Oxide scale formation and isothermal oxidation behavior of Mo–Si–B intermetallics at 600–1000°C

Initial scale formation in the range 600–1000°C and isothermal oxidation behavior at 1000°C was investigated for Mo–Si–B intermetallics containing 81–88 wt% molybdenum. All compositions exhibited an initial transient oxidation period consisting of a mass gain due to MoO₃ and SiO₂ formation, followed by a rapid mass loss starting at 750°C due to MoO₃ volatilization. After the initial transient oxidation period, oxidation proceeded at a much slower rate. During isothermal oxidation at 1000°C the oxidation rate was found to vary inversely with the ratio of B/Si in the intermetallic, indicating that viscous flow of the scale was an important factor in determining the isothermal oxidation rate at 1000°C.     

Microstructure and creep properties of a near-eutectic directionally solidified multiphase Mo–Si–B alloy

Due to their excellent creep behavior and acceptable oxidation resistance at ultrahigh temperatures multiphase Mo-based alloys are potential candidates for applications in aerospace engines and the power generating industry. The resulting materials properties, as well as the microstructure of Mo–Si–B materials, strongly depend on the manufacturing process. In the following paper we report on a new Mo–Si–B alloy which was processed by crucible-free zone melting (ZM) from cold pressed elemental powders. SEM investigations of the zone molten microstructure showed well-aligned arrangements of a three-phase microstructure consisting of a Mo solid solution (Mo_SS), and the two intermetallic phases Mo₃Si and Mo₅SiB₂. First, high temperature mechanical properties, such as the compressive strength and creep strength at about 1100 °C, were evaluated and compared with a commonly used Ni-based superalloy and a PM processed Mo–Si–B material. In comparison to the PM processed reference alloy, the creep resistance of ZM materials was found to be substantially improved due to the relatively coarse directionally solidified microstructure. Thus, ZM alloys show great potential for applications at targeted application temperatures of around 1200–1300 °C.     

Environmental Resistance of Mo–Si–B Alloys and Coatings

For high temperature application beyond the range of Ni-base superalloys, multiphase Mo–Si–B alloys with compositions, that yield the ternary intermetallic Mo₅SiB₂ (T₂) phase as a key microstructure constituent together with the Mo and Mo₃Si phases, offer an attractive balance of high melting temperature, oxidation resistance and mechanical properties. The investigation of reaction kinetics involving the T₂ phase enables the analysis of oxidation in terms of diffusion pathways and the design of effective coatings. From this basis kinetic biasing is used together with pack cementation to develop multilayered coatings and in situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. While a combustion environment contains water vapor that can accelerate attack of silica based coatings, the current pack cementation coatings provide oxidation resistance in water vapor up to at least 1500 °C. An exposure to hot ionized gas species generated in an arc jet confirms the robust coating performance in extreme environments.     

Influence of zirconium content on microstructure and creep properties of Mo–9Si–8B alloys

Mo–Si–B alloys with a molybdenum solid solution accompanied by two intermetallic phases and Mo₅SiB₂ are a prominent example for a potential new high temperature structural material. In this study the influence of 1, 2 and 4 at.% zirconium on microstructure and creep properties of Mo–9Si–8B (at.%) alloys produced by spark plasma sintering is investigated. Creep experiments have been carried out at temperatures of 1100 °C up to 1250 °C in vacuum. The samples exhibit sub-micron grain sizes as small as 450 nm due to the chosen production route. With addition of 1 at.% zirconium, formation of SiO₂ on the grain boundaries can be prevented, thereby enhancing grain boundary strength and creep properties significantly. Moreover ZrO₂ particles also enhance creep resistance of the molybdenum solid solution. Creep deformation is a combination of dislocation creep in the grains including dislocation-particle interaction and grain boundary sliding leading to intergranular fracture surfaces. It is promising to use grain size adjustments in order to balance the creep and oxidation resistance of the investigated material.     

Mechanical and wear properties of Mo5Si3–Mo3Si–Al2O3 composites

Mo₅Si₃ and Mo₅Si₃–Mo₃Si–Al₂O₃ composite were synthesized use MoO₃, Mo, Si and Al as raw materials by mechanically induced self propagating reaction and then consolidated by hot-pressing. The microstructure of the materials was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with X-ray energy dispersive spectroscopy (EDS). The effects of the Al₂O₃ on the mechanical and tribological properties of Mo₅Si₃–Mo₃Si–Al₂O₃ composite have been studied. It was found that benefits associated with the addition of the Al₂O₃ to Mo₅Si₃ and Mo₃Si include finer microstructure, higher strength, higher fracture toughness and higher hardness. The dry sliding wear properties of the composite were investigated using against GCr15 bearing steel in ball-on-disk system at room temperature. The results indicated that the friction coefficients and specific wear rates of Mo₅Si₃–Mo₃Si–Al₂O₃ composite were significantly reduced by the addition of Al₂O₃, and its specific wear rates decreased by an order of magnitude compare with the monophase Mo₅Si₃. The friction coefficients of test materials decrease with an increasing load. The dominant wear mechanism of the composites was interpreted by several different wear models involving plastic deformation, adhesion, brittle fracture and reaction to form a tribo-oxidation layer.     

Tensile creep of Mo–Si–B alloys

Multiphase Mo–Si–B alloys containing a Mo solid solution matrix and brittle Mo₃Si and Mo₅SiB₂ (T2) intermetallic phases are candidates for ultra-high-temperature applications_. The elevated temperature uniaxial tensile response at a nominal strain rate of 10^?4 s^–1 and the tensile creep response at constant load between 1000 °C and 1300 °C of a (i) single phase solid solution (Mo–3.0Si–1.3B in at.%), (ii) two-phase alloy containing ～35 vol.% T2 phase (Mo–6Si–8B in at.%) and (iii) three-phase alloy with ～50 vol.% T2 + Mo₃Si phases (Mo–8.6Si–8.7B in at.%) were evaluated. The results confirm that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. A Larson–Miller plot of the creep data showed improved creep resistance of the two- and three-phase alloys in comparison with Ni-based superalloys. The extent of Si dissolved in the solid solution phase varied in these three alloys and Si appeared to segregate to dislocations and grain boundaries. A stress exponent of ～5 for the solid solution alloy and ～7 at 1200 °C for the two multiphase alloys suggested dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation was observed and the precipitation kinetics appear to be affected by the test temperature and applied stress.     

Correlation between microstructure and properties of fine grained Mo–Mo3Si–Mo5SiB2 alloys

Three phase α-Mo–Mo₃Si–Mo₅SiB₂ alloys of various compositions, namely Mo–6Si–5B, Mo–9Si–8B, Mo–10Si–10B and Mo–13Si–12B (at.%) were processed by a powder metallurgical (PM) route. Increasing the Si and B concentration in these Mo–Si–B alloys resulted in increasing volume fractions of the intermetallic phases Mo₃Si (A15) and Mo₅SiB₂ (T2) and the distribution of the three phases present in these alloys was dependent on the volume fractions of the individual phases. Above volume fractions of about fifty percent, bcc Mo solid solution (α-Mo) formed the matrix. Consequently, Mo–6Si–5B and Mo–9Si–8B alloys, which possessed a continuous α-Mo matrix provided increased fracture toughness at ambient temperatures. Additionally, a decreased BDTT of about 950 °C is caused by the homogeneous α-Mo matrix. In contrast, Mo–13Si–12B with 65 vol.% of the intermetallic phases that formed the matrix phase in this material had a BDTT value higher than 1100 °C, while the strength at elevated temperatures up to 1300 °C was significantly increased compared to alloys that have the α-Mo matrix. Alloy compositions with ≥50 vol.% of intermetallic phases (corresponding to alloys containing a minimum of 9 at.% Si and 8 at.% B) were oxidation resistant with minimal mass loss under cyclic conditions for 150 h at 1100 °C due to the formation of a dense borosilicate glass layer that protects the material surface.     

Modeling the effects of microstructure on the tensile properties and micro-fracture behavior of Mo–Si–B alloys at elevated temperatures

A computational framework is developed to study the role of microstructure on the deformation behavior of Mo–Si–B alloys. A parametric range of idealized multi-phase microstructures of Mo–Si–B alloys are instantiated in 2D using Voronoi tessellation schemes and their deformation behavior modeled with the use of the finite element method. Continuum elements are used to model the constituent phases, while cohesive elements are used to model debonding at the interfaces of the intermetallic (A15 and T2) phases with the solid solution-strengthened Mo_ss matrix and cleavage fracture within the intermetallic phases. The deformation behavior of Mo–Si–B alloys is studied in terms of the simulated stress-strain response and microstructure evolution characteristics. Effects of various microstructure parameters, such as composition and clustering of intermetallic phases, on the tensile strength and ductility are also studied.     

Fracture and fatigue resistance of Mo–Si–B alloys for ultrahigh-temperature structural applications

Fracture and fatigue properties are examined for a series of Mo–Mo₃Si–Mo₅SiB₂ alloys, which utilize a continuous α-Mo matrix to achieve unprecedented room-temperature fracture resistance (>20 MPa√m). Mechanistically, these properties are explained in terms of toughening by crack trapping and crack bridging by the more ductile α-Mo phase.     

Phase stability and structural defects in high-temperature Mo–Si–B alloys

For high-temperature application beyond the range of Ni-base superalloys, Mo–Si–B alloys with composition that yield the ternary intermetallic, Mo₅SiB₂, T₂, phase as a key microstructure constituent, offer an attractive property balance of high-melting temperature, oxidation resistance, and useful high-temperature mechanical properties. With the T₂ ternary phase as the focal point of the microstructure designs, the fundamental basis of the alloying behavior in T₂ including the mutual solid solution with a wide range of transition metals has been established in terms of the governing geometric and electronic factors. For non-stoichiometric compositions, constitutional defects such as vacancies for Mo-rich compositions and anti-site defects for Mo-lean compositions control the homogeneity range. Moreover, the aggregation of constitutional vacancies has been discovered to play a key role in the development of dislocation and precipitation reactions in the T₂ phase that directly impact high-temperature structural performance. The characteristically sluggish diffusion rates within the T₂ phase have also been quantified and applied to the materials processing strategies. The materials design based on the phase stability, diffusion and defect structure analysis in the Mo–Si–B system can also be applied to the design of new multiphase high-temperature alloys with balanced environmental and mechanical properties.     

Viscosity and fragility of the supercooled liquids and melts from the Fe–Co–B–Si–Nb and Fe–Mo–P–C–B–Si glass-forming alloy systems

The dynamic viscosity of four Fe-based bulk metallic glass-forming alloys, [(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]₉₆Nb₄ (alloy A), {[(Fe_0.5Co_0.5)_0.75B_0.2Si_0.05]_0.96Nb_0.04}_99.5Cu_0.5 (alloy B), Fe₇₄Mo₄P₁₀C_7.5B_2.5Si₂ (alloy C) and (Fe_0.9Ni_0.1)₇₇Mo₅P₉C_7.5B_1.5 (alloy D), was investigated as a function of temperature in the supercooled liquid region, as well as above the melting point. The alloy B is Cu-added alloy A, while the alloy D was obtained upon fine-tuning the alloy C composition. All these alloys may form bulk metallic glasses upon copper mold casting. The viscosities in the supercooled liquid region were calculated using the data obtained upon parallel plate rheometry measurements, as well as upon differential scanning calorimetry (DSC). The values of the supercooled fragility parameter m, 61, 66, 52 and 60 for the alloys A, B, C and D respectively, indicate that these alloys are intermediate glass formers. The behavior of the same alloys, in the molten state, was studied using a high temperature torsional oscillation cup viscometer. The values of the corresponding fragility parameter M was calculated as 5.03, 5.91, 4.25, 4.93 for the alloys A, B, C and D, respectively. They confirm the supercooled liquid behavior and predict that the alloys A and C may form glasses easier than the fine-tuned compositions B and D. Angell plot is constructed for the entire range of viscosities and the values from both regions, i.e. above melting point and supercooled liquid region, fit well with the model.     

Microstructural evolution of mechanically alloyed Mo–Si–B–Zr–Y powders

Elemental powder mixtures with compositions of Mo–13.8Si, Mo–20B and Mo–12Si–10B–3Zr–0.3Y (at.%) were respectively milled in a high energy planetary ball mill at a speed of 500 rpm. Microstructural evolution of powder particles during milling processes was evaluated. The results show that B can hardly be dissolved into Mo under present milling conditions and the additions of B and Si both accelerate the refining rate of Mo crystallites. For Mo–12Si–10B–3Zr–0.3Y system, the morphology and internal structure of powder particles change significantly with milling time. After 40 h of milling, an almost strain-free super-saturated molybdenum solid solution with a grain size of about 6.5 nm forms. The grain refinement mechanism and dissolution kinetics of solute atoms are highlighted. Both thermodynamic calculation and experimental results reveal that for the present alloy composition it is more favorable to form solid solution than amorphous phase.     

Processing and mechanical properties of a molybdenum silicide with the composition Mo–12Si–8.5B (at.%)

Alloys with the nominal composition Mo–12Si–8.5B (at.%) were prepared by arc-melting or powder-metallurgical processing. Cast and annealed alloys consisted of approximately 38 vol.% α-Mo in a brittle matrix of 32 vol.% Mo₃Si and 30 vol.% Mo₅SiB₂. Their flexure strengths were approximately 500 MPa at room temperature, and 400–500 MPa at 1200°C in air. The fracture toughness values determined from the three-point fracture of chevron-notched specimens were about 10 MPa m^1/2 at room temperature and 20 MPa m^1/2 at 1200°C in air. The relatively high room temperature toughness is consistent with the deformation of the α-Mo particles observed on fracture surfaces. Three-point flexure tests at 1200°C in air and a tensile test at 1520°C in nitrogen indicated a small amount of high temperature plasticity. Extrusion experiments to modify the microstructure of cast alloys were unsuccessful due to extensive cracking. However, using powder-metallurgical (PM) techniques, microstructures consisting of Mo₃Si and Mo₅SiB₂ particles in a continuous α-Mo matrix were fabricated. The room temperature fracture toughnesss of the PM materials was on the order of 15 MPa m^1/2.     

Microstructure and oxidation resistance of Mo–Si–B coating on Nb based in situ composites

The development of robust high temperature oxidation resistant coatings for Nb–Si based alloy was evaluated for a Mo–Si–B coating system that was applied by a two step process. It is observed that the coating is composed of an outer layer of MoSi₂ containing boride dispersoids and an inner layer of unreacted Mo. The mass gain of substrate and Mo–Si–B coating is 190.08 and 1.28 mg cm⁻² after oxidation at 1250 °C in dry air for 100 h, respectively. The good oxidation resistance of the coating is attributed to the formation of a continuous borosilicate glass coverage.     

Structure and mechanical properties of as-cast Ti–Si alloys

In the present work, the microstructure and mechanical properties of as-cast Ti–Si alloys with a Si content ranging from 1 to 12.5 wt% prepared using a dental cast machine were investigated and compared with commercially pure titanium (c.p. Ti). X-ray diffraction (XRD) for phase analysis was conducted using a diffractometer. Three-point bending tests were performed to evaluate the mechanical properties of all specimens and their microstructure and fractured surfaces were observed using scanning electron microscopy (SEM). Experimental results indicated that the diffraction peaks of the Ti–Si alloys matched those of α-Ti and Ti₅Si₃. All the Ti–Si alloys had higher bending strengths and bending moduli than those of c.p. Ti. For example, the bending strength of Ti–5Si was about 2.6 times that of c.p. Ti, and both Ti–10Si and Ti–12.5Si had the highest bending moduli, which were about 1.8 times higher than that of c.p. Ti. Additionally, Ti–1Si exhibited ductile properties and Ti–3Si and Ti–5Si had a combination of brittleness and ductility. When the Si content was 7.5 wt% or greater, the alloys showed brittle properties. Judging from the results of the mechanical properties and deformation behavior, Ti–1Si, Ti–3Si, and Ti–5Si can be considered highly feasible alloys for prosthetic dental applications if other properties necessary for dental casting are obtained.     

Microstructure and properties of Cu–2.8Ni–0.6Si alloy

The phase transformation behavior and heat treatment response of Cu-2.8Ni-0.6Si (wt%) alloy subjected to different heat treatments were studied by X-ray diffraction, transmission electron microscopy observation, and measurement of hardness and electrical conductivity. The variation of hardness and electrical conductivity of the alloy was measured as a function of aging time. On aging at the temperature below TR (500-550°C) in Cu-2.8Ni-0.6Si alloy, the transformation undergoes spinodal decomposition, DO22 ordering, and d-Ni2Si phase. On aging at the temperature above TR (500-550 °C), the transformation products were precipitations of d-Ni2Si. The free energy versus composition curves were employed to explain the microstructure observations.     

Oxidation-protective and mechanical properties of SiC nanowire-toughened Si–Mo–Cr composite coating for C/C composites

Oxidation-protective SiC nanowire-toughened Si–Mo–Cr composite coating prepared on the carbon/carbon (C/C) composites by chemical vapor deposition and pack cementation was investigated in this study. After incorporating SiC nanowires, the hardness, elastic modulus and fracture toughness of the composite coating were increased by 6.12%, 20.89% and 35.78%, respectively, due to the toughening and strengthening mechanisms including nanowire pullout, nanowire bridging, microcrack deflection and good interaction between nanowire/matrix interface. Thermogravimetric analysis revealed that the maximum weight loss of the coated C/C samples was decreased from 5.87% to 3.93% by incorporating SiC nanowires from room temperature to 1500 °C.     

Microstructure and mechanical properties of hot pressed Mo–Cr–Si–Ti in-situ composite,and oxidation behavior with silicide coatings

The present study deals with the synthesis of Mo–16Cr–4Si–0.5Ti (wt.%) alloy by means of the reactive hot pressing method. The microstructure of the synthesized alloy consisted of (Mo, Cr, Ti)₃Si, and the discontinuous α-(Mo, Cr, Ti)_SS phases. The isothermal oxidation behavior of the alloy was investigated in air at 1273 K for 50 h. The alloy exhibited superior oxidation behavior in comparison with single phase molybdenum alloys, because of the formation of SiO₂ and Cr₂O₃ over the alloy surface. The flexural strength determined from three-point bend testing of single edge notch bend specimens was 615 ± 15 MPa. The dominant mechanism of fracture was identified as transgranular mode of crack propagation. To extend the life of the alloy under oxidizing atmosphere, silicide based oxidation resistant coatings were developed, using halide activated pack cementation process. The kinetic behavior of growth of the coating was established and the activation energy of the coating process was determined to be 52.5 kJ/mol. Isothermal oxidation tests of the coated alloy at 1273 K for 50 h, revealed a small weight gain at the initial stages of oxidation followed by no change of weight, indicating the protective nature of the coating.