Thermal stability of a supersaturated solution of hafnium in aluminium

Abstract

A number of single crystals of aluminium were implanted with 110 keV hafnium ions to doses of 2 × 10 14 – 2 × 10 15 cm −2, corresponding to a maximum local concentration of about 0.13 –1.3 at.%, which is far above the solubility limit. A small dose of radioactive 181Hf atoms was co-implanted. The samples were studied by means of Rutherford backscattering spectrometry-channelling and perturbed angular correlations (PACs) . Both channelling and PAC measurements show that all hafnium atoms in the asimplanted 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 channelling dips. The temperature dependence of this amplitude strongly deviates from the prediction of a simple Debye model. PAC measurements on the samples with 2 × 10 15 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 strong, well-defined quadrupole interaction was observed by PAC, which may be attributed to oxygen association of hafnium atoms at the interface between aluminium and the thin surface layer of Al 2O 3. In channelling, precipitate formation shows up as a very slight narrowing of the channelling dips, which means that the precipitates are coherent.