Microstructure and Oxidation Behavior of Nb-Si-Based Alloys for Ultrahigh Temperature Applications: A Comprehensive Review

Nb-Si-based superalloys are considered as the most promising high-temperature structural material to replace the Ni-based superalloys. Unfortunately, the poor oxidation resistance is still a major obstacle to the application of Nb-Si-based alloys. Alloying is a promising method to overcome this problem. In this work, the effects of Hf, Cr, Zr, B, and V on the oxidation resistance of Nb-Si-based superalloys were discussed. Furthermore, the microstructure, phase composition, and oxidation characteristics of Nb-Si series alloys were analyzed. The oxidation reaction and failure mechanism of Nb-Si-based alloys were summarized. The significance of this work is to provide some references for further research on high-temperature niobium alloys.

Application of Flaky Reduced Iron Powders and Their Composites for Highly Effective Microwave Absorption Materials

On the basis of replacing carbonyl iron powders with reduced iron powders which is environmentally friendly in the production process and has low cost, the problems of poor impedance matching and poor oxidation resistance of iron powders are improved by anisotropy and core-shell structure construction. The results show that the microwave absorption performance of the flaky reduced iron powders are improved obviously after coating of SiO2 and carbon shell successively. When the absorber thickness is 2.00 mm and the frequency is 13.3 GHz, the minimum RL value reaches −47.72 dB, and the effective absorption bandwidth is 4.9 GHz (10.6 GHz to 15.5 GHz).

Preparation of MoSi2 Coating on Mo Substrate for Oxidation Resistance by a Facile Method

A MoSi2 coating layer with a MoB protected layer was simply prepared by using a combined method, which involved slurry painting and an embedded method via a hot press sintering process. The microstructure and phase composition of the coatings were investigated and determined. The experimental time and the B addition are considered as important factors to be discussed in this study. All the results proved that when B is added, it was found that the formation of MoB played an important role of a diffusion barrier to Si and hindered the production of the poor oxidation resistance phases (Mo5Si3 and Mo3Si).

Effect of Cr and Al alloying on the oxidation resistance of a Ti5Si3-incorporated MoSiBTiC alloy

The effects of adding Cr and Al on the oxidation behavior of a Ti5Si3-incorporated MoSiBTiC alloy (46Mo–28Ti–14Si–6C–6B, at%) were investigated at 800 and 1,100°C. The addition of Cr and Al largely improved the oxidation resistance of the MoSiBTiC alloy at 800°C due to the formation of Cr2(MoO4)3 and Al2(MoO4)3 in the oxide scales. These protective molybdates mainly formed on the molybdenum solid solution (Moss) and Mo3Si phases that show poor oxidation resistance in the Cr- and Al-free alloy and consequently increased the oxidation resistance of the alloys. However, accelerated oxidation occurred on the 10Al alloy after the long-term oxidation test, suggesting that the formed oxide scale has limited protection ability. At 1,100°C, the addition of Cr and Al also enhanced the oxidation resistance to some extent by forming Cr2O3 and Al2O3 in the oxide scales.

Protection of Orthorhombic Alloy by AlCrN Coating

The oxidation behavior of AlCrN coating deposited on Ti-25Al-12.5Nb-6Mo-0.48V alloy was investigated at 700 °C and 800 °C in static air. Oxidation in air indicated that the uncoated alloy exhibited poor oxidation resistance due to the formation of TiO2 scale, but the major oxide in the oxidized coating was chromia and alumina and the TiN layers were also observed beneath the coating. No sign of degradation and no scaling of the coating was observed after oxidation.

On high-temperature oxidation and protection of 2:17-type SmCo-based magnets

Sm2(Co,Cu,Fe,Zr)17 are the best high-temperature permanent magnets because of their high Curie temperature (800°C–850°C). However, irreversible and unacceptable coercivity losses retard their use in applications at temperatures over 550°C. The coercivity loss has been correlated with poor oxidation resistance at high temperatures. The current research progress on the effect of oxidation and its prevention, for 2:17-type magnets, is reviewed. Oxidation in air at 500°C–700°C causes the magnets to form three regions: (1) an external oxide scale mainly consisting of (CoxFe1-x)3O4, (2) a thicker internal oxidation zone where the typical cellular precipitation (2:17R cell and 1:5H cell boundary) structure has been completely collapsed due to the Sm oxidation into Sm2O3, and (3) an oxidation-free zone where the cellular precipitates remain unchanged in lattice structure. No unacceptable coercivity loss is seen in the oxidation-free zone. Its thickness can be impressively increased within the magnets at high temperature, when they are covered with surface diffusion barriers for oxygen from the atmosphere, such as thin films of Cr2O3, Al2O3, and the metals with the ability to thermally grow these oxides.

Phase Equilibria and Oxidation Behavior of C40 Disilicides in the Nb-Cr-Si System

ABSTRACTThe phase equilibrium and oxidation behavior of the disilicides that form in the Nb-Cr-Si ternary system have been investigated. Although NbSi2 and CrSi2 both exhibit a C40 crystal structure, they form separate ranges of compositional homogeneity in the ternary system. Their phase boundaries at 1300 °C have been experimentally determined in this study. The binary NbSi2 exhibited poor oxidation resistance, showing pest-like behavior during oxidation at temperature above 800 °C. In contrast, the alloys containing Cr showed much better oxidation resistance up to 1200 °C.

Use of CrN Passivation for Fabricating Al Micro-Materials by Electromigration

The technique for fabricating Al micro-materials using a conductive passivation film by electromigration (EM), which is the physical phenomenon of atomic transport with high-density electron flow, has been reported. Conductive passivation film precludes the unplanned hillock formation and substantially simplifies the sample preparation time for fabricating Al micro-materials by EM. To date, TiN that is electrical conductive material has been used as a passivation film. However, the TiN passivation oxidizes during heat and current test for fabricating Al micro-materials by EM because of inherent poor oxidation resistance of TiN. Oxidation of passivation causes a problem that applying current occasionally becomes difficult. The present paper proposes a new conductive passivation made of CrN for fabricating Al micro-materials by EM. CrN is used as a countermeasure against the oxidation problem. Additionally, the growth of Al micro-materials by EM is investigated in the relation with the experimental conditions of current and substrate temperature. As a result, we report that the fabrication of Al micro-materials using the CrN passivation is successfully demonstrated in the relation with the experimental conditions.

High-Temperature Oxidation of WC-20%TiC-10%Co Carbides

The oxidation characteristics of WC-20%TiC-10%Co sintered carbides were studied by oxidizing at 700, 800 and 900 °C for 3 h in air. The samples oxidized fast with large weight gains, displaying quite poor oxidation resistance. The formed oxide scales that consisted primarily of CoWO4, WO3, and TiO2 were porous, and prone to cracking.

High Temperature Oxidation of CrAlSiN Thin Films

Nano-multilayered, crystalline CrAlSiN thin films were deposited on either steel or WC-10%Co substrates by the cathodic arc plasma deposition. Their oxidation characteristics were studied at 800-1000°C for 50 h in air. The film deposited on steel displayed good oxidation resistance, due mainly to formation of Cr2O3 and α-Al2O3. The film deposited on WC-10%Co displayed poor oxidation resistance, due mainly to the oxidation of the substrate.