Which statement about how the shape of the solid–liquid interface and the fineness of the structure change with the temperature gradient G and growth rate V in alloy solidification is correct?

Answer: C. As G/V is decreased, the interface changes from planar to cellular to columnar dendritic to equiaxed dendritic, and the larger G×V, the finer the structure

As the condition for constitutional supercooling shows, the interface shape is set mainly by G/V, and a larger G/V makes a planar interface more stable. Decreasing G/V increases the constitutional supercooling, and the interface changes to cells, then columnar dendrites, and then equiaxed dendrites, where new crystals form in the liquid ahead of the interface. G×V, on the other hand, is the cooling rate (K/s) and sets the size of the structure: the faster the cooling, the less time solute has to diffuse and the finer the cell and dendrite spacings. The option in which a smaller G/V approaches a planar interface has the order reversed, and faster cooling does not give a coarser structure. Saying that the shape is set by G×V alone confuses the roles of the two quantities that set shape and size.