Which sequence of solidification structures is generally seen in the cross section of an ingot made by pouring molten metal into a mold?

Answer: C. From the outside: chill grains, columnar grains, equiaxed grains

When the melt touches the cold mold wall, the large undercooling nucleates many crystals at once, forming a layer of fine equiaxed chill grains at the surface. Next, grains whose orientation favors growth extend selectively in the direction opposite to heat flow toward the mold wall (inward, as heat flows outward), forming a zone of elongated columnar grains pointing toward the center. In the center the temperature gradient becomes small, and equiaxed grains with no preferred direction grow from crystal fragments drifting in the liquid. The choices that start from columnar or equiaxed grains on the outside rearrange this order. Columnar grains have direction-dependent properties, and impurities tend to collect where columnar grains meet, so castings are sometimes inoculated or stirred to increase the equiaxed zone.