THERMAL-STABILITY OF A SUPERSATURATED SOLUTION OF HAFNIUM IN ALUMINUM

Single crystals of aluminum were implanted with 110 keV hafnium ions to doses of 2×1014-2×1015 cm-2, corresponding to a maximum local concentration of about 0.13-1.3 at.%, which is above the solubility limit. A small dose of radioactive 181Hf atoms was co-implanted. The samples were studied by means of Rutherford backscattering spectrometry-channeling and perturbed angular correlations (PACs). Both measurements show that all hafnium atoms in the as-implanted samples occupy substitutional sites in a damage-free close environment. The r.m.s. thermal vibration amplitude of the hafnium atoms was determined from a comparison of measured and simulated channeling dips. The temperature dependence of this amplitude deviates from the prediction of a simple Debye model. PAC measurements on the samples with 2×1015 Hf atoms cm-2 showed that precipitation in the implanted layer starts during annealing at 380°C. The precipitates grow during prolonged annealing at 420°C. After annealing at 535°C, most hafnium atoms are diffused to the surface. At the same time a quadrupole interaction was observed, which may be attributed to oxygen association of hafnium atoms at the interface between aluminum and the thin surface layer of Al2O3. In channeling, precipitate formation shows up as a slight narrowing of the channeling dips, which means that the precipitates are coherent.