稀土Er和Ce复合改性对过共晶Mg-Si合金组织与性能的影响

通过金相、SEM、EDS和XRD等方法研究了稀土元素Er、Ce复合改性对过共晶Mg-3.2Si合金中初生Mg_2Si相的影响,并探讨了改性机制。结果表明,在过共晶Mg-3.2Si合金中,添加约0.6%(质量比,下同)Er时,初生Mg_2Si相的平均尺寸由150μm减小到40μm,其形态从粗大树枝状变为不规则多面体;在此基础上,继续添加1.0%Ce,可获得5~10μm大小的多面体或球状初生Mg_2Si相,改性效果最佳;但稀土添加过量,会出现过改性现象。改性机制:稀土Er和Ce吸附或富集在初生Mg_2Si相表面,降低固液界面张力,减小临界形核功,有利于更多初生Mg_2Si晶核的生成;稀土Er和Ce在初生Mg_2Si相的表面富集,减小了晶向之间的相对生长速度,使初生Mg_2Si相从粗大树枝状变为不规则多面体形状;合金凝固时,稀土Er与Ce的晶体结构相同,形成了连续固溶体。当稀土添加量为0.6%Er和1.0%Ce时,Mg-3.2Si合金试样的抗拉强度σb与伸长率δ分别提高到127 MPa和3.7%最大值。

Effect of Er and Ce on modification of primary Mg2Si phase in hypereutectic Mg-Si alloys

The primary Mg2Si phase of hypereutectic Mg-3.2Si alloys,in which rare earth Er and Ce were added,was studied by OM,SEM,EDS and XRD.And the metamorphic mechanism was also discussed.When 0.6% (mass ratio,the same below)Er to Mg-3.2Si alloys added,the size of the primary Mg2Si phase decreases from 150 μm to 40 μm,and its shape transformes from the bulky dendritic into the irregular polyhedron shape.Then continuely adding 1.0% Ce,the polyhedron or globular primary Mg2Si phase of 5-10 μm can be obtained.When the addition of rare earth increases higher,the primary Mg2Si phase grows into a coarse.Rare earth Er and Ce on the primary Mg2Si phase reduce the critical nucleation work and the relative growth rate between the crystallographic orientation,more primary Mg2Si crystal nucleus form,and its shape transformes into the irregular polyhedron shape.The rare earth Er and Ce form a continuous solid solution during solidification.When 0.6% Er and 1.0% Ce added,the tensile strength σb and elongation δ of Mg-3.2Si reach to maximum value of 127 MPa and 3.7%.