Research On Microstructure Evolution Of Pb-Sn And Ni-Nb Eutectic Alloy In Rapidly Changing Velocity

Non-stability solidification process is one of the most important part that control and affect the solidified microstructure remarkably. Two alloy systems of Pb-Sn and Ni-Nb eutectics were selected and their composition were Sn-38.1wt%Pb, Sn-20wt%Pb, Ni-24.19wt%Nb and Ni-53.98wt%Nb. Directional solidified microstructures were investigated at non-stability and stability solidification conditions. Microstructure features including the lamellar spacing selection, primary phase formation and evolution, as well as halo formation in Ni-Ni₃Nb eutectic have been researched. The experiment results are as follows:In Sn-38.1wt%Pb eutectic alloy, the microstructure splitting takes place both of Sn and Pb phases when the lamellar spacing ratio increased at the non-stability directional solidification. A new way of the lamellar spacing adjustment has been observed. In this case the lamellar firstly merges and then splits smaller. When the lamellar spacing coarsen there are four ways to result in coarsening the lamellar spacing of Pb-Sn eutectic, which are Sn phase merging, Pb phase merging, Sn phase growth and Pb phase growth.There is the value of νλ² = 284.03μm³/s which is greater than the value ofνλ_m² = 261.6μm³/s calculated by JH model. When the drawing velocity increasesthe irregular microstructure formed with α-Ni growing up from the eutectic interface.Most of the splitting and merging of the lamellar spacing take place in a-Ni at rapidly changing velocity, but the a-Ni lamellar at the two sides of β-Ni₃Nb will grow up outside when the velocity increased, β-Ni₃Nb keeps its growth direction and shape at rapidly changing velocity.At the constant directional velocity 10μm/s the directionally solidified may contains 20% volume fraction. When the velocity increases rapidly from 1 to 10μm/s, there is no volume fraction. At different velocities from rapidly changing of 2μm/s, the primary phase will decrease and disappear. The ways of the primary phase disappearance are the splitting at an initial transient zone, remelting at the rapidly increasing velocity and firstly growing up and then disappearing at the rapidly decreasing velocity. In Ni₃Nb-Ni₆Nb₇ eutectic alloy Ni₆Nb₇ primary phase may get smaller, disappears or keeps its shape at the rapidly changing velocity, but the volume fraction of Ni₆Nb₇ primary phase does not decrease greatly or disappear compared with that of the Ni-Ni₃Nb eutectic alloy.