Journal of Crystal Growth, Vol.498, 85-92, 2018
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 T-m(p) and the temperature at the dendrite tip T-tip, as expressed by Delta T-eff = T-m(p) - T-tip, whereas secondary dendrite arm spacing increased with the re-growth and re-melting cycle. The relationship of the velocity of the dendrite tip nu with the effective undercooling Delta T-eff and pressure change Delta P was derived and expressed as nu = A Delta T-eff + B Delta P-2 + C where the effective undercooling plays a primary role in determining the tip velocity.