Study On Solidification Structure Of Undercooled Ag-Cu Alloy

Rapid development and outstanding advantages of deep undercooling rapid solidification technology make it not only realize rapid solidification of threedimensional bulk liquid metal,but also artificially control the degree of nucleation undercooling in the slow cooling process of liquid metal,and real-time observation and recording of melt solidification process.It makes it possible to quantitatively study many laws of rapid solidification process,which provides an important experimental basis for the study of rapid crystal growth under the condition of nonequilibrium solidification,and also provides a new idea for the preparation and research of ultrafine microstructure materials,amorphous alloys and other new materials.For Ag-28.1Cu eutectic alloy,the transformation of solidification microstructure of single-point and multi-point nucleation samples has not been considered in the current study of undercooling,and the mechanism of influence of alloying elements on solidification microstructure is also lacking.Therefore,it is particularly important to study the transformation of solidification structure of single-point and multi-point nucleation samples with the undercooling degree,and to discuss the influence of the third element on the eutectic solidification structure of undercooling.The Cu-rich AgCu single-phase alloys are widely used in high-speed train conductors and strong magnetic field winding materials because of their high strength and high electrical conductivity.However,the preparation of Cu-Ag alloys with both high strength and high electrical conductivity requires tedious process treatment,while the increase in hardness significantly reduces the electrical conductivity.Therefore,if we can find a process that can increase the hardness of Cu-Ag alloy while reducing the decrease of electrical conductivity,it will provide new possibilities for the development and design of high-strength and high-conductivity materials.The Ag-28.1Cu eutectic alloy specimens with different undercooling degrees were prepared by a process combining cyclic superheat and molten glass purification,and the effects of superheat degree,cycle number and holding time on the undercooling degree level were explored.It is found that when the superheat degree is less than400 K,the undercooling degree increases with the increase of the superheat degree,and when the number of cycles is less than 9,the undercooling degree also increases with the increase of the number of cycles.Controlling the holding time between 5 and7 min is conducive to obtaining a larger undercooling degree.The results of the LZ model showed that the eutectic growth rate V increased with the increase of undercooling degree,and the eutectic lamellae spacing λ and eutectic dendrite tip radius R decreased with the increase of undercooling degree.Meanwhile,the larger the equilibrium solute partition coefficient K,the faster the eutectic growth rate increases,and the faster the eutectic lamellae spacing and eutectic dendrite tip radius decrease.For the Ag-28.1Cu eutectic alloy with single-point nucleation,only one nucleation point exists in the macrostructure,and the microstructure changes from cellular to dendritic with increasing undercooling degree.For Ag-28.1Cu eutectic alloy with multi-point nucleation,there are multiple nucleation points in the macrostructure,and obvious dendrite collision interface can be observed,and with the increase of undercooling degree,the eutectic lamellae in the microstructure gradually refine,and the microstructure at the collision interface gradually becomes chaotic due to the dendrite collision.Small amounts of Sn and Ni elements were added to the undercooled Ag-28.1Cu eutectic alloy,and the effects of the alloying elements on the solidification structure of the undercooled eutectic were investigated.It was found that the macroscopic solidification structure of the Ag-28.1Cu eutectic alloy consisted of eutectic regions and anomalous eutectic regions,with obvious zoning characteristics.The addition of Sn elements does not change the zoning characteristics of the macrostructure,and with the increase of Sn content,the phenomenon of increased anomalous eutectic and coarsening in the microstructure with random orientation distribution appears.At the same time,the Ag-28.1Cu eutectic alloy is destabilized from cellular to dendritic because Sn is enriched at the solidification interface front and diffuses perpendicular to the solid-liquid interface.The addition of Ni disappears the zoning feature of the macrostructure,and with the increase of Ni content,the single-phase dendrites in the microstructure increase and show a specific orientation distribution,which is caused by the diffusion of Ni parallel to the solid-liquid interface,resulting in the eutectic two phases no longer maintaining coupled growth,but changing to decoupled growth.Cu-Ag alloy specimens with different undercooling degrees were successfully prepared.The maximum undercooling degree obtained in the experiments was 178 K for Cu-1Ag and 191 K for Cu-4Ag,and only the Cu phase existed in the specimens with different undercooling degrees,and no new substable phase was formed.The microstructure of the Cu-1Ag and Cu-4Ag alloys was significantly refined as the undercooling degree increased,which was the main reason for the increase in hardness,as well as the increase in lattice distortion and grain boundary area inside the crystals,which increased the chance of scattering free electrons and therefore decreased the electrical conductivity.The presence of Ag in Cu-Ag alloys in the form of replacement solid solution increases the hardness,but also leads to lattice distortion and increased scattering of free electrons,resulting in a significant decrease in conductivity.