Shigenari Hayashi

The oxidation behavior of CuNi, CuFeNi, CrCu_0.3FeNi, and Al_0.4CrCuFeNi₂ Co-free Cu-containing concentrated solid solution alloys was investigated in steam conditions under 500, 600, and 700°C for 25 h. All the alloys have a face-centered cubic structure. The kinetic curves of oxidation were measured, and the microstructure and elemental distribution of oxide scales were analyzed. The oxidation of all the Cu-containing alloys indicated parabolic behavior, and those appeared to have better corrosion resistance than normal 316 SS. The parabolic rate constant increased with increasing temperature. The oxidation resistance of CuNi and CuFeNi were relatively poor due to the formation of unprotective NiO and Fe₃O₄ oxide scales, which were thicker than that of CrCu_0.3FeNi and Al_0.4CrCuFeNi₂ at all testing temperatures. Cross-sectional electron probe microanalysis (EPMA) and X-ray diffraction (XRD) results revealed that, by the addition of Cr and Al, CrCu_0.3FeNi and Al_0.4CrCuFeNi₂ had the enhanced oxidation resistance, which is probably due to the formation of Cr₂O₃ and Al₂O₃ inner oxide layer.

The study focuses on obtaining Inconel 625-NbC composites for high-temperature applications, e.g., jet engines, waste-to-energy combusting systems or gas engine turbines, and characterizing them in terms of their microstructure and hardness improvement. Synthesis was performed utilizing Spark Plasma Sintering (SPS) at 1150 °C under the load of 45 MPa in medium vacuum (under 10⁻³ MPa) for a total time of 60 min. Four sets of samples with different Inconel 625 to NbC weight ratios were prepared (5, 10, 20, and 30 wt.%), followed by a reference sample containing no ceramic reinforcement. Obtained materials were hot-rolled at 1150 °C with a 10% reduction step and later cut and polished to perform characterization utilizing scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) module and microhardness testing device equipped with Vickers indenter. Hardness was improved proportionally to NbC addition achieving an increase of up to 20% of reference values. Additional heat treatment was conducted on the hot-rolled samples at 1200 °C in an argon atmosphere to further observe the interaction between reinforcement and alloy. Their microstructure revealed the coarsening of precipitates within the metal matrix and partial reinforcement dissolution, which proved to be crucial to obtaining the highest quality composites with homogenous hardness improvement.