Ion beam mixing of U-based bilayers
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M. Nastasi, M. Cohen, and C. Olsen Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
J. R. Tesmer Physics Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Chuck Egert Martin Marietta Yl2 Plant, Oak Ridge, Tennessee 37831 (Received 22 December 1989; accepted 14 February 1991)
Bilayer samples of U/Al, U/Ti, U/Si, and U/C have been ion beam mixed with 400 keV Ar and U/Al with Xe ions at doses from 2 x 10 15 to 1 x 10 17 ions/cm 2 . Mixing experiments were performed at various temperatures between 77 and 420 K. The amount of interfacial mixing, 4Dt, follows a linear dose dependence below a critical temperature, depending on the system studied. Below this temperature, the mixing efficiency, defined as d(4Dt)/d§ where ADt is the mixing and $ is the dose, is temperature independent. Its value, as well as the value of the transition temperature, agrees well with the thermodynamical model of chemically biased diffusion in a thermal spike for the four systems tested. The transition between the thermal spike regime and the temperature enhanced mixing regime was interpreted on the basis of an intracascade mechanism. The formation of an intermetallic compound in the U/Al system was detected and interpreted on a qualitative basis by crystallographic considerations.
I. INTRODUCTION Ion beam mixing of uranium with C, Si, Ti, and Al has been studied as a function of temperature and dose to test the applicability of the thermodynamic model proposed by Johnson1 and Cheng.2 These experiments are of interest first because of the very different ballistic behavior of the two components of the bilayers. Uranium, with a high density and atomic number, tends to form dense cascades; on the other hand, Si, Ti, C, and Al tend to form dilute cascades and it is of interest to determine if the mixing at the interface is changed compared to the other systems studied in the literature. Second, these elements can form intermetallic compounds with uranium. It is important to know which compound forms, and to check if the model still predicts correctly the mass transport during irradiation. II. EXPERIMENTAL Uranium/M bilayer samples, where M = (Ti, Si, Al, and C), on SiO2 substrates were produced by sequential sputter deposition of a 2000 A U film, followed by the deposition of the other element. The thickness of the asdeposited layers was measured by Rutherford Backscata'Current
address: Centre d'Etudes et Recherches sur les Materiaux, CEN Grenoble BP85X 38041, Grenoble Cedex, France.
tering Spectroscopy (RBS). Backscattering was carried out using a 2.2 MeV and 3 MeV helium ion beam, the backscattered particles being detected at a 167° angle. The measured values for the top layers, Ti, Si, Al, and C, were 1600, 6000, 3000, and 1600 A, respectively. An additional set of U/Al samples was also evaporated on top of a U-oxide layer with a U thickness of 1000 A with an Al top layer of 1200 A for mixing experiments with different ions. The first bilayers were ion beam mixed at
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