Nanoscaled in-situ dispersoid formation mechanism and additive manufacturing of oxide dispersion-strengthened CoCrNi-Based alloy system

Abstract

Oxide dispersion-strengthened (ODS) alloys, characterized by a microstructure in which nano-sized oxides are homogeneously dispersed in a high number density in an alloy matrix, are known as promising structural materials for extreme environments based on excellent high temperature properties and radiation resistance. In general, oxide dispersion-strengthened alloys are manufactured via powder metallurgy, including mechanical alloying and sintering after mixing Y$_2$O$_3$ particles and metal powders. The mechanical alloying process is a key process for ODS alloy manufacturing to achieve the refinement and homogeneous distribution of oxide particles within the matrix. During the mechanical alloying, the added Y$_2$O$_3$ particles are severely deformed and refined due to high-energy collisions with the milling media. However, the non-uniform microstructure is expressed even after the long mechanical alloying process. Although the powder metallurgical approach is known as an optimized method for ODS alloy fabrication, microstructural inhomogeneity and the time and cost consumption of the process make it difficult to utilize ODS alloys as structural materials. Since the dispersed oxides in the matrix directly affect the microstructure formation and final properties of ODS alloys, it is very important to understand the oxide formation mechanisms and behavior of ODS alloys. However, because oxide formation is influenced not only by various factors such as alloying elements, manufacturing methods, and process conditions, but also by the analytical limitations of nanoscale oxides, a sufficient understanding of the oxide formation mechanism is still lacking. Therefore, a systematic understanding of the oxide formation mechanism is necessary to overcome the manufacturing limitations of ODS alloys. In this paper, the oxide formation mechanism and behavior during the powder metallurgical and laser-based additive manufacturing processes on CoCrNi-based medium- and high-entropy alloy (MEA and HEA) matrix were studied to understand the effect of complex matrix composition on oxide formation and the ex situ and in situ formation mechanisms. A multistep sintering process was designed to systematically understand the effect of oxide formation behavior during mechanical alloying and sintering processes on microstructure formation and the final properties of alloys. The study revealed that the oxide-forming elements dissolved from the alloy powder during the mechanical alloying were induced to undergo in situ synthesis during the multistep sintering process, and the resulting oxides exhibit high resistance to agglomeration compared to those formed through the conventional method. These oxides formed by multistep sintering effectively contribute to the strengthening of the alloy matrix. Then, in situ ODS–HEA were prepared using pre-alloyed powder with yttrium instead of adding Y$_2$O$_3$ to promote the robust in situ synthesis of oxides, and the oxide formation mechanism was systematically analyzed by multiscale analyses covering the atomic to the macroscopic scale. Finally to fabricate the ODS alloy using laser-based additive manufacturing, a newly designed ODS feedstock for additive manufacturing was developed. The feedstock consists of alloy powder coated with oxide-forming element. The oxide-forming elements coated on the powder react with each other to undergo in situ oxide synthesis in the melts during additive manufacturing, which results in the formation of fine oxide particles. By implementing the concept of in situ oxide synthesis during additive manufacturing, the mechanism of oxide formation and the effect of oxide-forming elements were studied. Based on the understanding of oxide behavior in ODS-MEA and HEA prepared by powder metallurgy and additive manufacturing, a breakthrough in ODS alloy manufacturing was suggested.