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根据杂质原子诱发共生的成对孪晶理论,变质剂原子在Si晶体内诱发的共生成对孪晶大大降低了初生Si生长时的各向异性,使得变质后的过共晶Al-Si合金中的初生Si最终生长为球团状。Na原子吸附在共生成对孪晶间的晶格紊乱区——亚晶界,形成了初生Si内部的富Na带。亚晶界对初生Si强度的消弱、富Na带对亚晶界强度的进一步降低、以及α(Al)、Si二相间热膨胀系数的差异造成的热应力是初生Si开裂的原因。球团状初生Si内部的裂纹是在合金凝固结束后的固态收缩阶段形成的。
According to twin theory of induced twinning by impurity atoms, the co-formation of twin induced by modifier atoms in Si crystals greatly reduces the anisotropy during the growth of primary Si, so that the degenerate hypereutectic Al-Si alloy The nascent Si eventually grows into pellets. Na atoms are adsorbed on the disordered lattice-intergranular intergranular-subgrain boundaries, forming Na-rich bands inside the primary Si. The subgrain boundaries weaken the strength of primary Si, the further decrease of strength of Na subband with respect to the subgrain boundaries, and the thermal stress caused by the difference between the thermal expansion coefficients of α (Al) and Si phases are the reasons for the rupture of primary Si. The cracks inside the pellet-like primary Si are formed during the solid-state contraction after the alloy has been solidified.