Microstructure, phase transformation temperatures and long-term stability of ex-situ Alloy 718 + TiB2 metal-ceramic nanocomposites as a repair coatings for aerospace applications

Abstract In this study, Ni-based superalloy Alloy 718 + xTiB2 (x = 1.25; 2.5; 3.75; 5.0%) composites were fabricated by suction casting to improve hardness, wear resistance, and corrosion resistance. The microstructure and properties were analyzed using synchrotron X-ray diffraction, light microscopy, scanning electron microscopy, scanning transmission electron microscopy, dilatometry, differential scanning calorimetry, Vickers hardness, wear resistance measurements, and corrosion tests. The results show that the addition of TiB2 to Alloy 718 has a strong influence on the primary microstructure. All composites are characterized by a dendritic microstructure with irregular distribution of strengthening precipitates. In the reference Alloy 718, the dendritic regions consist of the γ-phase, while interdendritic areas contain Nb-rich carbides and Laves phase precipitates. The TiB2 particles added to a powder mixture reacts strongly with the liquid Alloy 718 during suction casting, resulting in the decrease of the Laves phase and the formation of the M3B2 and M23B6 phases. Increasing the initial TiB2 content shifted the phase transformation temperatures (solidus and liquidus) to lower temperatures. Hardness values increased significantly from 198 HV10 in the reference Alloy 718 to 403 HV10 (5.0% TiB2), which contributed to improved wear resistance. Finally, high temperature exposures in rich oxygen atmosphere (steam) and reduced oxygen atmosphere (Ar + 0.25 vol% SO2) at 704 °C for 1000 h indicated that the Alloy 718 + TiB2 composites were characterized by a lower mass gain than the reference Alloy 718.