The spacing between neighboring arms of a dendrite. The secondary arm spacing is roughly inversely proportional to the cube root of the cooling rate, and smaller spacings give finer segregation and defects and better mechanical properties.
Japanese name: デンドライトアーム間隔
Dendrite arm spacing appears in 1 practice question across 1 topic, including Solidification and cast structures.
Answer: B. About 10 µm
λ₂ is inversely proportional to the cube root of the cooling rate, so when the cooling rate increases 125-fold, λ₂ becomes 1/125^(1/3) = 1/5, giving 50/5 = 10 µm. About 0.40 µm assumes that λ₂ is inversely proportional to the cooling rate and divides by 125, and about 4.5 µm uses an exponent of 1/2 and divides by √125 ≈ 11.2. The exponent is about 1/3 because coarsening during solidification, in which thin arms dissolve and are absorbed into thicker ones, proceeds in proportion to the cube root of the local solidification time. λ₂ serves as a guide to the cooling rate in each part of a casting, and the smaller λ₂, the finer the interdendritic segregation, microshrinkage and intermetallic compounds, which improves the tensile strength, elongation and fatigue strength.
Chvorinov's rule Constitutional supercooling Dendrite Equilibrium partition coefficient Eutectic modification Macrosegregation Scheil equation