Pest Oxidation Research Articles

High Temperature Wear Property of Ta-10W Alloy

This study investigated the high-temperature wear characteristics of Ta-10W alloy, composed of 90% tantalum and 10% tungsten, known for its high melting point, excellent corrosion resistance, and oxidation resistance. Wear tests were conducted using a ball-on-disc method with alumina balls at temperatures of 200°C, 400°C, 600°C, and 700°C. The results indicated that wear loss decreased (increasing wear resistance) from 200°C to 600°C due to the lubrication effect by oxide particles induced by Ta2O5, which enhanced wear resistance. However, at 700°C, a significant increase in wear loss was observed due to excessive oxide formation and the occurrence of cracks. Calculation of the wear rate revealed that it decreased as the temperature increased to 2.12×10-4 mm3· N-1· m-1 at 200 °C, 2.67×10-5 mm3·N-1·m-1 at 400 °C, and 4.21×10-6 mm3· N-1· m-1 at 600 °C. On the other hand, the wear rate increased at 700 °C to 3.87×10-4 mm3· N-1· m-1. The wear track analysis results revealed distinct wear mechanisms at different temperatures. At 200°C, abrasive wear characterized by pits and furrows was dominant. At 400°C, adhesive wear became more prevalent with cleavage worn surfaces compared to the wear pattern at 200°C. At 600°C, fragmented oxides indicating pest oxidation were noted, while at 700°C, deep pits and cracks along with fragmented oxides were prevalent. X-ray diffraction (XRD) analysis results showed the presence of the α-Ta phase at 200°C, 400°C, and 600°C, with peak shifts indicating lattice expansion. In contrast, at 700°C, Ta2O5 peaks were present. Based on the findings, the high-temperature wear mechanism of Ta-10W alloy was also discussed.

Exploring Nb[sbnd]Si based alloys with excellent fracture toughness and oxidation resistance

The main objective of this paper is to give a comprehensive discussion of the fracture toughness and oxidation resistance of Nb-16Si-20Ti-3Al-5Cr-xSn-yZr (x=0.5, 1.5; y=2, 4, 6) alloys, based on experimental evidence, phase formation and diffusion theory. Results demonstrate that the microstructure of Sn0.5Zr2 and Sn1.5Zr2 alloys consist of Nbss and α-Nb5Si3, and the other four alloys are composed of Nbss, α-Nb5Si3 and γ-Nb5Si3. At the same time, the area fraction of γ-Nb5Si3 increases with the increase of Zr content. The coarsening of silicides increases with the addition of Sn. The KQ value of the Sn0.5Zr4 alloy reaches the highest with the mean of 12.64 MPa·m1/2. After oxidation at 800 °C for 24 h, Sn0.5Zr4 alloy alloy did not undergo pest oxidation. Sn0.5Zr4 alloy forms a continuous and dense TiO2 oxide layer oxidized at 1200 °C, which makes it have the best high temperature oxidation resistance. Sn0.5Zr4 alloy has excellent fracture toughness, medium temperature and high temperature oxidation resistance.

Segregation mediated design of single-phase refractory medium and high entropy intermetallic compounds with complete suppression of pest oxidation

Refractory intermetallic compounds are potential candidates for high-temperature applications owing to their high thermal stability and excellent softening resistance. However, they suffer from the pest oxidation phenomenon at low temperatures, which highly restricts their applications. Here, after determining the stable segregated phases in complex mixtures of refractory elements, two novel refractory intermetallic compounds with complex compositions are engineered, (Ti1/5Zr1/5Nb1/5Hf1/5Ta1/5)Al3 and (Mo1/3W1/3Ta1/3)Si2. These two single-phase materials exhibit excellent oxidation resistance with complete suppression of pest phenomenon in the as-cast state. Exploring the underlying oxidation resistance mechanisms, it is found that the synergistic effect of high entropy lattice sites and the ability of Si and Al elements to develop a dense protecting layer are the main reasons for the complete elimination of the pest oxidation. Besides, we introduce and verify a sublattice electronegativity difference criterion as a new indicator for forming single-phase high and medium entropy materials. The approach to engineering single-phase entropy-stabilized intermetallic compounds reported here can be used to develop additional medium- and high-entropy intermetallic compounds with tailorable properties.

The Effect of Fe Addition in the RM(Nb)IC Alloy Nb-30Ti-10Si-2Al-5Cr-3Fe-5Sn-2Hf (at.%) on Its Microstructure, Complex Concentrated and High Entropy Phases, Pest Oxidation, Strength and Contamination with Oxygen, and a Comparison with Other RM(Nb)ICs, Refractory Complex Concentrated Alloys (RCCAs) and Refractory High Entropy Alloys (RHEAs).

In this work, the RM(Nb)IC alloy Nb–30Ti–10Si–5Cr–5Sn–3Fe–2Al–2Hf (NV2) was studied in the as-cast and heat-treated conditions; its isothermal oxidation at 700, 800 and 900 °C and its room temperature hardness and specific strength were compared with other Sn-containing RM(Nb)ICs—in particular, the alloy Nb–24Ti–18Si–5Cr–5Fe–5Sn (NV5)—and with RCCAs and RHEAs. The addition of Fe (a) stabilised Nbss; A15–Nb3X (X = Al, Si and Sn) and Nb3Si; metastable Nb3Si-m’ and Nb5Si3 silicides; (b) supported the formation of eutectic Nbss + Nb5Si3; (c) suppressed pest oxidation at all three temperatures and (d) stabilised a Cr- and Fe-rich phase instead of a C14–Nb(Cr,Fe)2 Laves phase. Complex concentrated (or compositionally complex) and/or high entropy phases co-existed with “conventional” phases in all conditions and after oxidation at 800 °C. In NV2, the macrosegregation of Si decreased but liquation occurred at T >1200 °C. A solid solution free of Si and rich in Cr and Ti was stable after the heat treatments. The relationships between solutes in the various phases, between solutes and alloy parameters and between alloy hardness or specific strength and the alloy parameters were established (parameters δ, Δχ and VEC). The oxidation of NV2 at 700 °C was better than the other Sn-containing RM(Nb)ICs with/without Fe addition, even better than RM(Nb)IC alloys with lower vol.% Nbss. At 800 °C, the mass change of NV2 was slightly higher than that of NV5, and at 900 °C, both alloys showed scale spallation. At 800 °C, both alloys formed a more or less continuous layer of A15–Nb3X below the oxide scale, but in NV5, this compound was Sn-rich and severely oxidised. At 800 °C, in the diffusion zone (DZ) and the bulk of NV2, Nbss was more severely contaminated with oxygen than Nb5Si3, and the contamination of A15–Nb3X was in-between these phases. The contamination of all three phases was more severe in the DZ. The contamination of all three phases in the bulk of NV5 was more severe compared with NV2. The specific strength of NV2 was comparable with that of RCCAs and RHEAs, and its oxidation at all three temperatures was significantly better than RHEAs and RCCAs.

Mechanical and high temperature oxidation response of Nb-18.7Si–5Ti–5Zr alloy

In the present work, the influence of addition of Ti and Zr elements (5 at.% each) on the microstructure, room temperature mechanical behavior and cyclic oxidation response (800–1400 °C) of the cast Nb-18.7Si alloy has been studied. The cyclic oxidation behavior of the alloy in both the uncoated and Fe,Cr modified silicide slurry coating conditions have been investigated at the pest oxidation temperature of 800 °C and at high temperatures of 1200 °C, 1400 °C. The microstructure of the Nb-18.7Si–5Ti–5Zr alloy exhibited the desirable α-Nb5Si3, Nbss phases in the cast alloy, which is otherwise needs high temperature heat treatment for longer duration. Further, a minor fraction of γ–Nb5Si3 phase is also present which resulted from the segregation of Ti, Zr elements during the solidification of the alloy. The alloy is measured with significantly high compressive strength (2236 ± 81 MPa) and fracture toughness (10.59 ± 0.35 MPa m1/2). The addition of Ti and Zr has not improved the oxidation resistance and the alloy is dimensionally degraded within 15min/1cycle of oxidation at all the test temperatures. Whereas, the Fe–Cr modified silicide coating provided the oxidation life of 350–425 min (23–28 cycles) during the oxidation test in the temperature range of 800–1400 °C. The formation of multi-layer oxide sale on the surface is attributed to the improvement in the oxidation life of the alloy.

Phase evolution and thermal stability of novel high-entropy (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2 ceramics

Owing to the high melting points and high-temperature stability, transition-metal disilicides are potential components for aerospace, automotive, and industrial engineering applications. However, unwanted oxidation known as PEST oxidation severely limits their application owing to the formation of volatile transition metal oxides, especially in the temperature range of 500–1000 °C. To overcome this problem, a new class of high-entropy disilicides, (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2, was selected by first-principles calculations and then successfully fabricated using a hot-pressing sintering technique. Furthermore, the phase evolution, thermal expansion behavior, thermal conductivity, and oxidation behavior were systematically investigated. Compared with MoSi2, (Mo0.2Nb0.2Ta0.2V0.2W0.2)Si2 possessed a lower thermal conductivity (10.9–14.7 W·m−1·K−1) at 25–1000 °C, higher thermal expansion coefficients (8.6 ± 1.3–6 K–1) at 50–1200 °C, and especially an excellent thermal stability at 500–1000 °C owing to slow diffusion and selective oxidation. This work provides a strong foundation for the synthesis and application of high-entropy disilicides.

Segregation Mediated Design of Single Phase Refractory Medium and High Entropy Intermetallic Compounds for Complete Suppression of Pest Oxidation

Segregation Mediated Design of Single Phase Refractory Medium and High Entropy Intermetallic Compounds for Complete Suppression of Pest Oxidation

On the Microstructure and Properties of the Nb-23Ti-5Si-5Al-5Hf-5V-2Cr-2Sn (at.%) Silicide-Based Alloy—RM(Nb)IC

The microstructure, isothermal oxidation, and hardness of the Nb-23Ti-5Si-5Al-5Hf-5V-2Cr-2Sn alloy and the hardness and Young’s moduli of elasticity of its Nbss and Nb5Si3 were studied. The alloy was selected using the niobium intermetallic composite elaboration (NICE) alloy design methodology. There was macrosegregation of Ti and Si in the cast alloy. The Nbss, αNb5Si3, γNb5Si3, and HfO2 phases were present in the as-cast or heat-treated alloy plus TiN in the near-the-surface areas of the latter. The vol.% of Nbss was about 80%. There were Ti- and Ti-and-Hf-rich areas in the solid solution and the 5-3 silicide, respectively, and there was a lamellar microstructure of these two phases. The V partitioned to the Nbss, where the solubilities of Al, Cr, Hf, and V increased with increasing Ti concentration. At 700, 800, and 900 °C, the alloy did not suffer from catastrophic pest oxidation; it followed parabolic oxidation kinetics in the former two temperatures and linear oxidation kinetics in the latter, where its mass change was the lowest compared with other Sn-containing alloys. An Sn-rich layer formed in the interface between the scale and the substrate, which consisted of the Nb3Sn and Nb6Sn5 compounds at 900 °C. The latter compound was not contaminated with oxygen. Both the Nbss and Nb5Si3 were contaminated with oxygen, with the former contaminated more severely than the latter. The bulk of the alloy was also contaminated with oxygen. The alloying of the Nbss with Sn increased its elastic modulus compared with Sn-free solid solutions. The hardness of the alloy, its Nbss, and its specific room temperature strength compared favourably with many refractory metal-complex-concentrated alloys (RCCAs). The agreement of the predictions of NICE with the experimental results was satisfactory.

Current Status of Research on the Oxidation Behavior of Refractory High Entropy Alloys

Refractory high entropy alloys (RHEA) are considered as novel promising high‐temperature materials for structural applications at ultrahigh temperatures primarily due to their attractive mechanical properties. By contrast, the oxidation behavior of RHEA has raised concern owing to pest oxidation, significant weight changes, scale spallation, or even complete oxidation at elevated temperatures. Herein, the currently available literature on high‐temperature oxidation behavior of RHEA is reviewed with respect to alloy composition, mass changes, corrosion products, and scale constitution. While many RHEA indeed suffer from poor oxidation resistance similar to that of pure refractory metals, some RHEA exhibit very good protectiveness, which is attributed to the formation of either well‐known protective scales such as α‐Al2O3 or rarely encountered complex oxides such as CrTaO4. Thermodynamic and kinetic aspects of oxide formation and growth are discussed to understand the oxidation mechanisms typical of RHEA. Further research directions with respect to additional in‐depth studies elucidating the oxidation mechanisms as well as the further consequent improvement of the oxidation resistance of RHEA are developed from the current intermediate stage of research in the field.

On the Microstructure and Properties of Nb-12Ti-18Si-6Ta-5Al-5Cr-2.5W-1Hf (at.%) Silicide-Based Alloys with Ge and Sn Additions.

The microstructures and properties of the alloys JZ3 (Nb-12.4Ti-17.7Si-6Ta-2.7W-3.7Sn-4.8Ge-1Hf-4.7Al-5.2Cr) and JZ3+(Nb-12.4Ti-19.7Si-5.7Ta-2.3W-5.7Sn-4.9Ge-0.8Hf-4.6Al-5.2Cr) were studied. The densities of both alloys were lower than the densities of Ni-based superalloys and many of the refractory metal complex concentrated alloys (RCCAs) studied to date. Both alloys had Si macrosegregation and the same phases in their as cast and heat treated microstructures, namely βNb5Si3, αNb5Si3, A15-Nb3X (X = Al, Ge, Si, Sn), C14-Cr2Nb and solid solution. W-rich solid solutions were stable in both alloys. At 800 °C only the alloy JZ3 did not show pest oxidation, and at 1200 °C a thin and well adhering scale formed only on JZ3+. The alloy JZ3+ followed parabolic oxidation with rate constant one order of magnitude higher than the single crystal Ni-superalloy CMSX-4 for the first 14 h of oxidation. The oxidation of both alloys was superior to that of RCCAs. Both alloys were predicted to have better creep at the creep goal condition compared with the superalloy CMSX-4. Calculated Si macrosegregation, solid solution volume fractions, chemical compositions of solid solution and Nb5Si3, weight changes in isothermal oxidation at 800 and 1200 °C using the alloy design methodology NICE agreed well with the experimental results.

Complete elimination of pest oxidation by high entropy refractory metallic silicide (Mo0.2W0.2Cr0.2Ta0.2Nb0.2)Si2

Molybdenum disilicide (MoSi2) is a competitive ultra-high-temperature structural material. However, it suffered from a fatal low-temperature oxidation, which extremely limited its application. To solve this problem, a novel high entropy refractory metallic silicide with composition of (Mo0.2W0.2Cr0.2Ta0.2Nb0.2)Si2 was synthesized by in-situ spark plasma reaction sintering in this work. The interrupted oxidation tests at 500 °C for 300 h have been performed, and shown that the prepared material has an excellent oxidation resistance at low temperatures without pest phenomenon. The mechanism for eliminating pest oxidation is that the high entropy of this material would suppress generation of bulk metallic oxides with formation of only an intact SiO2 film during oxidation.

The Oxidation Resistance of Nb-Si-Based Alloys at Intermediate and High Temperatures

The oxidation behavior of three Nb-Si-based alloys were evaluated at intermediate (800 °C) and high (1250 °C) temperatures for 100 h in air. At 800 °C, the Nb-24Ti-15Si-13Cr-2Al-2Hf (at. %) alloy suffered from catastrophic pest oxidation. This pest phenomenon was suppressed by the addition of Sn. However, Ta addition protected the Nb-Si-based alloys from pest oxidation for a short time. At 1250 °C, Sn could enhance the oxidation resistance of Nb-Si-based alloys due to the formation of a Sn-rich layer. In addition, the oxidation mechanisms of Nb-Si-based alloys at intermediate and high temperatures were discussed.

On the Microstructure and Isothermal Oxidation at 800 and 1200 °C of the Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn (at.%) Silicide-Based Alloy.

The research presented in this paper aspired to understand how the simultaneous addition of Ge and Sn in an Hf-free Nb-silicide-based alloy affected its oxidation resistance. Results are presented for the Nb-24Ti-18Si-5Al-5Cr-5Ge-5Sn alloy (at.%) which was studied in the as-cast and heat-treated (1400 °C/100 h) conditions and after isothermal oxidation in air at 800 and 1200 °C. There was macrosegregation in the cast alloy, in which the Nbss formed at a low volume fraction and was not stable after heat treatment at 1400 °C. The βNb5Si3, A15-Nb3Sn, and C14-NbCr2 were stable phases. The alloy did not undergo pest oxidation at 800 °C, and there was no spallation of its scale at 1200 °C. There was enrichment in Ge and Sn in the substrate below the scale/substrate interface, where the compounds Nb3Sn, Nb5Sn2Si, (Ti,Nb)6Sn5, and Nb5Ge3 were formed. After the oxidation at 1200 °C, the solid solution in the bulk of the alloy was very Ti-rich (Ti,Nb)ss. Improvement of oxidation resistance at both temperatures was accompanied by a decrease and increase, respectively, of the alloy parameters VEC (valence electron concentration) and δ, in agreement with the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration). The elimination of scale spallation at 1200 °C was attributed (a) to the formation of Ti-rich (Ti,Nb)ss solid solution and (Ti,Nb)6Sn5, respectively, in the bulk and below the scale, (b) to the low concentration of Cr in the scale, (c) to the absence of GeO2 in the scale, (d) to the formation of αAl2O3 in the scale, and (e) to the presence (i) of Nb5Ge3 below the scale/substrate interface and (ii) of oxides in the scale, namely, SiO2, Al2O3, TiO2, and SnO2, and Ti2Nb10O29,TiNb2O7, and AlNbO4, respectively, with a range of intrinsic thermal shock resistances and coefficient of thermal expansion (CTE) values that reduced stresses in the scale and the substrate below it.

A Study of the Effect of 5 at.% Sn on the Micro-Structure and Isothermal Oxidation at 800 and 1200 °C of Nb-24Ti-18Si Based Alloys with Al and/or Cr Additions.

This paper presents the results of a systematic study of Nb-24Ti-18Si based alloys with 5 at.% Sn addition. Three alloys of nominal compositions (at.%), namely Nb-24Ti-18Si-5Cr-5Sn (ZX4), Nb-24Ti-18Si-5Al-5Sn (ZX6), and Nb-24Ti-18Si-5Al-5Cr-5Sn (ZX8), were studied to understand how the increased Sn concentration improved oxidation resistance. In all three alloys there was macrosegregation, which was most severe in ZX8 and the primary βNb5Si3 transformed completely to αNb5Si3 after heat treatment. The Nbss was not stable in ZX6, the Nb3Sn was stable in all three alloys, and the Nbss and C14-NbCr2 Laves phase were stable in ZX4 and ZX8. The 5 at.% Sn addition suppressed pest oxidation at 800 °C but not scale spallation at 1200 °C. At both temperatures, a Sn-rich area with Nb3Sn, Nb5Sn2Si, and NbSn2 compounds developed below the scale. This area was thicker and continuous after oxidation at 1200 °C and was contaminated by oxygen at both temperatures. The contamination of the Nbss by oxygen was most severe in the bulk of all three alloys. Nb-rich, Ti-rich and Nb and Si-rich oxides formed in the scales. The adhesion of the latter on ZX6 at 1200 °C was better, compared with the alloys ZX4 and ZX8. At both temperatures, the improved oxidation was accompanied by a decrease and increase respectively of the alloy parameters VEC (Valence Electron Concentration) and δ, in agreement with the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration). Comparison with similar alloys with 2 at.% Sn addition showed (a) that a higher Sn concentration is essential for the suppression of pest oxidation of Nb-24Ti-18Si based alloys with Cr and no Al additions, but not for alloys where Al and Cr are in synergy with Sn, (b) that the stability of Nb3Sn in the alloy is “assured” with 5 at.% Sn addition, which improves oxidation with/out the presence of the Laves phase and (c) that the synergy of Sn with Al presents the “best” oxidation behaviour with improved scale adhesion at high temperature.

Complete Elimination of Pest Oxidation by High Entropy Refractory Metallic Silicide (Mo <sub>0.2</sub>W <sub>0.2</sub>Cr <sub>0.2</sub>Ta <sub>0.2</sub>Nb <sub>0.2</sub>)Si <sub>2</sub>

Molybdenum disilicide (MoSi2) is a competitive ultra-high-temperature structural material. However, it suffered from a fatal low-temperature oxidation, which extremely limited its application. To solve this problem, a novel high entropy refractory metallic silicide with composition of Mo0.2W0.2Cr0.2Ta0.2Nb0.2)Si2 was synthesized by in-situ spark plasma reaction sintering in this work. The interrupted oxidation testings at 500 °C for 300 h have been performed, and shown that this material has an excellent oxidation resistance at low temperatures without pest phenomenon. The mechanism for eliminating pest oxidation is that the high entropy of this material would suppress generation of bulk metallic oxides with formation of only an intact SiO2 film during oxidation testing.

Preparation and moderate temperature oxidation behavior of Ti- and Al-doped NbSi2-Si3N4 composite coatings on Nb alloy

In order to adjust the mismatch of coefficient of thermal expansion between NbSi2 coating and Nb alloy while suppressing the pest oxidation of NbSi2 coating at moderate temperatures, NbSi2-Si3N4 composite coatings with doped Ti and Al were prepared on Nb alloy by a spark plasma sintering process using an Nb-Si-Si3N4-Ti-Al powder mixture. The sintering reactions based on the thermodynamic feasibility for the constituent phase formation of the coatings were revealed, and the oxidation resistance of the coatings at 750 °C was investigated. The coatings had a multiple-phase structure and mainly consisted of (Nb,Ti)Si2, Si3N4, (Ti,Nb)5Si4, (Nb,Ti)5Si3, and a small number of other nitride phases. After 50 h of oxidation, pest oxidation was observed on both the NbSi2 coating and NbSi2-Si3N4 coating, while no indication of pest oxidation was present on the coatings after doping with Ti and Al.

The Effect of Ge Addition on the Oxidation of Nb-24Ti-18Si Silicide Based Alloys.

In this paper, we report research about the isothermal oxidation in air at 800 and 1200 °C for 100 h of the as-cast alloys (at.%) Nb-18Si-5Ge (ZF1), Nb-18Si-10Ge (ZF2), Nb-24Ti-18Si-5Ge (ZF3), Nb-24Ti-18Si-5Cr (ZF4), Nb-24Ti-18Si-5Al (ZF5), Nb-24Ti-18Si-5Al-5Cr-5Ge (ZF6), and Nb-24Ti-18Si-5Al-5Cr-5Ge-5Hf (ZF9), the microstructures of which were reported in previous publications. Only the alloys ZF1, ZF2, and ZF3 suffered from pest oxidation at 800 °C. The Ge addition substantially improved the oxidation resistance of the other alloys both at 800 °C and 1200 °C, which followed parabolic kinetics at 800 °C and in the early stages at 1200 °C and linear kinetics at longer times, where there was spallation of the scales. The Nb2O5 and TiNb2O7 oxides were formed in the scales of the alloys ZF4, ZF5, ZF6, and ZF9 at 800 °C and 1200 °C, the GeO2 was observed in all scales and the SiO2 in the scales of the alloys ZF4 and ZF5, the CrNbO4 in the scales of the alloys ZF4, ZF6, and ZF9 and the AlNbO4 in the scales of the alloys ZF5, ZF6, and ZF9. Diffusion zones were formed below the scale/substrate interface only in the alloys ZF4 and ZF5 where the Nbss and Nb5Si3 were contaminated by oxygen. However, these phases were not contaminated by oxygen in the bulk of the alloys ZF4, ZF5, ZF6, and ZF9. The alloys ZF9 and ZF6 exhibited the best oxidation behaviour at 800 °C and 1200 °C, respectively. The alloys were compared with Nb-silicide based alloys of similar compositions without Ge and Hf additions and the alloy ZF9 with refractory metal High Entropy Alloys. Compared with the former alloys, the addition of Ge reduced the vol % of the Nbss. Compared with the latter alloys, the superior oxidation behaviour of the alloy ZF9 was attributed to its higher and lower values respectively of the parameters δ and VEC.

The preparation of in-situ MoSi2-SiC-MoB three-phase composite completely eliminating the PEST phenomena

Because of high melting point (2300 K), rather low density (6.28g/cm3) and excellent elevated oxidation resistance (above 1600 °C), MoSi2 have been treated to be one of the most attractive high temperature materials. However, thus a MoSi2 materials have their intrinsic limitations, one of the worst things is it suffered from an accelerated oxidation at low temperatures (between 350 and 550 °C) which was first discovered by Fitzer in 1955 and was named PEST oxidation. The phenomena of PEST extremely hindered the application of MoSi2 in advanced engineering and aerospace, also limited its longevity and safety. In this paper, an in-situ MoSi2-SiC-MoB three-phase composite was prepared by reactive infiltration process which is known as low-cost and net shape method, and after the oxidation process in 500 °C for 3 h, it was found that there is no PEST occurred anymore. And the residual bending strength of this composite will still over 68%. The reasons for overcoming PEST would be the in-situ formed MoB phase will be stiffly covered over the surface of MoSi2 grains, and the volume expansion of SiC phase and volume contraction of MoB phase will be matched in the composite system 9.7%wt MoB/8.6%wt SiC/MoSi2.

Recycling Molybdenum Oxides from Waste Molybdenum Disilicides: Oxidation Experimental Study and Photocatalytic Properties

To recycle elemental molybdenum from waste molybdenum disilicide (MoSi2) heating elements, the MoSi2 was first disintegrated to MoO3 and SiO2 powders in air at a pest oxidation temperature of 500 °C. X-ray diffraction (XRD) patterns confirmed the completion of the pest oxidation reaction. The mixture of MoO3 and SiO2 powders were heated to 950 °C in a tube furnace to evaporate MoO3, and the XRD patterns of the residue showed that only SiO2 was left in the crucible, confirming that the MoO3 was removed through thermal evaporation. The collected MoO3 crystals had a striped morphology. Photocatalytic performance of MoO3 showed superior activity in comparison with commercial MoO3 and P25 for the degradation of methylene blue under visible light irradiation. The photocatalytic degradation activity of MoO3 synthesized by thermal evaporation at 950 °C was 99.25% in 60 min.

A Study of the Effects of Hf and Sn on the Microstructure, Hardness and Oxidation of Nb-18Si Silicide Based Alloys without Ti Addition.

The paper presents the results of an experimental study of large (≈0.6 kg) arc melted buttons of four Ti free Nb-silicide based alloys with Sn addition with nominal compositions (at.%) Nb-18Si-5Hf-5Sn (EZ1), Nb-18Si-5Al-5Sn (EZ7), Nb-18Si-5Cr-5Hf-5Sn (EZ3) and Nb-18Si-5Al-5Hf-5Sn (EZ4). The alloys were studied in the as-cast and heat treated conditions. In all the alloys there was macrosegregation of Si (MACSi). Among the single element additions Hf had the weakest and Sn the strongest effect on MACSi. The simultaneous presence of Cr and Hf in the alloy EZ3 had the strongest effect on MACSi. In all the alloys the βNb5Si3 was the primary phase and was present after the heat treatment(s), the Nb3Si silicide was suppressed and the A15-Nb3Sn intermetallic was stable. The Nbss was not stable in the alloys EZ7 and EZ4 and the C14-NbCr2 Laves phase was stable in the alloy EZ3. Very Hf-rich Nb5Si3 was stable in the alloy EZ4 after prolonged heat treatments. Eutectics were observed in all the alloys. These were binary eutectics in the alloys EZ1 and EZ7, where respectively they consisted of the Nbss and βNb5Si3, and βNb5Si3 and A15-Nb3Sn phases. Most likely ternary eutectics consisting of the Nbss, C14-NbCr2 and βNb5Si3, and Nbss, βNb5Si3 and A15-Nb3Sn phases were observed, respectively in the alloys EZ3 and EZ4. The addition of Al increased the vol% of the Nb5Si3 and A15-Nb3Sn phases, particularly after the heat treatment(s). The lattice parameter of Nb respectively increased and decreased with the addition of Hf, and Al or Cr and the latter element had the stronger negative effect. Pest oxidation was not suppressed in the alloys of this study.