Response of primary dendrite spacing to varying temperature gradient during directional solidification

The response of primary dendrite spacing λ to a change in solidification condition is examined through the directional solidification of a succinonitrile-acetone alloy. While the growth velocity was kept constant, the thermal gradient G was altermately varied between two extreme values. The measured average dendrite spacing is compared to a calculation involving a previously proposed model, revealing good correlation. The investigation results enable us to characterize some important features about dendrite spacing evolution: during alternately changing G, dendrite spacing λ varies along different paths; after changing G-variation direction at turning points, the incubation periods required for the initiation of λ variation are much longer than those at the initial stage; as G changes back to its initial state, λ cannot return to its original value, revealing an unclosed hysteresis loop for the λ-G diagram; and continuing the process of alternately changing G will result in the formation of a closed loop. This kind of hysteresis loop is repeatable and roughly the same for varying G in reverse sequences. Thus, becoming free of the effect of the initial state, λ varies in nearly the same way for the same change in process condition, regardless of the difference in previous history.