average cross section for irradiation damage, the dose rates used varied between 1.6. 10-2 dpals and 3.9.10-2 dpa/s. Further experimental details are given in [2]. RESULTS: Fig. 1 shows TEM bright field images taken in the course of electron irradiation of the TiCr 2 phase are seen to have Laves compound at 225 K. In Fig. lb, small precipitates of the P3 appeared in the center of the irradiated area after 3.9 dpa. The bend contours of the Laves compound weaken as they enter the irradiated area, indicating that some progress toward irradiation-induced amorphization has occurred. It is however noted that the prominent Laves phase bend contour of type < 1 12>C 14 is still faintly visible in the irradiated area of Fig. lb. This indicates that the anisotrop y associated with crystalline planes of this type is still present at the phase. This is supported by the diffraction patterns shown in Fig. 2, point of nucleation of the 13 and the Laves phases are present after 4.4 dpa. As is evident from in which reflections of both 13 phase is preferentially oriented with respect to the initial Laves compound. Fig. 2, the nucleating 13 phase during electron irradiation is presented A detailed analysis of oriented precipitation of the 13 elsewhere [3].
Fig. 1. Images of TiCr2 foil during irradiation at 225 K. a) 0.0 dpa, b) 3.9 dpa, c) 5.6 dpa. In
b), small 13-phase crystals are seen in the middle of the irradiated area. The prominent < 1 12>C 14type bend contour is still faintly observed traversing the irradiated area. In c), the center of the phase. Arrows mark a fixed position in the foil. irradiated area has largely transformed to the 13 b 124
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Fig. 2. Diffraction patterns of the area shown in Fig. 1. a) 0.0 dpa, b) 4.4 dpa, c) 5.6 dpa. In b)9 type 13 phase spots have appeared just inside of C 14 spots (arrows in b). In c), type 13 spots have appeared coincident with initial C 14 < 112> type spots (arrows in c).
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THE LAVES PHASE/BCC STRUCTURAL RELATIONSHIP: Two consistent features observed in diffraction patterns of oriented precipitation of 13from the Laves phase are 1) the coincidence of type spots of the precipitating 13phase with C14 diffraction spots, and 2) the alignment of 13 spots with C14. Both of these features are seen in Fig. 2, above. Based on these observations, a reciprocal lattice relationship involving three orientation variants of precipitating 13was determined, which is capable of accounting for all cases of oriented 13-phase precipitation [3]. This relationship is shown in stereographic projection in Fig. 3. The relationship pins 8 of the 12 13reciprocal lattice points to C14 points of type , and also accounts for the alignment of 1 with C14. The relationship imposes strain on the bcc lattice. Transforming to real space, and taking a--0.4932 nm and c=0.8005 nm for C14 TiCr2 results in non-cubic lattice which shall be referred to as 13' which has: a=b=0.3176 nm c=0.2847 nm z=13_-90 0 r
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