Crystal structures of small Al-rich Fe alloy particles formed by a gas-evaporation technique

Abstract

Crystal structures in small Al-rich Fe particles were studied by powder X-ray diffraction and transmission electron microscopy. The specimens of the small particles were made by evaporating an Al alloy material containing 5.0-33.3 at.% Fe in Xe gas at a pressure of 3.3 × 10³ Pa. The small particles were 70-150 nm in diameter. In the particles, f.c.c. Al-Fe solid solution, Al₁₃Fe₄, Al₂Fe and AlFe structures were observed. The Al₁₃Fe₄ structure, which is abundant in all specimens, was found to have many lattice defects and twins, specifically planar defects lying on (100), (201) and (001) planes and twins on (100), (201) and (001) planes. Three kinds of planar defect and the (100) twin were directly observed by high-resolution electron microscopy. The high-resolution image of the (100) twin supports Black's model. The observed facts and the atomic position proposed by Black suggest a similarity between the atomic arrangement around the (100) plane and that around the (201) plane. Also, two types of new structure were recognized one is a trigonal structure with a = 0.858 nm and α = 111.78°, belonging to the space group R3c or R3c, and the other is a modulated structure belonging to the Laue class 4/mmm. For the former structure it was found that the spacing of (110) plane varies with the position in a particle. The latter structure has modulation along the four-fold rotational axis and is considered to have a structural relation to the tetragonal Al _m Fe structure. The Fe concentrations in the trigonal and the modulated structures were estimated to be 19 and 17 at.% respectively.