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This is the 1st progressive report on the growth twin crystals in aluminum and aluminum-base alloys, and describes the morphology of them.A growth twin crystals in cast aluminum is found in the form of thin plate in thickness in which a planar twin plane lies between both boundary surgaces. It originates near the cooling end of the cast block and grows in the direction parallel but opposite to the heat flow, accompanying multiplication or paralleled piling of new growth twin crystals. The (111) planes conjugate the twin plane and facted (100) planes grow at an angle from the twin plane. It is therefore determined that prefered growth occurs on the (112) plane macroscopically.The growth twin dendrite in multi-phase alloys such as aluminum-magnesium alloy has a similar crystallographic orientation. Both the columnar and growth twin crystals have similar dislocation density Dislocations and/or faults, however are dispersed in the former homogeneously, while they are concentrated in the latter on the twin plane. Other features of the growth twin crystal include crystallographic stability at elevated temperature normal segregation of solutes such as magnesium and lesser coring.

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