Preparation of nanostructured high-temperature TZM alloy by mechanical alloying and sintering

Mechanical milling proceeded by sintering was used to synthesize nanostructured temperature-resistant TZM alloy. Milling under Ar for different times (1, 2, 3, 5, 10, 15, 20, 25, and 30 h) and sintering at 1500, 1600 and 1700 °C for 30, 45, 60 and 90 min resulted in increasing of low-energy grain boundaries (LEGBs) and dispersion of TiC and ZrC with a size of ~ 65 nm in the matrix near LEGBs. Morphology and grain size of the products were determined from scanning electron microscope (SEM) images and X-ray diffraction (XRD) patterns, almost precisely. Optimum density of nanostructured TZM alloy ~ 9.95 ± 0.01 g/cm³ was achieved by sintering at 1700 °C for 90 min.     

An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings

NiCrAlY bond-coat was coated on Inconel 718 substrate by air plasma spraying (APS) followed by APS ZrO₂-8 wt.%Y₂O₃ as top-coat. Using CO₂ laser of different energy densities, ceramic top-coat surface was remelted. Laser remelting with high energy density (4 J/mm²) produced a dense microstructure over the whole thickness of top-coat, while low energy density (0.67 J/mm²) laser remelting produced a ~ 50 μm thick dense layer on the top-coat surface. It was found that the volume fraction of monoclinic phase decreased from 9% in as-sprayed coating to 4% and 3% after laser remelting with high and low energy density respectively. After isothermal oxidation at 1200 °C for 200 h, the thickness of oxide layer (TGO) in the sample produced by low energy density laser remelting was ~ 5.6 μm, which was thinner than that of oxide layer in as-sprayed (~ 7.6 μm) and high energy density laser remelted (~ 7.5 μm) samples. A uniform and continuous oxide layer was found to develop on the bond-coat surface after low energy density laser remelting. Thicker oxide layer containing Cr₂O₃, NiO and spinel oxides was observed in both as-sprayed and high energy density laser remelted coatings. After cyclic oxidation at 1200 °C for 240 h, the weight gain per unit area of as-sprayed coating was similar to that of high energy density laser remelted coating while a significantly smaller weight gain was found in low energy density laser remelted coating.     

SiC nanowire-toughened MoSi2-SiC coating to protect carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a SiC nanowire-toughened MoSi₂-SiC coating was prepared on them using a two-step technique of chemical vapor deposition and pack cementation. SiC nanowires obtained by chemical vapor deposition were distributed random-orientedly on C/C substrates and MoSi₂-SiC was filled in the holes of SiC nanowire layer to form a dense coating. After introduction of SiC nanowires, the size of the cracks in MoSi₂-SiC coating decreased from 18 ± 2.3 to 6 ± 1.7 μm, and the weight loss of the coated C/C samples decreased from 4.53% to 1.78% after oxidation in air at 1500 °C for 110 h.     

Oxidation protection of gamma-titanium aluminide using glass-ceramic coatings

γ-TiAl intermetallic alloys are presently considered an efficient structural material for advanced turbine blades and aero-engine components due to their various advantages compared to the traditionally used superalloys. However, their poor oxidation resistance at temperatures > 750 °C severely limits their wider application. The present study dealt with the improvement of oxidation resistance of this alloy by applying impervious glass-ceramic coatings by vitreous enameling technique. Results showed that MgO-SiO₂-TiO₂ glass-ceramic coating could offer excellent oxidation resistance to γ-TiAl at 800 °C even up to 100 h with negligible weight gain (~ 0.10 mg/cm²) compared to that of the bare alloy (~ 1.3 mg/cm²). The coatings those were belonging from BaO-MgO-SiO₂, ZnO-Al₂O₃-SiO₂ and BaO-SiO₂ systems also extend appreciable improvement in the oxidation resistance of the alloy at 800 °C up to 100 h. At further higher temperature such as at 1000 °C, the ABK-13 and ABK-103 glass-ceramic coatings offered significant protection to the alloy up to 25 h of exposure in air with minimum weight gain (~ 0.34 mg/cm²). However, after that the coated layers started to peel off from the alloy surface.     

Multi-layered silicides coating for vanadium alloys for generation IV reactors

The halide-activated pack-cementation technique was employed to fabricate a diffusion coating that is resistant both to isothermal and to cyclic oxidation in air at 650 °C on the surface of the V–4Cr–4Ti vanadium alloy that is a potential core component of future nuclear systems. A thermodynamic assessment determined the deposit conditions in terms of master alloy, activator, filler and temperature. The partial pressures of the main gaseous species (SiCl₄, SiCl₂ and VCl₂) in the pack were calculated with the master alloy Si and the mixture VSi₂ + Si. The VSi₂ + Si master alloy was used to limit vanadium loss from the surface. The obtained coating consisted of multi-layered V_xSi_y silicides with an outer layer of VSi₂. This silicide developed a protective layer of silica at 650 °C in air and was not susceptible to the pest phenomenon, unlike other refractory silicides (MoSi₂, NbSi₂). We suggest that VSi₂ exhibits no risk of rapid degradation in the gas fast reactor (GFR) conditions.     

Formation of MoSi2 and Al doped MoSi2 coatings on molybdenum base TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy

Formation of aluminium (Al) doped molybdenum di-silicide (MoSi₂) coatings was studied to improve the high temperature oxidation behavior of TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy. The pack composition of the halide activated pack cementation process was successfully optimized to form silicide and Al doped silicide coatings on the TZM alloy substrates. Mo(Si, Al)₂ phase was found to form at the outer layer of the coating prepared by doping Al in MoSi₂. A change in composition of the phases with increase in coating temperature was detected with Al doping, whereas un-doped silicide coating process was dominated by the formation and growth of MoSi₂ phase. Oxidation test and the characterization studies using SEM, EDS, XRD, and micro-hardness measurements indicated the improved performance of Al doped silicide coating during high temperature oxidation in dry air due to the formation of the protective alumina scale.     

Oxidation resistance of TiAl3-Al composite coating on orthorhombic Ti2AlNb based alloy

The TiAl₃-Al composite coating on orthorhombic Ti₂AlNb based alloy was prepared by cold spray. Oxidation in air at 950 °C indicated that the bare alloy exhibited poor oxidation resistance due to the formation of TiO₂/AlNbO₄ mixture and intended to scale off at the TiO₂ rich zone. A nitride layer about 2 µm was formed under the oxide layer. The oxygen invaded deeply into the alloy and caused severe microhardness enhancement in the near surface region. The TiAl₃-Al composite coating exhibited parabolic oxidation kinetics and showed no sign of degradation after oxidized up to 1098 h at 950 °C in air under quasi-isothermal condition. No scaling of the coating was observed after oxidized at 950 °C up to the tested 150 cycles. The major oxide in the oxidized coating was Al₂O₃. The AlTi₂N, TiAl and small amount of TiO₂ were also observed in the oxidized coating. The EPMA and microhardness tests showed that inward oxygen diffusion was prevented by the interlayer, which was formed between the composite coating and the substrate during heat-treatment. Microstructure analyses demonstrated that the interlayer play a major role in protecting the substrate alloy from high temperature oxidation and interstitial embrittlement.     

Oxidation resistance of boronized CoCrMo alloy

Boronizing of CoCrMo alloy has been performed by means of a powder-pack method using commercial LSB powders at 850, 900 and 950 °C for 8 h, respectively. In this study, the boronized CoCrMo alloy before and after oxidation tests were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The distribution of alloy elements of boronized samples from surface to interior was determined using energy-dispersive X-ray spectroscopy (EDS). XRD study showed the boride layer formed at 950 °C/8 h consisted of the phases Co₂B and CrB. Depending on boronizing temperature, the thickness of boride layer ranged from 2 to 11 μm. Cyclic oxidation behavior of the boride layer has been investigated at an oxidation temperature of 950 °C with a total exposure time up to 50 h in air. The test results indicated that the boronized CoCrMo alloy had superior oxidation resistance compared to unboronized sample.     

Effect of alloying with Al on oxidation behavior of MoSi2 coatings at 1100 °C

MoSi₂ - 0, 15.3, 22, and 29.3 at.% Al coatings were prepared on the nickel-based super-alloy substrates by electro-thermal explosion ultrahigh speed spraying technology. The analysis showed that the coatings had fine microstructure with grain sizes ranging from 0.5 to 2 μm. The bonding between coating and substrate was typically metallurgical cohesion. The oxidation resistance of the coating was further studied at 1100 °C in air. Al alloyed in MoSi₂ coatings increased the oxidation resistance of the coatings, and the oxidation resistance of MoSi₂-15.3 at.% Al coating was higher than the other two MoSi₂–Al coatings. This suggested the oxidation resistance might have close relations to the obtained grain size.     

The corrosion of Fe3Al alloy in liquid zinc

A systematic study of the isothermal corrosion testing and microscopic examination of Fe₃Al alloy in liquid zinc containing small amounts of aluminum (less than 0.2 wt.%) at 450 °C was carried out in this work. The results showed the corrosion of Fe₃Al alloy in molten zinc was controlled by the dissolution mechanism. The alloy exhibited a regular corrosion layer, constituted of small metallic particles (diameter: 2-5 μm) separated by channels filled with liquid zinc, which represented a porosity of about 29%. The XRD result of the corrosion layer formed at the interface confirmed the presence of Zn and FeZn_6.67. The corrosion rate of Fe₃Al alloy in molten zinc was calculated to be approximately 1.5 × 10⁻⁷ g cm⁻² s⁻¹. Three steps could occur in the whole process: the superficial dissolution of metallic Cr in the corrosion layer, the new phase formation of FeZn_6.67 and the diffusion of the dissolved species in the channels of the corrosion layer.