TEM study on laser processed intermetallic phases: Al 16 Ti 6 (Fe or Ni) 7
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The structures of two laser processed ternary intermetallic phases, Al16Ti6(Fe or Ni) 7 , have been studied by using electron diffraction and microscopy. A convergent beam electron diffraction (CBED) technique for structure determination is described based upon the principle of kinematic interpretation of large angle systematic reflections in CBED disks. The structure type of phases Al16Ti6(Fe or Ni)7 is characterized as the Mn23Th6(cF116)type, the same as those of relevant phases Cui6Mg6Si7 and Al5oTi25Fe25 etc., by using the CBED technique. Various errors involved in the analyses are also discussed.
I. INTRODUCTION Various transition-metal Ti aluminide-based intermetallics have recently been intensively investigated for their potential of future structural materials1"6 and also for their basic alloy properties, which have attracted the attention of many solid state physicists.7^13 These intermetallics are light, hard, and have high melting temperatures and strong anti-oxidation, anti-corrosion properties. However, most of them are brittle, which becomes a major obstacle to the engineering application of these materials. Owing to the intrinsic brittleness found in various Al3Ti-based Ll 2 -structured ternary intermetallics,14'15 it became clear that brittleness of these intermetallics may well be related to their atomic bonds and electronic structures rather than microstructures. This further indicates the need for detailed crystallographic analyses of these thermodynamically related structures for the understanding of their mechanical behaviors. During the investigation of three laser processed Al3Ti-based ternary intermetallic systems, A162.5 ±xTi25±y (Fe, Ni, or Cu)i 2 5 ± z , 1 6 ' 1 7 two fee cubic phases with a space group Fm3m and compositions Al56.oTi2o.6Fe23.4 and A l s ^ T i ^ s N i ^ ! as determined by EDS (Energy Dispersive X-ray Spectroscopy) were found in equilibrium with two Ll 2 -type phases of compositions Al 650 Ti 230 Feii9 and Al65.iTi23oNi119. The almost identical compositions of the two face-centered phases correspond to stoichiometric compounds Ali 6 Ti 6 (Fe or Ni) 7 ; note that 1 6 : 6 : 7 = 55.2:20.7:24.1. This composition ratio is identical with intermetallic phase Cui 6 Mg 6 Si 7 , which has the Mn23Th6-type (cF116) structure, as determined by powder and single crystal x-ray diffraction.18 The structure of an intermetallic a
'Present address: 264 Materials Research Laboratory, University of Illinois at Urbana-Champaign, 104 South Goodwin Avenue, Urbana, Illinois 61801. J. Mater. Res., Vol. 8, No. 3, Mar 1993 http://journals.cambridge.org
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compound of approximated composition Al5oTi25Fe25 has previously been proposed as Mn23Th6(cF116)-type using powder x-ray diffraction,19'20 although no detailed structure analysis on the compound was reported. Another relevant intermetallic compound Al5oTi25Ni25 was also found to have fee cubic-type lattice with a unit cell of a size almost identical with Al5oTi25Fe25,21 but neither the space group nor the atomic structure of
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