Ba-Nd复合变质对Mg-3Si-4Zn铸造合金组织及性能的影响

Effect of Ba-Nd Composite Modification on Microstructure and Mechanical Properties of Mg-3Si-4Zn Cast Alloy

研究了Ba-Nd复合变质对Mg-3Si-4Zn铸造合金组织及力学性能的影响,用OM、SEM、EDS、XRD等方法对微观结构进行了表征,通过硬度测试研究了材料的力学性能。结果表明,当单一变质剂Ba在Mg-3Si-4Zn合金中的添加量为1.2%(质量分数)时,变质效果最好。形成的BaMg_(2)Si_(2)相可以作为初生Mg_(2)Si的异质形核核心,使初生Mg_(2)Si细化。Ba-Nd复合变质是通过在Mg-3Si-4Zn-1.2Ba合金中加入变质剂Nd来完成的。通过Miedema模型和对吉布斯自由能的线性拟合,发现由于Nd和/或Ba原子能与Si原子形成更加稳定的化合物NdSi、NdSi_(2)、Ba_(2)Si、BaSi_(2),防止Si原子在凝固的初始阶段与Mg原子结合,因此初生Mg_(2)Si的生长速度被抑制,微观结构中初生Mg_(2)Si相变小。当Nd含量为2.0%时,Ba-Nd复合变质效果最好,即初生Mg_(2)Si由平均面积约为600μm^(2)的树枝状变为平均边长约5μm的近似方形的组织,共晶Mg_(2)Si由平均面积约444μm^(2)的复杂粗大汉字状变为平均面积约89μm^(2)的简单汉字状组织。合金硬度由575.75 MPa提高到612.11 MPa,提高了6.31%。

The effects of Ba-Nd composite modification on the microstructure and mechanical properties of Mg-3Si-4Zn cast alloy were investigated.The microstructure was characterized by OM,SEM,EDS and XRD.The mechanical properties were tested by hardness test.The best modification effect was achieved when a single denaturant Ba of 1.2wt%was added to the Mg-3Si-4Zn alloy.Results show that the formed phase BaMg_(2)Si_(2)can act as a heterogeneous nucleation core for the primary Mg_(2)Si,refining the primary Mg_(2)Si.Ba-Nd composite modification is achieved by adding the modifier Nd to the Mg-3Si-4Zn-1.2Ba alloy.By the calculation of the Gibbs free energy using the Miedema model and a linear fit,it is found that the growth rate of primary Mg_(2)Si is suppressed and the primary Mg_(2)Si phase in the microstructure becomes smaller because more stable compounds like NdSi,NdSi_(2),Ba_(2)Si,and BaSi_(2)can be formed by Nd and/or Ba atoms with Si atoms,preventing Si atoms from binding to Mg atoms at the initial stage of solidification.The best Ba-Nd composite modification effect is achieved when the Nd content is 2.0wt%,i.e.the primary Mg_(2)Si changes from a dendritic shape with an average area of about 600μm^(2)to a nearly square-shape with an average side length of about 5μm and the eutectic Mg_(2)Si changes from complex and coarse Chinese-script shape with an average area of about 444μm^(2)to simpler shape with a mean area of about 89μm^(2).The hardness of the alloy is increased from 575.75 MPa to 612.11 MPa,increased by 6.31%.