Controlling microstructural gradients in laser-clad AlCoCrFeNi2.1 EHEAs: The role of faceted B2 and non-faceted FCC phase growth under rapid solidification
Huiting Zheng, Junjie Fu, Yongzhe Wang, Yong Dong
Abstract
This study investigates the potential of laser-clad AlCoCrFeNi 2.1 eutectic high-entropy alloy (EHEA) for mold repair applications, focusing on the microstructural evolution of faceted B2 phase and non-faceted FCC phase in multi-layer deposition. The results reveal that the laser cladded AlCoCrFeNi 2.1 EHEA exhibits a progressive transition from regularly aligned FCC columnar crystals adjacent to the substrate to coarse FCC/B2 eutectic lamellae in the intermediate region, and finally to fine FCC/B2 eutectic colonies in the topmost layer. The heterogeneous microstructure growth mechanisms is governed by distinct solidification kinetics. In the first layer, high thermal gradients promote directional growth of FCC columnar crystals, while B2 phase forms in intercolumnar regions due to its slower faceted growth. Subsequent layers exhibit coarser FCC/B2 lamellae due to reduced cooling rates, with the topmost region forming fine eutectic colonies under rapid solidification. The hardness of the single-layer clad and the H13 substrate was 3.62 GPa and 3.53 GPa, respectively. Thermal accumulation during multi-layer cladding resulted in cyclic annealing, which softened the substrate and first layer to 2.88 GPa and 2.77 GPa, respectively, while the new top layer achieved a higher hardness of 3.84 GPa. The competitive growth between FCC and B2 phases is attributed to differences in atomic diffusion rates and growth mechanisms. The growth of FCC phase is governed by diffusion-controlled mechanism, demonstrates significantly faster kinetics compared to the strongly anisotropic B2 phase, and solidified as the leading phase during the laser cladding. These findings demonstrate that modifying the growth mechanism of non-faceted FCC phase and faceted B2 phase during laser cladding can tailor phase distribution, offering a promising approach for fabricating graded EHEA coatings with optimized mechanical properties for mold repair.