Formation of feathery crystals depending on the solidifying conditions in Al-Mg alloys.

Automatic indexing of electron backscattered diffraction patterns, scanning electron microscopy, and optical microscopy observations have been carried out on aluminum-magnesium-silicon, aluminumcopper, and aluminum-silicon alloys directionally solidified or semicontinuously cast using the direct chill casting process. From these combined observations, it is shown that the feathery grains are made of ͗110͘ primary dendrite trunks (e.g., [01 ]) split in their centers by a coherent (111) twin 1 plane. The average spacing of the dendrite trunks in the twin plane (about 10 to 20 m) is typically one order of magnitude smaller than that separating successive rows of trunks (or twin planes). The [01 ] orientation of these trunks is close to the thermal gradient direction (typically within 15 deg)-1 a feature probably resulting from a growth competition mechanism similar to that occurring during normal ͗100͘ columnar dendrite growth. On both sides of these trunks, secondary dendrite arms also grow along ͗110͘ directions. Their impingement creates wavy noncoherent twin boundaries between the coherent twin planes. In the twin plane, evidence is shown that ͗110͘ branching mechanisms lead to the propagation of the twinned regions, to the regular arrangement of the primary dendrite trunks along a [ 11] direction, and to coherent planar twin boundaries. From these observations, it 2 is concluded that the feathery grains are probably the result of a change from a normal ͗100͘ to a ͗110͘ surface tension/attachment kinetics anisotropy growth mode. This change might be induced by the added solute elements, by the local solidification conditions (thermal gradient, growth rate, and melt convection), and possibly by the help of the twin plane itself. Convection in the melt could also play a role in the symmetrization of the ͗110͘ growth directions of the side arms. Finally, the proposed mechanisms of feathery grain growth are further supported by the observation of ͗110͘ dendrite growth morphologies in thin aluminum-zinc coatings.

Section: Resultsmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

〈110〉 dendrite growth in aluminum feathery grains

Henry

Rappaz

Jarry³

1998

“…Besides a high thermal gradient (typically 100 °C/cm) and growth rate (typically 1 mm/s), [9,10,11] feathery grains are known to be favored by the presence of strong fluid flow [2,12] and a minimal concentration of certain solute elements. [10] Despite these experimental facts, the complete mechanism of feather grain formation is still unclear.…”

Section: Introductionmentioning

confidence: 99%

Influence of convection on feathery grain formation in aluminum alloys

Henry¹,

Gruen

Rappaz

2004

The influence of convection on the formation of feathery grains, i.e., of columnar grains made of twinned dendrites growing along Ͻ110Ͼ directions, in AA1050 aluminum alloys has been studied. Round billets have been semicontinuously cast in a mold equipped with lateral liquid feeding. The fluid flow pattern in the liquid sump has been modeled using Fidap software. Feathery grains have been observed in the region opposite the mold entrance, i.e., in regions where the change of the velocity field (shearing rate) is the highest. Electron back scattered diffraction (EBSD) maps of two feathery regions, which were symmetric with respect to the liquid flow pattern, showed clear symmetry relationships. Furthermore, the Ͻ110Ͼ secondary dendrite arms had grown in directions opposite to the fluid flow. This experimental evidence brings more experimental support to the mechanism of feathery grain formation proposed earlier by Henry et al. [1]

“…From the ͗111͘ 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 ] and a [ 2] direction. The results clearly show that the ''feathery grains'' are made of twinned lamellae, which are parallel to a (111) twin plane.…”

mentioning

confidence: 99%

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

Henry

Jarry

Jouneau³

et al. 1997

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 ͗111͘ 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 ] and a [ 2] direction. Within a given feathery grain, small variations of the crystallographic 1 1 1 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.