Electron backscattered diffraction investigation of the texture of feathery crystals in aluminum alloys

Scanning electron microscopy (SEM), metallographic observations, and automated electron backscattered diffraction (EBSD) experiments were carried out on ''feathery crystals'' of a unidirectionally solidified (1D) Al-Cu alloy and of a direct-chill (DC) cast Al-Mg-Si alloy. The results clearly show that the ''feathery grains'' are made of twinned lamellae, which are parallel to a (111) twin plane. The contrast seen in the metallographic sections after a Barker etching or observed in an SEM is perfectly corroborated with the EBSD reconstructed microstructure. The lamellae are separated by an alternance of straight and wavy lines. Some equiaxed grains are also observed occasionally in the specimens. From the [111] pole figures of the various grains, it is concluded that the thermal gradient direction is close to, but not necessarily within, the (111) twin plane: its direction is in between a [01 (1) over bar] and a [<(11)over bar>2] direction. Within a given feathery grain, small variations of the crystallographic orientations (subgrain boundaries) are observed. The lamellae of one grain can sometimes penetrate into another one. Based upon this information, the mechanism of feathery grain growth previously proposed by Eady and Hogan is ruled out. Although no other growth mechanism is proposed yet, it is believed that feathery grains are the result of a competition similar to that occurring in normal dendritic alloys, but with twinned dendrites.