Solid solubility extension and nano-mechanical properties of rapidly solidified Fe-Zr eutectic alloys under free fall condition

Abstract

The liquid-solid phase transitions and solidified microstructures of highly undercooled Fe-Zr eutectic melts were investigated by the drop tube technique. The quantitative relationships among droplet diameter (D), undercooling (ΔT) and cooling rate (Rc) were calculated for three types of eutectic alloys (Fe-9.8 at. % Zr, Fe-6 at. % Zr and Fe-16 at. % Zr alloys). Furthermore, the evolutions of solidified microstructure with droplet diameter were studied. The results show that the eutectic microstructure for Fe-9.8 at. % Zr droplet transforms from regularly lamellar shape to anomalous shape as the diameter reduces. The α-Fe dendrite in Fe-6 at. % Zr droplet transforms from coarse dendrite to refined dendrite with the decrease of droplet diameter, while the Fe2Zr dendrite in Fe-16 at. % Zr droplet transforms from faceted dendrite to non-faceted grain. Besides, a remarkable solid solubility extension was found in both the α-Fe dendrites of Fe-6 at. % Zr alloy and the Fe2Zr dendrites of Fe-16 at. % Zr alloy. The solid solubility of the α-Fe dendrite increases from 3 at. % to 5.6 at. % when the droplet diameter reduces from 1069 to 212 µm, while it decreases from 31 to 22 at. % in the Fe2Zr dendrite as the diameter reduces from 991 to 228 µm. The microstructural morphology and solid solubility are directly related to the nano-hardness and Young’s elastic modulus of alloys. The nano-mechanical properties of the eutectic gradually increase with the refinement of the layers, and gradually decrease as the irregularity of the anomalous eutectic increases. With the reduction of droplet diameter, the nano-hardness rises and Young’s elastic modulus continuously diminishes for the α-Fe dendrite due to the increase in the solute content of Zr, while these two mechanical properties gradually decrease for the Fe2Zr dendrite because of the reduction in the solute content of Zr.