Discussion
During solidification of a pure molten metal, the grains in the casting near the mould wall are
Answer: B
The grains near the mould wall are fine and randomly oriented.
When molten metal touches the cold mould wall it is rapidly undercooled. Many nuclei form at once and each can grow only a little before meeting its neighbours, so this outer chill zone consists of small, equiaxed grains with random crystallographic orientations.
Inward from the chill zone, grains whose preferred growth direction lies along the heat flow outgrow the rest and form long columnar grains, which are coarse and ordered, so that description belongs to the next zone, not the one at the wall. Coarse random grains would need slow cooling with few nuclei, the opposite of conditions at the wall.
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