Yuhki Tsukada

The present study was undertaken to investigate changes in the solidification microstructure consisting of Zn(hcp), α-Al(fcc), and Zn2Mg phases in the hot-dip Zn–6Al–3Mg (mass%) alloy coating during isothermal holding at an elevated temperature of 200℃ (below the solidus temperature), in terms of the transformation from Zn2Mg metastable phase to Zn11Mg2 stable phase. The macroscopic Vickers hardness increased approximately from 150 HV to 180 HV after holding at 200℃ for 36 ks. The hardness change corresponded well with the occurrence of the Zn2Mg→Zn11Mg2 phase transformation identified by X-ray diffraction analyses. The Zn11Mg2 stable phase nucleated at the interface between Zn2Mg and Zn(hcp) phases, and grew by consuming both phases formed in the initial solidification microstructure. Nanoindentation measurements revealed that the Zn11Mg2 phase had a lower hardness than the Zn2Mg phase. Therefore, the significant growth of the Zn11Mg2 phase, which consumes a large part of the soft Zn(hcp) phase, could contribute to the observed hardening of the hot-dip Zn–Al–Mg alloy coating during isothermal holding.

An attempt was made to apply the laser powder bed fusion (L-PBF) process to Fe–Ni–Nb ternary alloys as a major composition of heat-resistant Fe-based alloys strengthened by the Fe2Nb intermetallic phase. This study focused on a composition of Fe–40Ni–15Nb (at%) near a eutectic composition of austenite (γ-Fe) and Fe2Nb in the Fe–Ni–Nb ternary system for the pre-alloy powder with an average particle size of approximately 17 μm, which was produced by the gas-atomizing process. The Fe–40Ni–15Nb pre-alloy powder exhibited insufficient L-PBF processability for fabricating fully dense centimeter-sized samples, whereas several alloy samples with relative densities above 90 % were manufactured. The L-PBF manufactured alloy samples exhibited a representative melt-pool structure in which regions had locally melted and rapidly solidified by scanning laser irradiation in the L-PBF process. The L-PBF sample exhibited a high hardness of approximately 900 HV compared to the slowly solidified alloy sample (about 550 HV). The high hardness could be attributed to the formation of nanoscale γ-Fe/Fe2Nb eutectic microstructure by the L-PBF process. Intriguingly, the observed melt-pool boundary had several tens of micrometers in width, which significantly varied depending on the applied laser condition. Nanoindentation tests demonstrated the melt-pool boundary region exhibited a relatively lower hardness than inside the melt pools. It was assumed that the unique melt-pool boundary region would consist of a relatively coarsened solidification microstructure formed at the interface of liquid with the solid and a locally coarsened microstructure affected by laser local heating, which can be controlled by manipulating the laser conditions.

The stability of η-Fe2Al5 phase at α-Fe/molten Zn–0.1Al (wt.%) (L) interface at 723 K in Fe–Zn–Al ternary system was investigated by phase-field simulations. Thin layers of intermetallic compound (IMC) phases (η，Г-Fe3Zn10，Г1-Fe5Zn21 and δ1-FeZn7) were placed between the α and L phases, and the growth of the IMC layers and the atomic diffusion of constituent elements along the direction perpendicular to the α/L interface were calculated by one-dimensional phase-field simulation. The simulation result showed that Г and Г1 phases dissolved, and thin η phase and thick δ1 phase remained stable at the α/L interface. Moreover, several phase-field simulations were performed by varying the values of interdiffusion coefficients in each phase. The simulation results showed that the diffusion and partitioning behaviors of Al have a significant effect on the stability of IMC layers at the α/L interface. It was found that the partitioning of Al to the α phase was suppressed due to the fact that the value of interdiffusion coefficient in the α phase was several orders of magnitude smaller than those in the IMC phases. The resultant Al partitioning to the IMC phases was the direct cause of the stabilization of the η phase and the destabilization of the Г and Г1 phases.

Grain boundary (GB) strengthening elements, such as B, C, and Zr have been added in small amounts to nickel-base superalloys. However, their strengthening effects have not been quantified and no specific design principles for GB chemistry have been reported. In this study, we propose a practical computational approach for the GB segregation engineering of nickel-base superalloys. Considering the partitioning of alloying elements into coexisting phases (strengthening phases, carbides, etc.), the equilibrium composition of a high-angle GB was computed for several nickel-base superalloys using a calculation of phase diagrams database. The computational results showed that B and Mo were enriched at the GB in most of the investigated alloys. The creep rupture strengths of the investigated alloys were predicted using the computed GB composition as a regression model feature. The regression coefficients for the features confirm that B segregation at the GB has a non-negligible strengthening effect on nickel-base superalloys.