Interface Related Strength Phenomena in Two-Phase Titanium Aluminides
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with respect to ( 1120)(x
,
the y phase has a
domain structure of six orientation variants. This results in three distinct y/7 interfaces which can 175 Mat. Res. Soc. Symp. Proc. Vol. 586 ©2000 Materials Research Society
formally described in terms of rotations of one lamella relative to its neighbour by multiples of 600 about (111), [7]. An exact matching of close packed orientations is only achieved when the rotation is 180°, so that the adjacent lamella have a [112(1 11) true twin orientation. In all the other cases a significant mismatch occurs due to the differences in lattice parameters. For the unrelaxed state of the interfaces misfit strain of about 1% are expected [7], which cannot be accommodated elastically. These lamellar boundaries therefore contain dense arrays of misfit dislocations with Burgers vectors entirely in the interface, which reduce the misfit strain energy and improve the interfacial coherency by producing local relaxations [2, 7]. The structural features of the interfaces observed in the Ti-48A1-2Cr alloy are demonstrated in Fig. 1. The thickness of the lamellae ranges from 50 nm to 1 gim and the fraction of a 2 larnelae is 5-10%. Most of the interfaces join differently oriented y lamellae, with 60' and 1200 rotational variants being predominant. One particular feature of the misfitting interfaces is that they often contain a significant tilt component (Fig. 1(b)). This misfit is accommodated by dislocations with a
Fig. 1. Sructural features of lamellar interfaces in Ti-48AI-2Cr. (a) Lamellar microstructure, foil orientation
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