Microstructure Evolution of Ni-Sn Anomalous Eutectic with Different Rapid Solidification

Author:Wang Zhi Tai

Supervisor:huang wei dong lin zuo

Database:Doctor

Degree Year:2018

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Pages:121

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Eutectic alloys are an important class of industrial material,the performance of eutectic alloys depends largely on the characteristics scale of regularity or irregularity eutectic microstructure,and orientations of eutectic phases.This makes the regularity of eutectic structure become the core content of eutectic theory research.Previous studies have shown that in several eutectic alloys,regular lamellar or rod eutectic transforms into anomalous eutectic via a rapid solidification process.However,there is still considerable disagreement among researchers on the understanding of the nucleation and growth competition mechanisms of the eutectic phases during the formation of anomalous eutectic.In order to clarify the mechanism of formation of anomalous eutectic and its growth behavior,in this paper,Ni-Sn eutectic alloy is choosed as the research object because it has been widely studied,different rapid solidification processes have been taken,such as deep undercooling,laser remelting and laser cladding,nucleation competition and epitaxial growth competition behavior of eutectic two phases had been comparative study.It attempts to clarify the Ni-Sn eutectic structure evolution and phase selection mechanism in different types of rapid solidification process,and to clarify the formation mechanism of Ni-Sn anomalous eutectic under different solidification conditions.The main achievements obtained are as follows:(1)Microstructure evolution of two phases under competitive nucleation in free solidification:always growth asα-Ni dendrite+lamellar eutectic(α-Ni+Ni3Sn),however,as the degree of undercooling increases,α-Ni dendrites gradually tend to mature and fuse.For the Ni-30wt.%Sn hypoeutectic alloy,as the degree of undercooling increases,when the undercooling reaches 50K,anomalous eutectic begins to appear in the solidification structure of the alloy,and when the undercooling is 130K,it turns into complete anomalous eutectic structure.When the undercooling continued to increase to180K,the anomalous eutectic appeared to be significantly refined.For the Ni-33%Sn hypereutectic alloy,the solidification structure of the Ni-33%Sn hypereutectic alloys shows the growth of eutectic dendrites in the form of algae when the undercooling is less than 70 K.When the undercooling reaches 130K,the structure changes to complete anomalous eutectic.Clarified the sufficient and necessary conditions to produce Ni-Sn anomalous eutectic:(2)Microstructure evolution of two phases under competitive growth in epitaxial solidification:The microstructure of Ni-Sn alloy epitaxially growth by laser rapid remelting is finer than that of as-cast Ni-Sn alloy substrate.The dendrite spacing of primary phase of Ni-Sn near-eutectic alloy and lamellar spacing of eutectic gradually decreases with the increase of laser scanning velocity,even when the scanning rate was much higher than that of the Ni-Sn eutectic decoupling growth rate of 300mm/s predicted by the TMK model,and the absolute stability velocity of 455mm/s,reached1000mm/s,the anomalous eutectic was still not obtained,only the lamellar spacing of eutectic was significantly reduced to about 0.1mm.This indicates that the anomalous eutectic can’t obtained in the condition of epitaxial solidification.(3)Microstructure evolution of eutectic under the of chilling nucleation and epitaxial competition growth:When the laser scanning velocity is 0.1 mm/s,the Ni-Sn eutectic alloy is completely penetrated and the anomalous eutectic appears at the bottom of the Ni-33wt.%Sn hypereutectic and Ni-30wt.%Sn hypoeutectic alloys.At the same time,the growth area of the anomalous eutectic increases with the increase of the chilled thermal conductivity of the bottom substrate.It can be inferred that when the sample penetrated,the free nucleation of eutectic and rapid growth of the eutectic phase produced by the bottom substrate on the bottom of the bath result in the generation of anomalous eutectic at the bottom,and the molten pool is solidified from the bottom to the top.The epitaxial solidification growth behavior on both sides of the weld pool and the rapid establishment of the epitaxial growth conditions of the bottom of the weld pool after undergoing high-speed free nucleation competition growth resulted in the formation of anomalous eutectic structure only in a small area at the bottom of the molten pool.This further shows that during eutectic solidification,the free nucleation of two phases should be necessary for the formation of anomalous eutectic.(4)Microstructure evolution of eutectic solidification under columnar/equiaxial crystal transformation(CET):During the laser cladding deposition of Ni-32.5wt.%Sn eutectic alloy,it was found that when the CET occurs on the top of the molten pool,irregular anomalous eutectic structure will appear at the top of the cladding layer.Further nucleation was induced by the introduction of extraneous particles.It was found that when Ni-32.5wt.%Sn alloy powder with 1%SiC particles added was used for laser cladding deposition,there was also anomalous eutectic cluster formed around the SiC particles on the top of the cladding layer.These further confirm that,during eutectic solidification,the free nucleation of two phases and their subsequent rapid growth are necessary and sufficient conditions for the formation of anomalous eutectic.(5)Microstructure and formation mechanism of anomalous eutectic:The EBSD analysis results of the anomalous eutectic structure obtained under different rapid solidification conditions show that the orientation of Ni3Sn is relatively uniform and forms a continuous structure in space,and the orientation ofα-Ni phase is divided into two types:uniform and dispersed.The large-scale anomalous eutectic formed under deep undercooling free solidification belongs to the former.The small-size anomalous eutectic formed under deep undercooling free solidification conditions and the anomalous eutectic formed under the conditions of laser remelting and laser cladding belong to the latter.By comparing the anomalous eutectic structure and orientation characteristics under different solidification conditions,it is shown that the anomalous eutectic with a uniformα-Ni orientation is derived from the melting ofα-Ni phase dendrite,and the anomalous eutectic with theα-Ni phase orientation dispersed is derived from the decoupled growth of the eutectic phases.