ELECTRON BACKSCATTERED DIFFRACTION INVESTIGATION OF THE TEXTURE OF FEATHERY CRYSTALS IN ALUMINUM-ALLOYS

Scanning electron microscopy (SEM), metallographic observations, and a utomated electron backscattered diffraction (EBSD) experiments were ca rried out on ''feathery crystals'' of a unidirectionally solidified (1 D) Al-Cu alloy and of a direct-chill (DC) cast Al-Mg-Si alloy. The res ults clearly show that the ''feathery grains'' are made of twinned lam ellae, which are parallel to a (111) twin plane. The contrast seen in the metallographic sections after a Barker etching or observed in an S EM is perfectly corroborated with the EBSD reconstructed microstructur e. The lamellae are separated by an alternance of straight and wavy li nes. Some equiaxed grains are also observed occasionally in the specim ens. From the [111] pole figures of the various grains, it is conclude d that the thermal gradient direction is close to, but not necessarily within, the (111) twin plane: its direction is in between a [01 (1) o ver bar] and a [<(11)over bar>2] direction. Within a given feathery gr ain, small variations of the crystallographic orientations (subgrain b oundaries) are observed. The lamellae of one grain can sometimes penet rate into another one. Based upon this information, the mechanism of f eathery grain growth previously proposed by Eady and Hogan is ruled ou t. Although no other growth mechanism is proposed yet, it is believed that feathery grains are the result of a competition similar to that o ccurring in normal dendritic alloys, but with twinned dendrites.