Pb-Bi-Sn准包晶合金定向凝固微观组织演化

采用液态金属冷却定向凝固结合液淬法,在温度梯度为18 K/mm、凝固速度为0.5～100 μm/s的条件下系统研究了Pb-30％Bi- 18％Sn准包晶合金的微观组织演化及溶质分布规律.主要从凝固路径、相组成、溶质分布及微观组织特征等几个方面来研究Pb-Bi-Sn准包晶合金定向凝固过程的组织演化.其中,准包晶合金的凝固路径通过DSC差热分析的方法进行研究,发现其凝固路径为:L→L+α-Pb→L+α-Pb+β-Pb7Bi3→L+α-Pb+β-Pb7Bi3+Sn→ (β-Pb7Bi3+Sn)共晶.结合XRD,分析定向凝固试样的相结构,结果表明:试样由具有面心立方结构的α-Pb相、密排六方结构的β-Pb7Bi3相和体心正方结构的(Sn)相构成.在Pb-Bi-Sn合金定向凝固过程中,当凝固速度为0.5～2 μm/s时,初生α-Pb相以胞状界面生长,而当凝固速度增大至5μm/s及以上时,初生α-Pb相将由胞状转变为枝晶状界面生长.利用能谱分析仪对定向凝固试样进行溶质分布测试,发现局部出现溶质偏析,导致共晶组织(β-Pb7Bi3+Sn)出现.

Microstructure Evolution of Directionally Solidified Pb-Bi-Sn Quasi-Peritectic Alloy

The Pb-30 wt.%Bi-18 wt.%Sn quasi-peritectic alloy was prepared and directionally solidified with liquid metal cooling and quenching technology under solidification rate of 0.5-100 μm/s and temperature gradient up to 18 K/mm.The microstructure evolution and solute distribution were investigated,which were based on solidification path,phase composition,solute distribution and microstructure characteristics.The DSC analysis was employed to investigate the solidification path,and the results indicate that the solidification is: L→L+α-Pb→L+α-Pb+β-Pb7Bi3→L+α-Pb+β-Pb7Bi3+Sn→(β-Pb7Bi3+Sn) eutectic.The XRD results show that the phase compositions of the directionally solidified samples consist of primary α-Pb phase(f.c.c),β-Pb7Bi3 phase(h.c.p) and(Sn) phase(b.c.t).The primary α-Pb phase grew in cellular solid/liquid interface with solidification rate 0.5-2 μm/s,but it grew in dendritic S/L interface as the solidification rate increased to 5 μm/s or more.The solute distribution in the directionally solidified sample was measured by using EDS.The results show that solute segregation occurs in local position,which results in the formation of eutectic structure(β-Pb7Bi3+Sn).