Nd变质过共晶Al-17.5%Si合金的微观组织和断口形貌

Microstructure and fracture morphology of hypereutectic Al-17.5%Si alloy modified with Nd

采用单一稀土元素Nd对过共晶Al-17.5%Si合金进行变质;采用扫描电镜、金相显微镜、电子探针、透射电镜及X射线衍射等方法对变质前后合金的微观组织、成分和相组成进行分析;并对变质前后合金的断口形貌进行分析。结果表明：加入0.3%Nd（质量分数）到过共晶Al-17.5%Si合金中可同时变质初生硅和共晶硅,经过变质处理后,初生硅的形状由星形和不规则形状变为块状,尺寸由40~60μm减小到10~30μm,共晶硅由长针状变成球状或短棒状;初生硅区域的主要成分是硅,中心边缘几乎检测不到Nd元素,Nd沿晶界分布;与未变质合金相比,经0.3%Nd变质后的Al-17.5%Si合金硅相上孪晶密度增大,变质后的合金中没有生成新相,α（Al）相和硅相的晶格常数变大;经0.3%Nd变质后,合金的整体性能大幅度提高,抗拉强度提高了35.8%,从120 MPa提高到163 MPa,伸长率从0.8%提高到2.2%,提高了175%。

Pure Nd was adopted to modify the hypereutectic Al-17.5%Si alloy.The morphology,composition and phase of alloys before and after modification were analyzed by SEM,OM,EPMA,TEM and XRD.The fracture morphologies of the alloy were analyzed by SEM before and after modification.The results show that 0.3% Nd,mass fraction） can effectively refine the primary and eutectic silicon in alloy.The morphology of primary silicon after modification transforms from pentalpha and irregular shape to block,and the grain size of primary silicon is refined from 40-60 μm to 10-30 μm.A majority of eutectic silicon changes from acicular or lamellar morphology to small short rod.No Nd is detected in the center and edge of silicon,Nd distributes along the grain boundary.The twinning density in modified Si crystals is much higher than that of the initial sample.No new phase forms,and the lattices of silicon and aluminum increase after modification.The tensile strength of the alloy modified by 0.3%Nd increases by about 35.8% from 120 MPa to 163 MPa,and its elongation increases by about 175% from 0.8% to 2.2%.