Irradiation Mixing of Al into U 3 Si
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were then polished flat with 0.05 gtm A12 0 3 grit. The polished wafers were mounted on copper blocks using Sauereisen and then coated with 1800 A of Al by electron beam evaporation in ultrahigh vacuum. Because of the exposure to air before Al coating, the specimens initially had an oxide layer between the Al and the U 3 Si. In-situ ion irradiations were performed at the 2 MV tandem National Electrostatics ion accelerator facility at Argonne National Laboratory. Irradiations were performed with 1.5 MeV Kr ions at dose rates of approximately 6 1011 Kr/cm 2 sec. TRIM95 damage calculations [5] using a 20 eV threshold energy yield 18 dpa / 1016 Kr/cm 2 at the interface between the Al and the U 3 Si. Average damage rate at the interface was 1 10-3 dpa/s. RBS with 1.5 MeV He with a detector angle of 450 and a sample angle of 200. The spectra were deconvoluted by standard curve fitting
techniques [6]. RESULTS A series of RBS spectra taken from one wafer of Al coated U 3 Si before and after increasing doses of 1.5 MeV Kr ion irradiation at 30'C and 3000C are shown in figures 1 and 2. Fitting of the RBS spectra indicate that the Al/U3Si interface initially had an oxide layer which SEM shows consists of a combination of small U02 precipitates and an overall oxide layer on the U 3Si. When the U 3Ai is amorphized at 300C, the Al mixing proceeds at a slow rate. However, it is clear from a cursory observation that at 3000C when the U 3 Si remains crystalline, Al mixing and diffusion is quite rapid after a threshold dose of approximately 1.0 1016 Kr/cm 2 and a new phase forms. Studies [8,9] of the U Si Al alloy system indicate at equilibrium the formation of U(SiAl) 3 . In our experiments, the averaged mixed phase contains approximately 33 at.% U corresponding to U(SixAll-x)2. The mixing of Al into U also resulted in the formation of UA12 at elevated temperatures [7]. Growth of the U(SixAll-x)2 layer at 3000C is shown in Figure 3. The growth occurs with a square root of dose dependence typical of phase formation under ion irradiation. There is evidence of the same phase forming in the RBS spectra after the 300 C irradiations, fig. 1. During neutron irradiation and uranium burn up, mixing of Al into U 3 Si 2 has been reported to occur with a square root of dose dependence and to form a phase of U(Si,A1)3 [2] that is similar in structure to UA13 . The similarity of ion beam mixing of Al into U and U 3 Si may indicate that the mixing is controlled by phase formation and the diffusion of Al through the phase. The amount of Al diffusion, extract by fitting the Al and U edges of the RBS spectra, is shown in figure 4 for both irradiation temperatures. At 3000 C after a dose of 21016 Kr/cm2, signal from the discrete Al layer has disappeared as the new phase formed. The rate of mixing, a(4Dt)/aO, of Al into U 3 Si is 0.05 nm 2/dpa at 30 0C. Due to the rapid mixing and phase formation, we can only estimate a lower limit of at least 2.07 nm 2 /dpa at 3000C. These values can be compared to mixing of Al into pure U by 400 keV Ar ions [7] whic
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