Establishing multilayered coating on Mo–12Si–8.5 B alloy by Si pack cementation and its oxidation behavior at 900°C–1300°C

A multilayered oxidation protection coating consisting of MoSi₂ outer layer, Mo₅Si₃ internal layer, and Mo₅SiB₂/MoB inner layer was developed on the surface of Mo–12Si–8.5B 1.0 wt% ZrB₂ alloy via Si pack cementation. The multilayered coating significantly enhanced the oxidation resistance of the alloy at 900°C, 1100°C, and 1300°C in the air by exhibiting negligible oxidation recession. MoSi₂ outer layer provided admirable oxidation protection for the alloy at high temperatures by forming a thin and protective SiO₂-rich glass scale on its surface. This was supplemented by the Mo₅Si₃ internal layer and Mo₅SiB₂/MoB inner layer that reduced the thermal expansion mismatch between the MoSi₂ outer layer and substrate, and therefore no obvious cracks were found in the MoSi₂ outer layer. More importantly, the Mo₅SiB₂/MoB layer as an in situ barriers of Si interdiffusion ensured the stable existence of MoSi₂ and Mo₅Si₃ layers without obvious thickness change during oxidation at 900°C and 1100°C. Mechanical property test indicated that the formation of the coating layers could not affect the fracture toughness of the alloy.     

Improvement of oxidation resistance of arc-melted Mo76Si14B10 by microstructure control upon minor Fe addition

Addition of Fe refines the microstructure of Mo_76-xSi₁₄B₁₀Fe_x (x = 0, 0.5, 1 at.%) composites containing α-Mo, Mo₃Si and Mo₅SiB₂ phases, increases the hardness from 950 Hv (x = 0) to 1031 Hv (x = 1), and improves the oxidation resistance at temperature in the range of 800–1300 °C. The hardness of the base alloy substrate decreases only by <7% than that of as-solidified ingots, indicating good microstructural stability of the composite for high temperature application.     

Annealing response of point defects in off-stoichiometric Mo5SiB2 phase

Dislocations and planar faults are found to develop in the Mo₅SiB₂ phase during elevated temperature annealing. The dislocations formed in Mo₅SiB₂ of annealed alloys are mostly edge dislocations with the Burgers vectors of 〈100], 1/2〈111], and 〈110]. For the Mo-rich Mo₅SiB₂ phase that formed in a Mo–10Si–20B alloy, a significant amount of constitutional vacancies is introduced during solidification, and the excess vacancies aggregate and collapse into dislocations. At the same time, these dislocations can act as the heterogeneous nucleation sites for the subsequent Mo precipitation. For the Mo-lean Mo₅SiB₂ phase that formed in a Mo–13Si–28B alloy, the constitutional defects are not significant for the development of dislocations.     

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.     

Microstructure,high-temperature deformability and oxidation resistance of a Ti5Si3-containing multiphase MoSiBTiC alloy

A Ti₅Si₃-containing multiphase MoSiBTiC alloy with a composition of 38Mo–30Ti–17Si–10C–5B (at.%) was designed and produced by arc-melting. The alloy was composed of five phases—Mo solid solution (Mo_ss), Mo₃Si, Mo₅SiB₂ (T₂), Ti₅Si₃ and TiC—and consistently has good thermal stability at least up to 1700 °C. The density of the alloy was ∼7.0 g/cm³, which is considerably smaller than that of Ni-base superalloys. Microstructure was carefully examined and microstructural anisotropy was confirmed. The anisotropy was considered to be generated by thermal gradient during the solidification process. Microcracking was remarkable across the primary Ti₅Si₃ phase, which was caused by thermal expansion anisotropy of the Ti₅Si₃ phase. High-temperature deformability was examined by high-temperature compression tests at 1500 °C. Two kinds of loading axes were chosen for the compression tests with respect to the microstructural anisotropy. The alloy exhibited a peak stress of 450–550 MPa, followed by good deformability at the testing temperature. Microstructure refinement and reduction in microcrack density were observed after hot working. Oxidation tests were conducted on the alloy at 1100 °C and 1300 °C for 24 h. The oxidation curves demonstrated that rapid mass loss finished within several minutes. After that, the mass loss began to slow down and then the specimens' mass decreased almost linearly with increasing testing time. Cross-section observation indicated that oxygen propagated through Mo_ss, whereas T₂ and Ti₅Si₃ phases acted as barriers against oxygen attack during the tests. In addition, it was found that the alloy gained better oxidation resistance after high-temperature deformation, suggesting a positive effect of phase refinement on its high-temperature oxidation resistance.     

Comparative studies on densification behavior,microstructure, mechanical properties and oxidation resistance of Mo-12Si-10B and Mo3Si-free Mo-26Nb-12Si-10B alloys

Two bulk Mo-Si-B based alloys (Mo-12Si-10B and Mo-26Nb-12Si-10B (at.%), abbreviated as 0Nb and 26Nb alloy respectively) were fabricated by mechanical alloying and then hot pressing. Comparative studies were carried out on the densification behavior, microstructure, room-temperature fracture toughness, elevated temperature compression and oxidation resistance of these two alloys. The results showed that alloy 0Nb was composed of (Mo), Mo₃Si and Mo₅SiB₂, while alloy 26Nb was free of Mo₃Si and had higher (Mo) content and a little γNb₅Si₃. Compared to the alloy 0Nb, alloy 26Nb presented better compactibility, higher room-temperature fracture toughness (8.84 ± 0.17 vs. 6.77 ± 0.20 MPa·m^1/2) and elevated temperature compression strength (851.7 ± 11.7 vs. 644.2 ± 10.2 MPa) but worse oxidation resistance.     

Effect of Ti (Macro-) Alloying on the High-Temperature Oxidation Behavior of Ternary Mo–Si–B Alloys at 820–1,300 °C

The aim of the present investigation was to gain an initial understanding of the effect of (macro-) alloying with Ti on the oxidation behavior of Mo–Si–B alloys in the ternary phase region of Mo_ss–Mo₃Si–Mo₅SiB₂ at 820–1,300 °C. Motivated by recent studies and thermodynamic calculations, the alloy compositions Mo–9Si–8B–29Ti (at.%) and Mo–12.5Si–8.5B–27.5Ti (at.%) were selected and synthesized by arc-melting. Compared to the reference alloy Mo–9Si–8B, superior initial oxidation rates at 1,100–1,300 °C as well as a significant density reduction by nearly 18 % were observed. Due to enhanced initial evaporation of MoO₃ and mainly inward diffusion of oxygen, a borosilicate-rutile duplex scale with a continuous TiO₂ phase had formed. Detailed investigations of the oxidation mechanism by SEM, EDX, XRD and confocal micro-Raman spectroscopy indicated that Ti alloying is promising with regard to further improvement of the oxidation resistance as well as the strength-to-weight ratio of Mo–Si–B alloys.     

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 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.     

Oxidation behavior of magnetron sputtered double layer coatings containing molybdenum,silicon and boron

Protective coating systems were applied to Mo–9Si–8B (at.%) alloys to prevent oxidation at elevated temperatures. The coatings produced by magnetron sputtering and subsequent annealing consisted of an outer oxidation protection layer and an interlayer between this and the substrate. Three amorphous outer layers with different compositions were deposited: Mo–45Si–25B, Mo–55Si–10B and Mo–29Si–15B (all in at.%). The interlayer was selected to give a diffusion barrier with the composition of the Mo₅SiB₂ (T2) phase. All coatings were dense and well-adherent. During vacuum annealing the amorphous as-deposited coatings became crystalline exhibiting mainly the intermetallic Mo₅SiB₂ compound as interlayer and the MoSi₂, Mo₅Si₃ and MoB phases in the top layers. The samples were exposed to dry laboratory air in the pesting regime at 800 °C and above, i.e. at 1000 and 1300 °C for up to 100 h under cyclic conditions. All coatings were protective at 800 and 1000 °C for at least 100 h and showed a marked improvement in mass change compared to the uncoated substrate. For protection at 800 °C higher boron content is preferential, while at higher oxidation temperatures a lower boron content provides improved oxidation protection. At 1300 °C stress induced failures like cracking, spallation and buckling occurred due to the relatively high CTE mismatch between PVD coating and substrate. Even though, the mass change was still markedly reduced as compared to the bare substrate.     

Synthesis of α-Mo-Mo5SiB2-Mo3Si nanocomposite powders by two-step mechanical alloying and subsequent heat treatment

A two-step mechanical alloying process followed by heat treatment was developed as a novel approach for fabrication of Mo-12.5 mol%Si-25 mol%B nanocomposite powders. In this regard, a Si-43.62 wt.% B powder mixture was milled for 20 h. Then, Mo was added to the mechanically alloyed Si-B powders in order to achieve Mo-12.5 mol%Si-25 mol%B powder. This powder mixture was further milled for 2,5,10 and 20 h. All of the milled powders were annealed at 1100 °C for 1 h. After first step of milling, a nanocomposite structure composed of boron particles embedded in Si matrix was formed. On the other hand, an α-Mo/MoSi₂ nanocomposite was produced after second step while no ternary phases between Mo, Si and B were formed. At this stage, the subsequent annealing led to formation of α-Mo and Mo₅SiB₂ as major phases. The phase evolutions during heat treatment of powders can be affected by milling conditions. It should be mentioned that the desirable intermetallic phases were not formed during heat treatment of unmilled powders. On the other hand, α-Mo-Mo₅SiB₂-Mo₃Si nanocomposites were formed after annealing of powders milled for 22 h. With increasing milling time (at the second step), the formation of Mo₃Si during subsequent heat treatment was disturbed. Here, an α-Mo-Mo₅SiB₂-MoSi₂ nanocomposite was formed after annealing of 30 and 40 h milled powders.     

Site-occupation behavior and solid-solution hardening effect of rhenium in Mo5SiB2

The site-occupation behavior of Re in Mo₅SiB₂ (T₂) was studied both theoretically and experimentally, and the effect of Re on the solid-solution hardening of T₂ was investigated by taking into account the off-stoichiometry of the T₂ phase. Mo–Si–B quaternary alloys containing 1.4 at.% Re were produced using a conventional Ar arc-melting technique, and the cast samples were homogenized at 1800 °C for 24 h in an Ar atmosphere. High-resolution high-angle annular dark-field scanning transmission electron microscopy observations strongly suggest that Re preferentially occupies the Mo sites in the Mo–B layers of the T₂ unit cell, which was confirmed by the site-occupation behavior predicted by first-principles calculations. Nanoindentation measurements indicate that the hardness of the T₂ phase is affected by both the off-stoichiometry and Re addition.     

Deformation behavior of Mo5SiB2

First-principles calculations were employed to analyze the possible slip systems in Mo₅SiB₂. A striking result was obtained that the three most favorable slip systems, <100>(001), <110>(001) and [001]{010}, have close stacking fault energies, and the preference among them cannot be established. This finding explains a large variety of experimentally observed slip systems in Mo₅SiB₂. The dislocations associated with these slip systems may dissociate into partials joined with stacking faults and separated by the large splitting width of 5–6 nm.     

Room-temperature fracture toughness of MoSiBTiC alloys

Room-temperature fracture toughnesses of TiC-added Mo-Si-B alloys were evaluated for samples of three different compositions prepared using a conventional Ar arc-melting technique. The first alloy (TiCp) had a primary phase during solidification of NaCl-type TiC including an amount of Mo, with a Mo solid solution (Mo_ss) volume fraction of approximately 49% and a TiC volume fraction of approximately 19%, while the volume fraction of Mo₅SiB₂ (T₂) was approximately 31% and the remaining 1% was Mo₂C including an amount of Ti. The second alloy (T2p) had a primary phase of T₂, with volume fractions of Mo_ss, TiC, Mo₅SiB₂ (T₂), and Mo₂C of approximately 38%, 4%, 45%, and 13%, respectively. The third alloy (Mop) had a primary phase of Mo_ss, with volume fractions of Mo_ss, TiC, Mo₅SiB₂ (T₂), and Mo₂C of approximately 55%, 8%, 32%, and 6%, respectively. Room-temperature fracture toughness was evaluated by three different bending tests using Chevron-notched specimens. Fracture toughness values obtained by the three methods were relatively close with good reproducibility. Consequently, the fracture toughness values of TiCp, T2p, and Mop were evaluated to be ∼15.2 MPa(m)^1/2, ∼10.5 MPa(m)^1/2, and ∼13.6 MPa(m)^1/2, respectively. Fracture surface observations indicated that the Mo_ss phase is subject to severe plastic deformation during the fracture process. The TiC phase was also noted to leave river patterns behind through crack propagation. These fractographic results suggest that not only the ductile-phase toughening by the Mo_ss phase but also an extra-toughening mechanism by the TiC phase are responsible for the goodness of the room-temperature fracture toughness of the MoSiBTiC alloys.     

双峰结构Mo-Si-B基体表面多层硅化物涂层的表征和抗氧化性能

由于在细晶Mo-Si-B合金中制备双峰分布的α-Mo晶粒能够在不显著降低合金强度的前提下大幅提高其断裂韧性,为了加强双峰结构合金的表面防护,同时保持其优异的力学性能,通过包埋渗在合金表面上制备了一个具有多层结构（MoSi₂,Mo₅Si₃和Mo₅SiB₂/MoB）的涂层。研究结果表明,相比在细晶结构基体上制备的涂层,双峰结构基体上的涂层表面较为粗糙,并且也表现出双峰分布的微观组织。此外,覆盖涂层后的双峰结构合金的断裂韧性依然良好,并且分布在涂层中的La₂O₃颗粒能够增韧涂层。具有涂层的双峰结构合金在1100~1300 ℃下展现出了卓越的抗氧化性,这是由于氧化过程中在涂层表面快速形成了一个薄且能自愈合的SiO₂-B₂O₃膜。随着氧化温度升高,SiO₂-B₂O₃膜的粘度降低,使得SiO₂-B₂O₃膜的厚度和氧化产物Mo₅Si₃均增加。并且,升高温度促进了Si和B的互扩散,加速了Mo₅Si₃和Mo₅SiB₂/MoB层的增长。在1300 ℃下,由于单峰结构的MoSi₂涂层拥有更多的晶界,使得含涂层的细晶合金相比含涂层的双峰结构合金表现出更多的氧化增重。     

Oxidation behavior of Mo5SiB2-based alloy at elevated temperatures

A Mo₅SiB₂-based alloy having composition of Mo–12.3 mol% Si–24.9 mol% B was produced by arc-melting in an Ar atmosphere, and its oxidation behavior was investigated at temperature between 973 and 1673 K. At and above 1273 K, transient and steady state oxidation stages were clearly observed. The occurrence of the transient and steady state oxidation is interpreted in terms of rapid volatilization of MoO₃ and B₂O₃ under ambient O₂ pressure at the initial stage and the passive oxidation after completely sealing the substrate by silicate glass. Development of two layers onto the substrate, i.e. SiO₂ glass scale and Mo solid solution interlayer including SiO₂ dispersions, strongly supports the interpretation. Dissolution of B into the SiO₂ scale was not confirmed because of low B concentration that was under a detectable limit of EPMA and TEM-EDS. It is suggested that the SiO₂ glass scale formed on the Mo₅SiB₂-based alloy is more protective than as expected.     

On the subsurface deformation of two different Fe-based bulk metallic glasses indented by Vickers micro hardness

Bonded interface technique was employed to examine the nature of subsurface deformation during Vickers micro indentation in two iron-base bulk metallic glasses, (Fe_0.9Ni_0.1)₇₇Mo₅P₉C_7.5B_1.5 (BMG-1) and Fe₄₀Co₈Cr₁₅Mo₁₃Y₂C₁₆B₆ (BMG-2). Quantitative information such as the subsurface deformation zone size was recorded for indentation loads ranging from 200 to 5000 gr. The results showed that the BMG-2 specimens had an average hardness value higher than those observed in the BMG-1 specimens. The trends of the hardness vs. indentation load in the BMG-1 specimens were found to be related to the pressure sensitive index, while in the BMG-2 specimens, the cracking events and deformation-induced creation of free volume were responsible for the hardness tendency change. Observations of the deformation zones indicated that they deformed noticeably through two types of semi-circular and radial shear bands and the density of the radial shear bands was much more in the annealed specimens compared to the as-cast specimens. The relaxed and partially crystallized BMG-2 specimens exhibited cracking, ripple-like pattern as well as cracking and fragmentation, respectively. A simplified R = CP^0.5 model was used to analyze the shear band zone size in the subsurface and specimens brittleness.     

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.     

Erosion-corrosion behavior of nano-particle-reinforced Ni matrix composite alloying layer by duplex surface treatment in aqueous slurry environment

The present study concerns a duplex surface treatment of AISI 316L stainless steel to enhance the erosion-corrosion resistance. The duplex surface treatment consisted of Ni/nano-SiC and Ni/nano-SiO₂ predeposited by brush plating and a subsequent surface alloying with Ni-Cr-Mo-Cu by double glow process of the substrate. Results showed that under alloying temperature (1000 °C) condition, the amorphous nano-SiO₂ particles still kept the amorphous structure, whereas the nano-SiC particles had been completely decomposed and Ni, Cr reacted with SiC to form Cr_6.5Ni_2.5Si and Cr₂₃C₆. The electrochemical corrosion behaviors of composite alloying layers compared with the single alloying layer and 316L stainless steel were measured under a range of hydrodynamic conditions by recording the current response, open circuit potential, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). Results showed that the increase of the impact velocity had significant influence on the current density of composite alloying layer with brush plating Ni/nano-SiC particles interlayer obtained under flowing condition at a potential of 200 mV, whereas there were only small fluctuations occurred at current response of composite alloying layer with brush plating Ni/nano-SiO₂ particles interlayer. The results of potentiodynamic polarization indicated that, with increasing impact velocity under slurry flow conditions, the corrosion potentials of test materials decreased and the corrosion current densities of test materials increased. The corrosion resistance of composite alloying layer with brush plating Ni/nano-SiO₂ particles interlayer was prominently superior to that of single alloying layer under slurry flow conditions; the corrosion resistance of composite alloying layer with brush plating Ni/nano-SiC particles interlayer was evidently lower than that of single alloying layer, but higher than that of 316L stainless steel under slurry flow conditions. The results of EIS indicated that, with respect to the R_tot obtained in sand-free flow, the impacts of sand particles dramatically decreased the R_tot values of composite alloying layer with brush plating Ni/nano-SiC particles interlayer, single alloying layer and 316L stainless steel, whereas the impact action slightly decreased that of composite alloying layer with brush plating Ni/nano-SiO₂ particles interlayer. The weight loss rate studies suggested that the highly dispersive nano-SiO₂ particles were helpful to improve the erosion-corrosion resistance of composite alloying layer, whereas the carbides and silicide phase were deleterious to that of composite alloying layer due to the fact that preferential removal of matrix around the precipitated phase takes place by the chemical attack of aggressive medium.     

The microstructural engineering of Mo-Si-B alloys produced by reaction synthesis

Mo-Si-B alloys are candidate materials for next-generation jet engine turbine blades and have the potential to increase the service temperature of the base metals 200°C higher than nickel superalloys. These refractory alloys form a composite microstructure of molybdenum solid Solution (Mo_ss and two intermetallic phases, Mo₃Bi and Mo₅SiB₂, where the Mo_ss phase enhances toughness and the intermetallic phases provide oxidation resistance. The properties of the alloys are highly dependent on the morphology of the microstructure. A powder processing approach has been developed to synthesize the three-phase alloys through the reaction of molybdenum, Si₃N₄ and BN powders. Electron backscatter diffraction imaging has been used to map the location of individual phases and provide a method for quantifying the Cluster size distribution of a secondary phase to examine the effect of BN reactant powders on the dispersion of the intermetallic phases.