High-resolution electron microscopy of amorphization of Cu 4 Ti 3
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High-resolution electron microscopy of amorphization of Cu4Ti3 D. E. Luzzi and M. Meshii Department of Materials Science and Engineering and the Materials Research Center, Northwestern University, Evanston, Illinois 60201 (Received 25 March 1986; accepted 14 July 1986) The electron irradiation-induced, crystalline-to-amorphous transition was studied in the intermetallic compound Cu 4 Ti 3 by high-resolution electron microscopy. Using highresolution maps from the crystalline region into the amorphized region, the amorphization process and the amorphous structure were examined. The extent of chemical order in crystalline regions just prior to amorphization was studied by simultaneously imaging superlattice and fundamental lattice fringe contrast. The chemical order continuously decreased in these regions but faint superlattice contrast was recognized as long as the crystalline feature remained on the image, supporting the theory that chemical disordering is the major driving force for amorphization. The amorphization process appears to be evolutionary, leading to a nanocrystalline type of amorphous structure. A model of the amorphization process is proposed based on the present results and those from previous studies.
I.INTRODUCTION Electron irradiation induces a crystalline-to-amorphous (C-A) transition in many intermetallic compounds. ' In contrast to the case of ion irradiation, which can produce displacement cascades, each collision event imparts an energy sufficient to displace only an atom or two. Therefore the C-A transition occurs due to a gradual increase in the energy of the crystal until it becomes unstable with respect to the amorphous state.2"5 Recently it has been proposed that this energy increase is primarily due to irradiation-induced chemical disordering in Cu4Ti3.3~5 This chemical driving force mechanism has been supported by more in-depth experiments in the compound CuTi2 (Refs. 5 and 6) and qualitatively in other Cu-Ti intermetallic compounds. Fujita7 has analyzed the C-A transition in NiTi, but no attempt was made to investigate the role of chemical disordering in the amorphization process. In a previous report of the C-A transition in Cu 4 Ti 3 (Ref. 4), a high-resolution electron microscopy (HREM) study of the crystalline/amorphous interface was used to support the chemical disordering energy mechanism. By examining the presence or absence of the superlattice fringe intensity (due to some residual chemical order) simultaneously with the existence of a bend contour due to the fundamental Bragg reflection, it was shown that, although the intensity of the superlattice fringe decreased in the irradiated crystal that has not yet become amorphous, in every area in which the J. Mater. Res. 1 (5), Sep/Oct 1986
fundamental bend contour was visible (indicating the presence of crystal), the superlattice fringe was also visible. Thus it was shown that chemical disordering induces the C-A transition since no areas existed in the irradiated region that contained stable crystalline Cu 4 Ti 3 in the chemica
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