Investigation on solidification structure evolution of deeply undercooled single-phase Ni-Cu-Co alloys

In this work, the main focus was on exploring the effect of increasing the gradient content of Co element on the solidification process of Ni-Cu-Co alloy under deep undercooling conditions. The effects on its solidification rate, recalescence effect, and critical undercooling range were also explored. In the experiment, a combination of molten glass purification and cyclic superheating technology was mainly used to obtain gradient undercooling of Ni88Cu10Co2, Ni88Cu8Co4, and Ni88Cu6Co6 alloy samples at intervals of 10 K. By analyzing the microstructure evolution under different undercooling degrees through metallographic diagrams, and combining it with highspeed cameras to record the phenomenon of recalescence during the solidification process, the influence of the addition of Co element on the solidification rate, recalescence effect, and the critical undercooling range appearing in the microstructure during the overall solidification process was explored. Conduct EBSD analysis on the alloy samples under the maximum undercooling of Ni88CuCo6, and analyze the representative significance behind their characterization data. At the same time, TEM transmission analysis was conducted on the Ni88Cu6Co6 alloy sample with the highest Co content, and it was found that there are high-density dislocation and stacking fault networks as well as obvious twinning structures in the substructure area. The standard FCC diffraction pattern also represents that it is still a single-phase structure. Based on the above metallographic diagram, EBSD, and TEM data analysis, it is proven that the occurrence of grain refinement under high undercooling is mainly due to stress induced recrystallization.