凝固过程液相区内晶体形成的模拟研究

 通过NH_4C1水溶液二维凝固过程的模拟实验,研究了凝固过程中液相区内的晶体形成条件。结果表明,随凝固条件和时间的不同,型壁上晶体的游离、表面凝固产生“晶雨”及枝晶熔断均可在液相区形成晶体。

A SIMULATION STUDY OF CRYSTAL FORMATION IN LIQUID ZONE DURING SOLIDIFICATION

Crystal formation in liquid zone during solidification, which is necessary for equiaxed crystal growth, has been studied by direct observation of two dimensional solidification of NH4Cl-H2O solution. It is shown that crystals in liquid zone can be formed in three ways under appropriate conditions: （ 1 ） crtstals nucleated initially at the mold wall may be separated away from it and taken into the liquid zone by flowing liquid; （ 2 ） dendrite arms separated from their trunks due to rentet in mushy zone may be taken into the liquid zone by interdendritic liquid flowing from mushy zone to liquid zone; （3） crystals formed at free surface showered down.Crystal separation from the mold wall is observed at the beginning of solidification. If superheat of the solution and/or cooling rate are low, more crystals will be separated from the mold wall and this will be an important way for equiaxed crystal growth. Only when the condition of remeltgiven by Flemings is satisfied and velocity of liquid flow from mushy zone to liquid zone is significant, can separated dendrites caused by re-melt be a source of crystals in liquid zone. Crystal shower or iginates at top free surface and appears when under cooling of the solution near the surface is large enough to make it solidify.