采用高温度悌度定向凝固装置进行了Al-40%Cu(质量分数)过共晶合金的定向凝固实验,研究了凝固速率跃迁过程中的凝同组织演变.结果表明,当定向凝固速率从10 μm/8跃迁减速到2μm/s时,由于固/液界面附近的液相成分向共晶点成分变化以及耦合共晶组织的界面生长温度高于初生Al_2Cu相的界面生长温度,合金凝固组织从初生Al_2Cu枝晶和Al/Al_2Cu共晶组织转变为全耦合层片共晶组织.组织转变过程中,板条状的初生Al_2Cu相先分解成小尺寸的初生相,然后小的初生相逐渐被共晶组织所取代,这种组织转变是凝同界面前沿液相中溶质扩散不足造成的,而不是由合金中存在的热及溶质对流引起的.在初生相生长形态中,由于凝固速率跃迁引起的界面前沿液相中Cu成分富集,造成凝固界面生长温度升高,Jackson因子σ变小,Al_2Cu初生相由小平面相向非小平面相转变.
Intermetallics combining metal and ceramic properties exhibit colorfully complicated growth morphologies in solidification. Different from the solidus phase solidification, intermetallic solidification remains less understanding of the correlation between processing parameters and microstructure morphologies. In this paper, considering that the intermetallic compound Al_2Cu phase has well-known thermophysical properties available for the theoretical and experimental research, we carried out the directional solidification experiments and focused on an Al-Al>_2Cu hypereutectic alloy,where the Al_2Cu phase was solidified as a primary phase. The primary Al_2Cu phase growth behavior in the experiments included phase competition growth between coupled eutectic and primary phase,faceted phase transition and its change in growth morphology. By using a high thermal gradient directional solidification apparatus, the directionally solidified microstructures of Al-40%Cu (mass fraction)hypereutectic alloy were investigated and discussed based on the competition growth model. As the growth rate was changed abruptly from 10 μm/s to 2 μm/s, the microstructure transition from a primary Al_2Cu dendrite plus interdendritic eutectic to an entirely coupled eutectic occurred due to the interface growth temperature of the coupled eutectic exceeding that of the primary Al_2Cu dendrite. Also, the alloy liquid composition ahead of the solid/liquid interface approaching the eutectic point caused this microstructure transition. Simultaneously, in the changing-growth rate experiments, the primary Al_2Cu dendrites were firstly broken into small ones and then became the eutectic microstructure; it was interpreted rather by the decrease in liquid solute concentration ahead of the solid/liquid interface than by the effect of the thermal-solutal convection. Moreover, the morphology change in phase growth from faceted primary Al_2Cu phase to non-faceted phase was observed by reducing abruptly the growth rate from 10 μm/s to 2 μm/s, which can be explained by the Jackson factor α decreasing with increasing the interface growth temperature of primary Al_2Cu phase.
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