Study on the response of dendritic growth to periodic increase-decrease pressure in solidification via in situ observation using succinonitrile

A novel experimental apparatus for in situ observation of dendritic growth in directional solidification under dynamic pressure and thermal conditions was built, based on the principles of direct squeeze-casting, and the response of dendritic growth to an increase-hold-decrease pressure and a periodic increase-decrease pressure was investigated using succinonitrile. The results showed that, when continuous increase-hold-decrease pressure was applied, the dendrites underwent a growing and re-melting process where the dendritic re-melting phenomenon with fish-bone-like dendrites in the pressure decline stage was observed for the first time. When periodic increase-decrease pressure was applied, dendrites re-grew and were re-melted periodically owing to the change in effective undercooling, which is the difference between the equilibrium melting temperature under pressure TmP and the temperature at the dendrite tip Ttip, as expressed by ΔTeff=TmP-Ttip, whereas secondary dendrite arm spacing increased with the re-growth and re-melting cycle. The relationship of the velocity of the dendrite tip v with the effective undercooling ΔTeff and pressure change ΔP was derived and expressed as v=AΔTeff+BΔP2+C where the effective undercooling plays a primary role in determining the tip velocity.