Xiao Huang

In this study, the microstructure and solidus and liquidus of several Ni-Co-Hf-Zr-Co-Ti-Al braze alloys were first examined with the objective to develop a B and Si free low melting braze alloy for narrow gap (NGB) and wide gap brazing (WGB) and repair applications. Among various alloys examined, DSC was used to measure the solidus and liquidus during heating and cooling cycles.

As steam power plants continue to move towards higher operating temperatures in order to improve efficiency, materials exposed to the working fluid are subjected to accelerated degradations in the forms of surface oxidation and reduced mechanical properties. In this study, the oxidation behavior of two cobalt base alloys, CoCrMoSi (T14) and CoCrNiMoSi (T19), was evaluated in superheated steam (SHS, 0.1MPa) at 800 °C for up to 500 hours. After the exposure, both T14 and T19 alloys experienced weight gain caused by oxidation. Visual observation and SEM surface analysis revealed that T19 had greater extent of surface oxide spallation than that seen on T14. From the cross-sectional evaluation, however, a thin, adherent oxide layer was found to have formed on T19. T14 in fact had suffered from excessive internal oxidation and the surface oxide was uneven. Based on the results obtained so far, it is believed that the finer Laves phase combined with greater amount of Cr in alloy T19 have enabled the formation of a protective oxide layer and thus reduced the extent of internal oxidation. Due to the extensive oxidation ingress along the large Laves phase, it is concluded that T14 is not suitable for applications in SHS at 800 °C.

In this study, the microstructure and solidus and liquidus of several Ni-Co-Hf-Zr-Ti-Al braze alloys were first examined with the objective to develop a B and Si free low melting braze alloy for narrow gap (NGB) and wide gap brazing (WGB) and turbine component repair applications. Among various alloys examined, DSC was used to measure the solidus and liquidus during heating and cooling cycles. Following the measurements of liquidus and solidus, the microstructure was evaluated using SEM. Equations for calculating solidus and liquidus based on alloy’s compositions were established and the functions of each elements on these two characteristic temperatures were discussed. One selected alloy with a liquidus of 1201 °C was further employed for NGB and WGB experiments. The results showed that it was able join CMSX-4 at 1240°C without interfacial voids; and with the use of externally applied pressure and extended homogenization treatment the interfacial intermetallic compounds were substantially removed. Furthermore, the same braze alloy was used to fill a large artificial cavity in a WGB scheme at a reduced temperature of 1200°C. The braze alloy was able to fully bond the filler powder alloy in addition to join the two alloys to a IN 738 substrate. Finally, oxidation test was conducted at 1050°C (isothermal in static air) for 100 hours after NGB of CMSX-4 and WGB of IN 738. The results showed that the oxide formed on the standalone braze alloy is very dense and there is no sign of spallation. It contained primarily NiO (+CoO) with no other elements measured. For the NGB joints, large amount of scale spallation was observed on base alloy CMSX-4 while the NGB joint remained spallation free. The oxide formed on the NGB was NiO with partitions of Co, Al, Ti, Cr, and W. The WGB joint region in IN 738 showed oxide scale spallation on the IN 738 substrate side, leaving behind steps and depression on the sample surface. In the WGB joint itself, there were three notable phases after oxidation test, however, no scale spallation could be found. For the majority part of the surface, a Ni-rich oxide covered the surface. There were areas of smaller oxide particles with higher Cr content. Overall, the new boron/silicon free braze alloy was found to be able to join several superalloys in both WGB and NGB schemes without occurrence of defects and the oxidation resistance was superior to both substrate alloys examined in this study.

In this study, three alumina-forming austenitic (AFA) steels (Fe-18Ni-12Cr-AlNbC, Fe-18Ni-12Cr-Al, and Fe-18Ni-16Cr-AlNbC) were exposed to superheated steam (SHS) at 700 °C for 1000 h to study their oxidation behavior. The Fe-18Ni-16Cr-AlNbC alloy showed the best performance as it had the least weight gain due to a denser surface oxide layer and the lack of internal oxidation. The Fe-18Ni-12Cr-AlNbC alloy suffered from internal oxidation and had a greater weight gain than Fe-18Ni-16Cr-AlNbC, while Fe-18Ni-12Cr-Al experienced weight loss due to perhaps external oxide spallation, leaving a less protective Fe-oxide on the external surface.

The performance of S16, a CoCrWC alloy, was evaluated in superheated steam (SHS) at 625°C and 800°C (at 0.1 MPa), and in supercritical water (SCW) at 625°C and 26 MPa for 500 hours. S16 experienced weight loss under the SHS conditions (625°C and 800°C) and weight gain under the SCW condition. Weight loss was associated with scale spallation and pitting while oxide growth on the surface contributed to weight gain. Formation of CoCr₂O₄ and Cr₂O₃ was observed in the oxide layer of samples tested under the three conditions, however, the Cr-rich oxide on SCW tested samples was found to be continuous. A thin, continuous Co₃W subscale was believed to have provided an oxidation barrier under the SCW test condition. Based on the limited weight change and surface oxide formation, S16 has the potential to be used as a coating material for ultra-supercritical power generation systems at temperatures up to 625°C.

Additive manufacturing (AM) is a transformative technology that has rapidly grown over the past decade. AM processes build parts layer by layer and have found applications in the aerospace, biomedical, and automotive fields. The technology holds particular promise for the aerospace industry due to the reduced process time, weight savings of parts, and opportunities for new material development. Herein, a review of laser‐based AM processes, existing AM systems, and aerospace parts being fabricated is presented. It further explores the material properties and microstructure of printed samples with both powder bed fusion and direct energy deposition processes. The benefits and challenges associated with the widespread use of the technology are discussed with emphases on the aerospace sector. Finally, the steps required for parts produced by AM processes to become certified for use in aerospace applications are presented.

Coating application to superalloys provides oxidation and corrosion resistance. One of the key elements in gas turbine coatings is aluminium (Al) which forms a thin, dense alumina layer upon exposure to oxygen and impedes further oxygen diffusion into the alloy. However, Al in the coating depletes due to inward diffusion and oxide formation. The rate of Al depletion is a function of the coating composition/structure, substrate alloy composition/structure and service conditions. In this study, a PtAl diffusion coating, was applied onto polycrystalline IN738LC and single crystal CMSX-4. The coated systems were exposed to 1080°C for 1000 h. Both SEM and XRD analyses were carried out on the as-coated and tested samples. It was found that despite the much higher Al content in CMSX-4, Al depletion was greater. With the higher Cr in IN738LC, a diffusion barrier enriched in Cr/Mo was able to form and reduce the Al depletion.

The choice of materials is of great concern in the construction of Gen IV supercritical water reactors (SCWR), particularly the fuel cladding, due to the harsh environment of elevated temperatures and pressures. A material's performance under simulated conditions must be evaluated to support proper material selection by designers. In this study, aluminide and Cr-modified aluminide coated 304, as well as bare stainless steel 304 as a reference material, were tested in supercritical water (SCW) at 700 °C and 25 MPa for 1000 h. The results showed that all three samples experienced weight loss. However, the aluminide coated 304 had 20 to 40 times less weight loss compared to Cr-modified aluminide coated and bare stainless steel 304 specimens, respectively. Based on scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis results, spinel and hematite Fe2O3 formed on bare 304 after 1000 h in SCW while alumina was observed on both coated specimens, i.e., aluminide and Cr-modified aluminide surfaces. Oxide spallation was observed on the bare 304 and Cr-modified aluminide surface, contributing to a larger weight loss. Based on the results from this study, pure aluminide coating with Al content of 10–11 wt % demonstrated superior performance than bare 304 and Cr-modified aluminide coated 304.

In this study, single crystal superalloy René N5 was exposed in air at 1150 °C for up to 16 hours to evaluate the alloy’s short-term oxidation behaviour and the potential for developing a no-bond coat TBC system. The results showed that after 1 hour of exposure, a three layered oxide developed on the surface, consisting of spinel, Ta-containing oxide and alumina just above the substrate. After 4 hours of exposure, oxide spallation occurred; this became more pronounced after 16 hours. The oxide spallation took place between the top spinel layer and alumina layer, where Ta-rich oxide was more abundant. All samples tested for 0.5, 1, 2, 4 and 16 hours developed alumina near the substrate while the occurrence of NiO, spinel and Ta-oxides varied, depending upon the exposure time.