Strengthening, Corrosion and Protection of High-Temperature Structural Materials

This Special Issue presents a series of research papers and reviews about the second-phase enhancement, surface coating technology, high-temperature corrosion, wear, erosion, and protection of high-temperature structural materials. The effects of alloying and surface coating technology on the microstructure, mechanical properties, and oxidation resistance of materials were systematically introduced. In addition, this Special Issue also summarizes the strengthening mechanism of the second relatively refractory metal alloy and carbonized ceramic materials, compares the advantages and disadvantages of different surface coating technologies, and analyzes the oxidation behavior and failure mechanism of the coating in order to provide valuable research references for related fields.

High-temperature structural materials are characterized by high melting points, highstrength and high-temperature creep resistance, low thermal expansion coefficients, and excellent corrosion resistance. Such materials are widely used in metallurgy, chemical applications, aerospace, nuclear reactors, and other situations where extreme environments are encountered.
At present, the main high-temperature structural materials include nickel base superalloys, refractory metal alloys, carbides, and nitride ultra-high temperature ceramics. However, during high-temperature service, high-temperature structural materials are exposed to extremely harsh high-temperature environments, bear mechanical and thermal loads, and are exposed to high-temperature oxidation, erosion, corrosion, etc. Therefore, high stress can be easily concentrated at a defect site, especially near the phase interface [1][2][3]. Thermal expansion stress will drive the nucleation and propagation of cracks. At the same time, friction, oxidation, and corrosion will also aggravate crack propagation and material failure, which will pose a catastrophic threat to the high-temperature components [4,5]. Therefore, the characterization, strengthening, and corrosion protection of high-temperature structural materials are very important. This Special Issue focuses on second phase enhancement, surface coating technology, high temperature corrosion, wear, erosion, and protection with respect to high temperature structural materials.
Among these refractory metal alloys, Mo-based alloys and Nb-based alloys are considered to have the most potential for new ultra-high temperature structural material and are favored by researchers [6][7][8]. However, low-temperature oxidation pulverization and high-temperature oxidation volatilization limit their further application [9][10][11][12]. A large number of studies show that alloying is an effective way to solve this problem [13][14][15]. This Special Issue provided a comprehensive review for the microstructure and oxidation resistance of Mo-based alloys and Nb-based alloys. Moreover, the influence of metallic elements and rare earth elements on the microstructure, phase compositions, oxidation kinetics and behavior of refractory metal alloys were also studied systematically. Finally, the modification mechanism of metallic elements was summarized in order to obtain refractory metal alloys with superior oxidation performance.
It is worth noting that the surface-coating technology can improve the oxidation resistance of the alloy at high temperature with as little impact on the mechanical properties as possible [16][17][18][19][20][21]. Therefore, it is favored by the majority of researchers. This Special Issue provides a summary of surface modification techniques for Mo-based and Nb-based alloys under high-temperature aerobic conditions of nearly half a century, including slurry sintering technology, plasma spraying technology, chemical vapor deposition technology, and liquid phase deposition technology. The growth mechanism and micromorphology of the coatings access by different preparation methods are evaluated. In addition, the advantages and disadvantages of various coating oxidation characteristics and coating preparation approaches are summarized. Finally, the coating's oxidation behavior and failure mechanism are summarized and analyzed, aiming to provide valuable research references in related fields.
In addition, TiC ceramics have become one of the most potential ultra-high temperature structural materials because of its high melting point, low density, and low price [22,23]. However, the poor mechanical properties seriously limit its development and application. In this Special Issue, the mechanism of the second-phase (particles, whiskers, and carbon nanotubes) reinforced TiC ceramics was reviewed. In addition, the effects of the second phase on the microstructure, phase composition, and mechanical properties of TiC ceramics were systematically studied: the addition of carbon black effectively eliminates the residual TiO 2 in the matrix, and the bending strength of the matrix is effectively improved by the strengthening bond formed between TiC; SiC particles effectively inhibit the grain growth through pinning, the obvious crack deflection phenomenon is found in the micrograph; the smaller grain size of WC plays a dispersion strengthening role in the matrix and makes the matrix uniformly refined; the addition of WC forms (Ti, W) C solid solution, and WC has a solid solution strengthening effect on the matrix; SiC whiskers effectively improve the fracture toughness of the matrix through bridging and pulling out, and the microscopic diagram and mechanism diagram of the SiC whisker action process are shown in this paper. The effect of new material carbon nanotubes (CNTs) on the matrix is also discussed: the bridging effect of CNTs can effectively improve the strength of the matrix; during sintering, some CNTs were partially expanded into nano-graphene ribbons (GNR); and in the process of crack bridging and propagation, more fracture energy is consumed by flake GNR. Finally, the existing problems of TiC-based composites are pointed out, and the future development direction is prospected.
To conclude, this Special Issue focuses on the reinforcement, surface coating technology, high-temperature corrosion, wear, erosion and corrosion protection of high-temperature structural materials. The significance of this Special Issue is to provide some references for further research on high-temperature structural materials.