A directionally-disordered precursor to amorphization in electron irradiated Cu 4 Ti 3
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Evidence of directional structural disordering prior to complete amorphization of Cu4Ti3 during 2 MeV electron irradiation is presented. It is shown that the observed disordering is intrinsic to the irradiated material and is incommensurate with the structure of the crystalline compound. The results point to the presence of significant anisotropy in the response of the material to irradiation. A model based on local rotation of clusters of atoms within a partially chemically-disordered matrix is developed. These results are discussed in relation to recent proposed mechanisms of amorphization based on volume expansion. The questions raised by this work indicate a need for detailed studies of the atomic level structural evolution of a material during solid state amorphization.
I. INTRODUCTION Due to the simple nature of the damage process, electron and light ion irradiation provide a highly controllable means1 with which to study solid state amorphization transformations (SSAT; see reviews2"5). Much of the early work on the electron irradiation induced SSAT in intermetallic compounds was carried out by Fujita, Mori, and co-workers.6"11 Largely because of that work and other efforts that have built on those studies, it is generally accepted that there exist two types of irradiation-induced SSAT.5"12 At low temperatures, the SSAT is homogeneous and occurs at fairly low levels of irradiation damage.6-9 At higher temperatures up to the maximum temperature for amorphization (approximately room temperature in most compounds), the SSAT follows a heterogeneous path3 nucleating at dislocations, grain boundaries, the surface,7 antiphase boundaries,11 defects related to martensitic transformations,13 etc. and growing into the surrounding materials. This transformation requires a larger total dose. In the early studies of homogeneous SSAT, it was experimentally shown that there is a linkage between electron irradiation induced chemical disordering of the crystal lattice and amorphization in the bcc-based ordered intermetallic compounds of the Cu-Ti alloy system.10'11 This was the first documented connection between an atomic level process and the loss of crystal stability. These experimental results have been supported by Monte Carlo and molecular dynamics simulations of the SSAT process.14"16 Recent work in fcc-based Zr3Al has supported the connection between chemical disordering and amorphization.17'18 Of course, chemical disordering will make one contribution to the energy increase of the lattice with irradiation and others will be made by point defects as pointed out by Pedraza,19 yet it appears J. Mater. Res., Vol. 6, No. 10, Oct 1991 http://journals.cambridge.org
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that irradiation induced chemical disordering can play a major role in the destabilization of the crystalline lattice. Apart from the above discussion that centers on the driving force behind the irradiation induced amorphization, is the issue of the actual mechanisms whereby the crystal structure is lost. One approach to this explanatio
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