Effect of Microstructure Evolution on Micro/Nano-Mechanical Property of Fe–Co–Ni Ternary Alloys Solidified under Microgravity Condition

Rapid solidification of highly undercooled Fe–Co–Ni alloys is realized by drop tube technique. The microstructures of Fe–10%Co–10%Ni, Fe–15.6%Co–12%Ni, and Fe–10%Co–20%Ni alloys are all composed of single α(Fe) solid solution phase, which are confirmed by the results of XRD and DSC. With the decrease of droplet diameters, the cooling rate and undercooling increase, meanwhile, the microstructural characteristic of α(Fe) phase transforms from coarse dendrites to equiaxed grains. The size of coarse dendrites decreases to one tenth for the refined equiaxed grains. Employing the Vickers hardness and nano indenter techniques, the mechanical properties of α(Fe) dendrites are investigated. The average Vickers microhardness of α(Fe) phase is remarkably enhanced with the decrease of the grain size. Due to the more homogenous solute distribution in the dendrite trunks, the nanohardness presents a gentle fluctuation. Once the droplet diameter exceeds a critical value, the solutes concentrate at the grain boundaries. As a result, the closer the indenters to the grain boundary, the larger the nanohardness.