An investigation of aluminium intermetallic phases using 57Fe Mossbauer spectroscopy and complementary techniques.

Pure intermetallic compounds Al3Fe, AlmFe, AlxFe, ac-AlFeSi, and Al6(Fe,Mn) have been extracted from Bridgman grown model aluminium alloys by dissolving the aluminium matrix in butanol. The resultant transmission Mdssbauer spectra for each intermetallic compound were interpreted according to their crystal structure. Variable temperature 57Fe Mdssbauer studies have enabled the Debye temperature thetaD of each compound to be determined. The crystal structure of Al3Fe contains five different Fe sites within the unit cell. Four of the iron, Fe(l)-Fe(4), sites are approximately identical and produced a thetaD = 434 +/- 5 K. The remaining Fe site, Fe(5), produced a thetaD = 488 +/- 5 K, and the combined spectral areas a 3D = 452 +/- 5 K. There is only one individual site within the crystal structures of AlmFe, AlxFe, and Al6(Fe,Mn), which produced a thetaD of 358 +/- 5 K, 360 +/- 5 K, and 352 +/- 5 K respectively. The ternary intermetallic compound, ac-AlFeSi, has two different Fe sites within the unit cell. Fe(l) had a thetaD - 291 +/- 5 K, and Fe(2) thetaD = 329 +/- 5 K. The combined spectral areas of these two sites produced a thetaD = 311 +/- 5 K. The variation in the OD values was attributed to changes in the Al-Fe shortest bond within the Fe centred A1 polyhedra. The Fe centred A1 polyhedra are a common feature of all the intermetallic compounds studied. The iron atom in all the intermetallic compounds may have existed in a Fe2+ oxidation state. A Direct Chill-cast ingot was grown and two samples, A and B, were taken from regions within the ingot containing a mixture of two intermetallic compounds. Alloy sample A was found to contain the intermetallic compound combination Al3Fe + Al6Fe. The intermetallic combination Al6Fe + ac- AlFeSi was found to exist in alloy sample B. Transmission Mdssbauer spectroscopy was performed on the extracted phases and the insitu phases to determine the relative proportions of the intermetallic compounds within the two alloy samples. Alloy sample A had 50:50 +/- 5 % Al3Fe + Al6Fe, whereas alloy sample B had 30:70 +/- 5 % Al6Fe + ac-AlFeSi. The surface of alloy sample B was investigated using several surface techniques, CEMS, SAAES, and SAXPS, to determine whether the same relative proportions existed in the surface, and near surface, regions of the sample. A region of very fine amorphous iron super-paramagnetic grains were to dominate the near surface region of the sample, which was present due to selective oxidation of the Al6Fe intermetallic compound. This was then removed when the surface of the alloy sample was KI electro-etched, which had the effect of leaving the intermetallic particles standing proud of the surface. The CEMS technique identified that the Al6Fe + ac-AlFeSi existed in a 80:20 +/- 5 %. This change in phase ratio after the KI electro-etch process was attributed to the preferential etching of the ac-AlFeSi aluminium intermetallic compound.