Control of microstructure and properties of Cr-Fe-Ni based multicomponent alloys

Traditional metal alloys have been well developed for years, while less and less are left to be explored. In this context multicomponent alloys present an innovative alloy design strategy that breaks through the limitations of traditional design framework, and is shown to result in excellent physical and chemical properties. This thesis focuses on two kinds of multicomponent alloys, i.e., equiatomic CrMnFeNi alloy and non-equiatomic AlCoCr0.75Cu0.5FeNi alloy. Laser powder bed fusion (LPBF) is employed to carry out the fabrication of bulk alloys from gas- atomized powder. The influence of process parameters on the microstructure has been determined, and properties in relation to microstructural changes have been evaluated. Particularly, concerning possible target applications, hydrogen effects and magnetic behavior have been investigated on LPBF-built CrMnFeNi and AlCoCr0.75Cu0.5FeNi alloy, respectively. It is found that CrMnFeNi alloy produced by LPBF shows a single face-centered cubic (FCC) phase structure, owing to the favored FCC formation in rapid solidification during LPBF process. A hierarchical microstructure constituting of melt pools, grains, cellular structures including dendritic, elongated and equiaxed cells, as well as ultrafine sub-cells, has been observed. A large density of dislocations in association with segregation of Mn and Ni has been detected in the boundaries of melt pools, grains and cellular structures. When employing the same laser power, a faster scanning speed leads to a formation of more homogeneous and refined microstructure, accordingly, a higher Vickers hardness of 248 ± 8 HV0.5 is achieved. In hydrogen-charged TEM sample, hydrogen-induced planar faults, cracks and nanocrystals are identified. Absorbed hydrogen concentration is nearly 20 times higher than conventional austenitic stainless steel, which is attributed to both multicomponent alloy design strategy and LPBF process. AlCoCr0.75Cu0.5FeNi alloy is fabricated by LPBF applying two sets of process parameters. Laser power, scanning speed and volumetric energy density contribute to different aspects during the manufacturing, thus they need to be properly optimized. Typical rapidly solidified hierarchical microstructure is also characterized. A high Vickers microhardness of 604.6 ± 6.8 HV0.05 is measured regardless of the defects present. The degree of spinodal decomposition into two types of body-centered cubic (BCC) phase structures, i.e., A2 and B2 phase, has been found to depend on cooling rate, that is, A2 phase formation is promoted under a relatively slower rate during rapid solidification. As the slowest scanning speed is employed under the same laser power, the largest saturation magnetization of 65.3 emu/g at 9 T and 300 K is reached. The control and enhancement of magnetic properties can be easily conducted via spinodal decomposition by optimizing process parameters in LPBF process. Findings of this thesis demonstrate the feasibility and potentiality of manufacturing multicomponent alloys by LPBF process, which provides benefits for the development of new alloys for advanced applications in the future.