Growth Mechanism Of Ni-Sn Anomalous Eutectic During Non-equilibrium Solidification

Eutectic solidification is an important solid-liquid phase transformation process,and it has been studied widely.The performance of eutectic materials depends largely on the regularity of eutectic microstructure,characteristics scale and orientations of their eutectic phases.Previous studies have shown that in several eutectic alloys,regular lamellar or rod eutectic transforms into anomalous eutectic via a rapid solidification process.Nevertheless,there has been considerable controversy regarding the formation mechanism of the anomalous eutectic.Ni-Sn eutectic alloy is choosed because it has been widely studied and has relatively complete physical parameters.In this paper,different rapid solidification processes have been taken,such as casting which has lower solidification speed,powder atomization which has rapid cooling speed and free solidification,laser remelting block and powder bed.The microstructure evolution and phase selection mechanism for eutectic alloys in the process of nearly balance to rapid non-equilibrium solidification conditions had been studied.The main achievements obtained are as follows:?1?Clarified the sufficient and necessary conditions to produce Ni-Sn anomalous eutectic:The solidification behaviors of Ni-Sn eutectic alloys in as-cast solidification process,powder atomization and laser remelting block or powder beds were revealed.It was found that Ni-Sn anomalous eutectic microstructure formed in different solidification conditions can be summed up in two categories:one is spherical anomalous eutectic,and the other one is reticular band anomalous eutectic.These two kinds of anomalous eutectic not only have obviously different forms,but also the growth sequences for lamellar eutectic and anomalous eutectic are different.The main difference between these two kinds of anomalous eutectics lies in the competitive nucleation and growth during solidification.In conclusion:free nucleation of the two eutectic phases and rapid solidification are the sufficient and necessary conditions for the formation of anomalous eutectic.?2?Revealed the competition behavior of two eutectic phases and microstructure evolution of Ni-Sn eutectic:Ni-30wt.%Sn hypoeutectic,Ni-32.5wt.%Sn eutectic and Ni-33wt.%Sn hyper-eutectic alloy powder beds were remelted by a laser beam in different laser powers and scanning velocities.It is found that,the microstructure at the top of the molten pool is regular lamellar eutectic with lower laser scanning velocity.In the middle of molten pool,it is composed by primary?-Ni dendrites?Ni-30wt.%Sn hypoeutectic?or Ni₃Sn dendrites?Ni-33wt.%Sn hypereutectic?and interdendritic regular lamellar/rod eutectic.At the bottom of molten pool,it is nearly spherical?-Ni phase?Ni-30wt.%Sn hypoeutectic and Ni-32.5wt.%Sn eutectic?or nearly spherical Ni₃Sn phase?Ni-33wt.%Sn hypereutectic?and band anomalous eutectic.With the increase of laser scanning velocity,completely lamellar/rod eutectic microstructure gradually changed into primary phase and interdendritic regular lamellar/rod eutectic at the top of the molten pool.The overall microstructure in the molten pool gradually became thinning and anomalous eutectic microstructure at the bottom of molten pool gradually reduced.The volume fractions of nearly spherical primary?-Ni and Ni₃Sn particles above and adjacent the anomalous eutectic area decreased with the increase of scanning velocity.With the increase of laser power,the overall microstructure in the molten pool became coarsening.The KGT model describing dendrite growth and the TMK model describing eutectic growth were used to calculate the change of the interface temperature between primary dendritic tips and eutectic interface with growth velocity——the interface response function.The competition growth behavior between primary phases and lamellar eutectic at the top of the molten pool for laser remelting Ni-Sn eutectic powder bed under different laser remelting velocity were revealed.The influence of laser remelting number on the microstructure of the molten pool was considered through the Ni-30wt.%Sn hypoeutectic alloy powder bed remelted by the laser beam once and twice with the scanning velocity of 1 and 2 mm/s.It is found that there is anomalous eutectic at the bottom of the molten pool remelted twice by the laser beam when V_S=1 mm/s.But there is still regular lamellar eutectic at the bottom of the molten pool remelted twice by the laser beam when V_S=2 mm/s.It is speculated that the remelting of primary?-Ni dendrite has close relationship with the formation of anomalous eutectic.?3?Clarified the crystal orientation relationship of two eutectic phases in anomalous eutectic and its growth mechanism:For laser remelting Ni-30wt.%Sn hypoeutectic powder bed,it is found that the?-Ni particles in anomalous eutectic can be divided into two types based on the crystal orientation.One kind of?-Ni particles have consistent orientations,which may come from the remelting of primary?-Ni dendrites or lamellar eutectic.The other kind of?-Ni particles have random orientations,which may come from the freedom nucleation of?-Ni phase.For anomalous eutectic formed in laser remelting Ni-32.5wt.%Sn eutectic and Ni-33wt.%Sn hypereutectic powder bed,it is found that there is a peak near 60°in the misorientation angle distribution of?-Ni phase map.Through statistics of the Euler angles of adjacent?-Ni particles,it is found that many of the misorientation angles between two groups of?-Ni particles are 60°<111>.This means that the remelting of the twin lamellar?-Ni phases during the laser remelting process is one likely reason for the formation of anomalous eutectic microstructure.The three dimensional microstructure morphology of Ni-Sn anomalous eutectic was clarified by 3-D reconstruction technology.It is found that?-Ni phase is discontinuous granular and Ni₃Sn phase is a continuum as a whole.Through the CA simulation,it is found that the rapid transition of growth velocity can lead to instability of regular lamellar eutectic.In front of the solid-liquid interface,the?-Ni phase first nucleated freely and subsequent Ni₃Sn phase grabbed?-Ni phase to form anomalous eutectic.During laser remelting process,the solidification speed indeed rapidly changed from zero to close to the scanning speed from the bottom to the top of the molten pool.It is believed that this is a possible reason that anomalous eutectic can be observed in the process of laser remelting eutectic alloy.