Oxidation protective silicide coating on Mo-Si-B alloys

A MoSi₂ coating was successfully formed on a Mo-9Si-18B alloy, consisting of Mo₅SiB₂ (T₂) and Mo solid solution (Mo _ss ) phases, using pack cementation with Si. Isothermal and cyclic oxidation tests of pack-cemented Mo-9Si-18B alloys were performed at 1300 °C and 1500 °C. Steady-state oxidation rates at both temperatures are almost equal to those of pure MoSi₂. The MoSi₂ layer is completely transformed into Mo₅Si₃ (T₁) containing B after oxidation at 1500 °C for 24 hours. Thermal expansion of the T₁ phase is anisotropic, but a [001] texture in the growth direction for the columnar grains in the T₁ layer reduces thermal stresses generated around the phases. Evolution of T₁ layers during oxidation between 1300 °C and 1500 °C was investigated; their growth rate constants and the interdiffusion coefficient of Mo and Si in the Mo-Si-B system have been evaluated and compared with those in the binary Mo-Si system. Furthermore, we have studied phase transformations in a simpler system MoSi₂ vs T₂ using MoSi₂/T₂ diffusion couples. Layers of T₁ and MoB + T₁ were formed in the diffusion zone during oxidation at temperatures between 1400 °C and 1600 °C. This behavior is different from that of the pack-cemented Mo-9Si-18B alloy. Pack-cemented T₂ single crystals show a diffusion structure similar to that of MoSi₂/T₂ diffusion couples, but the ratio of layer thickness is different. Based on these diffusion results, a method for extending the lifetime of the MoSi₂ layer is proposed. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Toughening of MoSi2 with a ductile (niobium) reinforcement

The effect of interfacial reactions and Y₂O₃ coatings on toughening of MoSi₂ by ductile phase Nb reinforcements has been investigated. In the absence of coating the interfacial reaction layer exhibits parabolic growth with Mo₅Si₃, (Mo, Nb)₅Si₃, (Nb, Mo)₅Si₃ and Nb₅Si₃ phases forming. In precracked laminates subjected to tensile loads the ductile phase deformation is partially constrained, with debonding occurring within the interfacial reaction zone. Dense Y₂O₃ coating inhibits interdiffusion and results in more extensive debonding. In either case, significant toughening is expected with measured work of rupture values χ ≈ 5.7 to 6.3. Bulk composite MoSi₂ reinforced with 20 vol.% Nb particles subjected to a chevron-notched three point flexure test had a work of rupture almost five times larger than the unreinforced MoSi₂ matrix.     

Phase relations in the Mo-Si-C system relevant to the processing of MoSi2-SiC composites

Phase relations in the Mo-Si-C system were evaluated at 1200 °C and 1600 °C within the composition range delimited by the phases Mo₅Si₃, MoSi₂, Mo_≤5Si₃C_≤1, and SiC. The evaluation included estimation of possible equilibria from known thermodynamic data of the binary phases as well as experimental work. For the experimental evaluation, high-purity powders were hot-pressed, heat-treated, and characterized by X-ray diffraction and electron microprobe analysis. It is shown that MoSi₂ is in equilibrium with Mo_≤5Si₃C_≤1 at 1600 °C, as previously established by Nowotnyet al, (Monatsh. Chem., 1954, vol. 85, pp. 255-72), and at 1200 °C, in contrast to the Mo₅Si₃-SiC equilibrium reported by van Looet al. (High Temp.- High Press., 1982, vol. 14, pp. 25-31). The thermodynamic estimation suggests that these phase relations should extend to lower temperatures in the range of compositions investigated. Thus, the third phase in silicon-lean MoSi₂-SiC composites should be the Nowotny phase (Mo_≤5Si₃C_≤1) instead of Mo₅Si₃. The Gibbs free energy of formation at 298 K of the idealized compound Mo₅Si₃C is estimated as -40.2 kJ/mol.     

Role of Si on the Diffusional Interactions Between U-Mo and Al-Si Alloys at 823?K (550?��C)

U-Mo dispersions in Al-alloy matrix and monolithic fuels encased in Al-alloy are under development to fulfill the requirements for research and test reactors to use low-enriched molybdenum stabilized uranium alloy fuels. Significant interaction takes place between the U-Mo fuel and Al during manufacturing and in-reactor irradiation. The interaction products are Al-rich phases with physical and thermal characteristics that adversely affect fuel performance and result in premature failure. Detailed analysis of the interdiffusion and microstructural development of this system was carried through diffusion couples consisting of U-7 wt pct Mo, U-10 wt pct Mo and U-12 wt pct Mo in contact with pure Al, Al-2 wt pct Si, and Al-5 wt pct Si, annealed at 823 K (550 °C) for 1, 5 and 20 hours. Scanning electron microscopy and transmission electron microscopy were employed for the analysis. Diffusion couples consisting of U-Mo in contact with pure Al contained UAl₃, UAl₄, U₆Mo₄Al₄₃, and UMo₂Al₂₀ phases. Additions of Si to the Al significantly reduced the thickness of the interdiffusion zone. The interdiffusion zones developed Al- and Si-enriched regions, whose locations and size depended on the Si and Mo concentrations in the terminal alloys. In these couples, the (U,Mo)(Al,Si)₃ phase was observed throughout the interdiffusion zone, and the U₆Mo₄Al₄₃ and UMo₂Al₂₀ phases were observed only where the Si concentrations were low.     

Effect of composition of starting materials of Mo–Si on self-propagating high temperature SHS reaction

Abstract

An experimental study of the self-propagating high temperature synthesis of Mo–Si alloys was conducted from elemental powder compacts. Test specimens with seven compositions, including Mo/Si?=?1∶1·25, 1∶1·50, 1∶1·75, 1∶2·00, 1∶2·25, 1∶2·50 and 1∶2·75 respectively, were employed. Experimental evidence showed that a combustion wave featuring a spinning reaction zone can be observed. When the powder compacts are from Mo∶1·25Si to Mo∶1·75Si, the combustion temperature and the propagation velocity of combustion wave increase with increasing silicon in the sample, and the combustion products are composed of MoSi₂, Mo₅Si₃ and Mo. However, when the powder compacts are from Mo∶2·25Si to Mo∶2·75Si, the combustion temperature and the propagation velocity decrease rapidly as the silicon in the compact increases, and the combustion products are composed of MoSi₂ and Si. The sample with Mo/Si?=?1∶2·00 possesses the highest combustion temperature (1628·9 K) and propagation velocity (3·13 mm s^?1). A single-phase MoSi₂ is synthesised by the Mo/Si?=?1∶2·00 sample.     

Ternary alloying study of MoSi2

Ternary alloying of MoSi₂ with adding a series of transition elements was investigated by X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Iron, Co, Ni, Cr, V, Ti, and Nb were chosen as alloying elements according to the AB₂ structure map or the atomic size factor. The studied MoSi₂ base alloys were prepared by the arc melting process from high-purity metals. The EDS analysis showed that Fe, Co, and Ni have no solid solubility in as-cast MoSi₂, while Cr, V, Ti, and Nb exhibit limited solid solubilities, which were determined to be 1.4±0.7, 1.4±0.4, 0.4±0.1, and 0.8±0.1. Micro-structural characterization indicated that Mo-Si-M_VIII (M_VIII=Fe, Co, Ni) and Mo-Si-Cr alloys have a two-phase as-cast microstructure, i.e., MoSi₂ matrix and the second-phase FeSi₂, CoSi, NiSi₂, and CrSi₂, respectively. In as-cast Mo-Si-V, Mo-Si-Ti, and Mo-Si-Nb alloys, besides MoSi₂ and C40 phases, the third phases were observed, which have been identified to be (Mo, V)₅Si₃, TiSi₂, and (Mo, Nb)₅Si₃.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 3. Stability of Silicide Coatings on Niobium Heated in Air at 1500-1800°C

The kinetics of phase redistribution in the (Mo, W)Si₂ Nb system at 1500-1800°C was investigated. The kinetic parameters for growth of the lower silicides (Mo, W, Nb) ₅Si₃ + Nb₅Si₃ and decrease in the layer thickness of the higher silicide (Mo, W)Si₂ as function of the oxidation temperature were determined. It was established that the stability of the multiphase and multicomponent system was more than twice that of the system MoSi₂ Nb, and 15-18 times that of MoSi₂ Mo.     

Interdiffusion studies between a Mo-based alloy and Ti

Interdiffusion between Ti and a Mo alloy (TZM) has been studied in the temperature range of 1273 to 1373 K. Boltzmann-Matano and Hall’s methods were used to evaluate the interdiffusion coefficients. The microstructure of the TZM/Ti interface showed excellent contact for all specimens bonded above 1273 K. The reaction index (n) evaluated by layer growth kinetics was close to 2. Interdiffusion coefficients increase with the increase in Ti concentration, and a quadratic relationship could be determined between the activation energy and concentration of Ti. Impurity diffusion coefficients of Mo in Ti and the activation energy for diffusion of Mo in Ti were evaluated. A linear dependence between the activation energy (Q) and pre-exponential factor (D ₀) for interdiffusion could be established. The diffusion parameters for TZM/Ti in the present studies were also compared with the earlier work on the Mo/Ti system and there seems to be a good correlation between these two studies.     

Solid-State diffusion reaction and formation of intermetallic compounds in the nickel-zirconium system

Chemical diffusion studies in the nickel-zirconium system are investigated in the temperature range of 1046 to 1213 K employing diffusion couples of pure nickel and pure zirconium. Electron microprobe and X-ray diffraction studies have been employed to investigate the formation of different compounds and to study their layer growth kinetics in the diffusion zone. It is observed that growth of each phase is controlled by the process of volume diffusion as the layer growth obeys the parabolic law. The activation energies for interdiffusion in NiZr and NiZr₂, which are the dominant phases in the diffusion zone, are 119.0 ±13.4 and 103.0 ±25.0 kJ/ mole, respectively. The formation and stability of compounds over the temperature range have been discussed on the basis of existing thermodynamic and kinetic data.     

Low-temperature oxidation of molybdenum disilicide

Cyclic oxidation rates of 95 to 97 pct dense, powder-source molybdenum disilicide (MoSi₂) in dry air, wet air, and oxygen have been measured between 400°C and 600°C. Dense MoSi₂ does not disintegrate catastrophically (pest) in these atmospheres for exposure times up to 688 hours. Between 400°C and 500°C, Mo and Si oxidize simultaneously to form amorphous SiO₂, monoclinic Mo₉O₂₆, and vapor-deposited MoO₃ plates, and the oxidation rate of MoSi₂ in air is influenced by its microstructure, composition, and surface defects. Rapid oxidation obeying a linear rate law occurs over a narrow temperature range near 500°C, where Mo vapor transport by (MoO₂) _n species is sufficiently rapid to produce large numbers of surface MoO₃ plates but simultaneously is slow enough to allow nucleation and growth of solid Mo oxides in conjunction with SiO₂. Addition of water vapor to the oxidant stream at 500°C retards nucleation and growth of solid Mo oxides by formation of MoO₃·H₂O (g), which has a high vapor pressure relative to those of (MoO₃) _n species. The transition from nonselective oxidation to high-temperature selective oxidation of Si to form a protective SiO₂ layer occurs between 500°C and 550°C. Preoxidation of MoSi₂ at 1200°C creates a SiO₂ barrier layer which prevents further oxidation upon subsequent exposure at 500°C. The oxidation kinetics and microstructural observations support the model of MoSi₂ pest in which oxidation in pores and cracks is required for disintegration. Based on these results, low-temperature oxidation phenomena are not expected to restrict the use of MoSi₂ as a high-temperature material. P.J. MESCHTER, formerly Senior Scientist, McDonnell Douglas Research Laboratories, St. Louis, MO, is Materials Scientist, General Electric Company, Corporate Research and Development, Schenectady, NY 12301.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 2. Stability and Heat Resistance of Silicide Coatings on Tungsten and Molybdenum at 1500-2000°C

The kinetics of diffusion redistribution of phases within the system WSi₂ W on heating tungsten silicide in air in the temperature range 1500-2000°C is studied. The stability and heat resistance of silicide coatings on tungsten is mainly governed by the diffusion of silicon towards the interphase boundaries W W₅Si₃, W₅Si₃ WSi₂, and WSi₂ SiO₂, formation at them of diffusion barriers of lower silicide W₅Si₃, and also a protective SiO₂ film at the outer boundary of the silicide coating. It is established that the transition rate for the higher to the lower tungsten silicide WSi₂ W₅Si₃ is on average four times slower than the transition rate for MoSi₂ Mo₅Si₃. It is shown that an increase in silicon concentration in the WSi₂ surface layer stimulates formation of diffusion barrier compounds at interphase boundaries. This leads to an increase in the stability of the phase composition and heat resistance of a silicide coating on metals. In particular at 1700°C the transition rate for molybdenum silicide on tungsten MoSi₂ (Mo, W)₅Si₃ is about twenty times slower than the transition rate for MoSi₂ Mo₅Si₃, and less by a factor of about eleven than the transition rate for WSi₂ W₅Si₃. Here there is also an increase in the heat resistance of silicide coatings on tungsten and molybdenum. It is shown that the SiO₂ film on tungsten silicide does not lose its protective properties up to 2000°C.     

Preparation of Mo–Si–B nanocomposite powders by mechanical alloying and heat treating

Abstract

In this study, an attempt has been made to synthesis Mo–Si–B nanocomposite alloys using a combination of mechanical alloying (MA) and heat treatment in various primary elemental compositions. For this purpose, Mo–14Si–10B, Mo–57Si–10B and Mo–47Si–23B (at.‐%) elemental powders were separately milled using an attritor mill. Mechanically alloyed (MAed) powders were annealed in an atmosphere controlled furnace under constant temperature for 10?h. Metallurgical characteristics of MAed and/or annealed powders were evaluated by atomic absorption spectrometry, SEM, TEM and X‐ray diffraction. The results did not show any formation of related intermetallics after MA. However, MoSi₂, Mo₅Si₃, Mo₅SiB₂, MoB and Mo were successfully formed, when the MAed Mo–57Si–10B powders were subjected to annealing at a high temperature.     

Phase Constituents and Microstructure of Interaction Layer Formed in U-Mo Alloys <Emphasis Type="Italic">vs</Emphasis> Al Diffusion Couples Annealed at 873 K (600 °C)

U-Mo dispersion and monolithic fuels are being developed to fulfill the requirements for research reactors, under the Reduced Enrichment for Research and Test Reactors program. In dispersion fuels, particles of U-Mo alloys are embedded in the Al-alloy matrix, while in monolithic fuels, U-Mo monoliths are roll bonded to the Al-alloy matrix. In this study, interdiffusion and microstructural development in the solid-to-solid diffusion couples, namely, U-15.7 at. pct Mo (7 wt pct Mo) vs pure Al, U-21.6 at. pct Mo (10 wt pct Mo) vs pure Al, and U-25.3 at. pct Mo (12 wt pct Mo) vs pure Al, annealed at 873 K (600 °C) for 24 hours, were examined in detail. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA) were employed to examine the development of a very fine multiphase interaction layer with an approximately constant average composition of 80 at. pct Al. Extensive TEM was carried out to identify the constituent phases across the interaction layer based on selected area electron diffraction and convergent beam electron diffraction (CBED). The cubic-UAl₃, orthorhombic-UAl₄, hexagonal-U₆Mo₄Al₄₃, and cubic-UMo₂Al₂₀ phases were identified within the interaction layer that included two- and three-phase layers. Residual stress from large differences in molar volume, evidenced by vertical cracks within the interaction layer, high Al mobility, Mo supersaturation, and partitioning toward equilibrium in the interdiffusion zone were employed to describe the complex microstructure and phase constituents observed. A mechanism by compositional modification of the Al alloy is explored to mitigate the development of the U₆Mo₄Al₄₃ phase, which exhibits poor irradiation behavior that includes void formation and swelling.     

Diffusion of silicon in aluminum

Interdiffusion coefficients in Al-Si alloys were determined by Matano’s method in the tem-perature range from 753 to 893 K with the couple consisting of pure aluminum and an Al-Si alloy. Temperature dependence of the impurity diffusion coefficients of Si in Al, obtained by extrapolation of the concentration dependence of the interdiffusion coeffi-cient to zero mole fraction of Si, is given by the following equation: DSi/Al = (2.02^+0.97 _-0.66 × 10^-4 exp [-(136 ±3) kJ mol^-1/RT] m²/s. p ] The Kirkendall marker was found to move toward the Si-rich side, indicating that the Si atom diffuses faster than the Al atom in Al-Si alloys. From the interdiffusion coeffi-cient and the marker shift, the intrinsic diffusion coefficients were calculated. The difference in the activation energies (ΔQ) between the impurity diffusion of Si in Al and the self-diffusion of Al was estimated by means of the asymptotic oscillating po-tential and the Le Claire theory. The calculated value of ΔQ is in fair agreement with the experimental value. The vacancy-solute binding energy for Si in Al was also dis-cussed based on the diffusion data. formerly Undergraduate Student, Tohoku University     

Kinetic analysis of the combustion synthesis of molybdenum and titanium silicides

The temperature profiles associated with the passage of self-propagating combustion waves during the synthesis of MoSi₂ and Ti₅Si₃ were determined. From these profiles, kinetic analyses of the combustion synthesis process for these two silicides were made. The synthesis is associated with high heating rates: 1.3 × 104 and 4.9 × 104 K·s₋₁ for MoSi₂ and Ti₅Si₃, respectively. The width of the combustion zone was determined as 1.3 and 1.8 mm for the silicides of Mo and Ti, respectively. The degree of conversion, η, and its spatial distribution and the conversion rate, ∂η/∂t, were determined. However, because of the inherent characteristics of wave propagation in MoSi₂, only in the case of Ti₅Si₃ could the activation energy be calculated. An average value of 190 kJ μ mol₋1 was determined for titanium suicide.     

On the creep of directionally solidified MoSi2-Mo5Si3 eutectics

The high temperature deformation behaviour of MoSi₂-Mo₅Si₃ eutectics has been investigated as a function of lamellar spacing over the temperature range 1100–1400°C and strain rates (∈) of 1 × 10⁻⁴ to 1 × 10⁻⁶ s⁻¹. Specimens with lamellar morphologies were produced by directional solidification using the Czochralski method at pull rates of 25–210 mm/h giving lamellar spacings (λ) of 2.6 to 1.09 μm. The measured flow stress was found to increase as the lamellar spacing decreased for a given strain rate. A constitutive model for creep that incorporates reinforcement spacing for creeping fibers in a creeping matrix was found to describe the creep behaviour of the eutectic, i.e. ∈αλ withm = 1. Creep deformation of the eutectic was controlled by ½〈110〉 (001) partial dislocations in the Mo₅Si₃ phase. The creep behaviour of a [314] oriented Mo₅Si₃ single crystal was also investigated.     

Interdiffusion in the Mg-Al System and Intrinsic Diffusion in β-Mg2Al3

Solid-to-solid diffusion couples were assembled and annealed to examine the diffusion between pure Mg (99.96?pct) and Al (99.999?pct). Diffusion anneals were carried out at 573?K, 623?K and 673?K (300?°C, 350?°C and 400?°C) for 720, 360, and 240?hours, respectively. Optical and scanning electron microscopes were used to identify the formation of the intermetallic phases, ??-Mg₁₇Al₁₂, and ??-Mg₂Al₃, as well as the absence of the ??-Mg₂₃Al₃₀ in the diffusion couples. The thicknesses of the ??-Mg₁₇Al₁₂ and ??-Mg₂Al₃ phases were measured and the parabolic growth constants were calculated to determine the activation energies for growth. Concentration profiles were determined with electron microprobe analysis using pure elemental standards. Composition-dependent interdiffusion coefficients in Mg-solid solution, ??-Mg₁₇Al₁₂, ??-Mg₂Al₃, and Al-solid solutions were calculated based on the Boltzmann-Matano analysis. Integrated and average effective interdiffusion coefficients for each phase were also calculated, and the magnitude was the highest for the ??-Mg₂Al₃ phase, followed by ??-Mg₁₇Al₁₂, Al-solid solution, and Mg-solid solution. Intrinsic diffusion coefficients based on Huemann??s analysis (e.g., marker plane) were determined for the ~ Mg-62 at. pct Al in the ??-Mg₂Al₃ phase. Activation energies and the pre-exponential factors for the interdiffusion and intrinsic diffusion coefficients were calculated for the temperature range examined. The ??-Mg₂Al₃ phase was found to have the lowest activation energies for growth and interdiffusion among all four phases studied. At the marker location in the ??-Mg₂Al₃ phase, the intrinsic diffusion of Al was found to be faster than that of Mg. Extrapolations of the impurity diffusion coefficients in the terminal solid solutions were made and compared with the available self-diffusion and impurity diffusion data from the literature. Thermodynamic factor, tracer diffusion coefficients, and atomic mobilities at the marker plane composition were approximated using the available literature values of Mg activity in the ??-Mg₂Al₃ phase.     

On the fracture and fatigue properties of Mo-Mo<Subscript>3</Subscript>Si-Mo<Subscript>5</Subscript>SiB<Subscript>2</Subscript> refractory intermetallic alloys at ambient to elevated temperatures (25 °C to 1300 °C)

The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above ∼1000 °C has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation (“pesting”), fracture toughness, and fatigue-crack propagation resistance of four such alloys, which consisted of ∼21 to 38 vol. pct α-Mo phase in an intermetallic matrix of Mo₃Si and Mo₅SiB₂ (T₂), were characterized at temperatures between 25 °C and 1300 °C. The boron additions were found to confer improved “pest” resistance (at 400 °C to 900 °C) as compared to unmodified molybdenum silicides, such as Mo₅Si₃. Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi₂, for the I/M processed microstructures with coarse distributions of the α-Mo phase, fracture toughness properties were far superior, rising from values above 7 MPa √m at ambient temperatures to almost 12 MPa √m at 1300 °C. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi₂, with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPa √m at 25 °C to ∼8 MPa √m at 1300 °C. These results, in particular, that the toughness and cyclic crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of toughening in Mo-Si-B alloys and the specific role of microstructure.     

Interdiffusion in the TiO2 oxidation product of Ti3Al

Oxidation experiments have been conducted on Ti₃Al in the temperature range 1023 to 1273 K in a one-atmosphere pure oxygen environment. The oxidation products were analyzed using the scanning electron microscope (SEM), energy-dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques and found to be predominantly TiO₂ (rutile). The oxidation rate was observed to obey the parabolic rate law. Diffusivity data were obtained using the parabolic rate constant for interdiffusion of Ti and O in the oxide layer. Parabolic rate constants were calculated from oxidation rate data, and Valenci equations for flat sheets were used to calculate diffusion coefficients. The activation energy, Q, was found to be 295.43±5.90 kJ/mol, and the frequency factor, D ₀, was calculated to be 0.68±0.01 m²/s for oxygen in the TiO₂. The activation energy obtained in this study matches closely with that of oxygen diffusion in TiO₂ reported in the literature.     

Effect of Oxygen Partial Pressure on the Cyclic Oxidation Behavior of Mo76Si14B10

Hot-pressed and arc-melted Mo₇₆Si₁₄B₁₀ (at. pct) exhibits α-Mo solid solution, Mo₃Si, and Mo₅SiB₂ in microstructures with varying morphologies. Cyclic oxidation tests performed at oxygen partial pressures of 0.21 and 1 atm show the mass loss of the hot-pressed alloy to be ≈1.5 and ≈4 times less, respectively, than that of the arc-melted alloy. The thickness of the protective silicate layer increases with an increase of both Mo_ss grain size and oxygen partial pressure in the environment.