Structural Evolution and Micromechanical Properties of Ternary Ni-Fe-Ti Alloy Solidified Under Microgravity Condition

Both the microgravity solidification mechanism and resultant micromechanical properties of ternary Ni₄₁Fe₄₀Ti₁₉ alloy were investigated by means of drop tube, nanoindentation, and nano-dynamic mechanical analysis (Nano-DMA) techniques. The microstructure of the Ni₄₁Fe₄₀Ti₁₉ alloy droplets consisted of γ-(Fe,Ni) dendrites and interdendritic pseudobinary eutectic phases. The average cooling rate and undercooling increased significantly as the droplet diameter decreased during free fall. Owing to the refinement of the rapidly solidified microstructure, and the Ti solute hardening of the primary γ-(Fe,Ni) dendrites, the microhardness of this alloy was remarkably increased with the decrease of droplet size. Moreover, the nanohardness of the γ-(Fe,Ni) dendrite increased as the indentation displacement decreased within the depth range of 40 to 244 nm, indicating a conspicuous indentation size effect (ISE). However, the ISE increased as the undercooling increased, because additional geometrically necessary dislocations (GNDs) were required, while intragranular dislocation motion was further hindered as the Ti content increased. The size effect factor increased linearly with the reduced droplet diameter.