Inclusions in aluminium crystals

Aluminium crystals, growing by recrystallization in fine-grained material, often do not consume some grains of the original material. They contain a number of inclusions, which do not vanish even after very long annealing. These reflect light simultaneously in the same direction. By means of (1) the etching method of Lacombe and Beaujard and (2) the X-ray method it is proved that these inclusions are deformed grains of the original material possessing spinel-twin relationship to the large surrounding crystal correct to within a few degrees. All four possible spinel-twin positions are represented. The above is not valid when the `inclusions' are larger than the average original grain size. In that case `inclusions' have an undeformed lattice and possess no special orientation. On the strength of these experiments it was expected that an aluminium crystal grown by recrystallization contains also inclusions with an orientation slightly different (by a few degrees) from its own orientation. The existence of such inclusions has been proved by X-ray methods. By means of the etching-method it was possible to determine the orientation of inclusions with dimensions down to 30 ; for the X-ray method the dimensions had to be of the order of 0.1 mm. To account for the observations the assumption is made that a grain of the original material having approximately (within a few degrees) a spinel-twin orientation towards a growing crystal of aluminium is left by the latter as an unconsumed inclusion, whereas if the spinel-twin orientation is perfect to within a fraction of a degree that grain may grow in exact twin position by `stimulation'.