采用双电偶热分析技术和SEM表征了Mg-6Al-xZn合金(简称AZ6x合金,x=0,2,4,6,质量分数,%)在砂型铸造过程中的凝固行为和显微组织;采用背散射电子衍射(EBSD)分析对合金的晶粒尺寸进行定量表征.利用Pandat热力学软件计算了合金的平衡截面相图、非平衡Scheil模型凝固过程,以及枝晶生长抑制因-(growth restriction factor,或称为Q值).结果表明,在AZ6x合金的砂型铸造凝固过程中,AZ60合金中只有非平衡凝固的γ-Mg17Al12,而AZ62~AZ66合金的铸态组织中除了γ-Mg17Al12相,还出现了Φ-Mg21(Al,Zn)17相,并且随着Zn含量的增加,γ-Mg17Al12相减少而Φ-Mg21(Al,Zn)17相增多.热力学计算结果表明,AZ60~AZ64合金中γ-Mg17Al12相和Φ-Mg21(Al,Zn)17相在一定温度下能够完全固溶到α-Mg中,而AZ66合金中的Φ-Mg21(Al,Zn)17相在任何温度下都不可能完全固溶.研究结果还表明,Zn含量高的合金具有高的Q值、小的晶粒尺寸及低的枝晶相干点固相分数fDCPs;并讨论了Q值、晶粒尺寸与fDCPs的关系.
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