Extrusion and selected engineering properties of Mo–Si–B intermetallics

In this study, an extrusion process has been developed to produce defect free, high-density rods of Mo–Si–B material. An initial powder composition (53.5 vol.%, 91 wt.%) of 66 vol.% Mo₅Si₃B_x (T1)–16 vol.% MoB–18 vol.% MoSi₂ was mixed with a paraffin-wax based binder (46.5 vol.%, 9 wt.%) and extruded using a twin-screw extruder. Following binder removal by a combination process of wicking and thermal degradation, the material was sintered at 1800°C. The bulk density of the sintered material was 90–92% of theoretical. Thorough binder removal was evidenced by low impurity levels: 258±6 ppm carbon and 772±10 ppm oxygen. The material demonstrated excellent high temperature oxidation resistance. The calculated parabolic rate constant is 1.1×10⁻² mg²/cm⁴/h at 1600°C. The extruded material was also successfully tested as a resistance heating element. These materials show promise for the development of heating elements with enhanced performance compared to current MoSi₂-based heating elements.     

Preparation and properties of sintered molybdenum doped with La2O3/MoSi2

Sintered Mo with the addition of La₂O₃/MoSi₂ was prepared via the process of solid–solid doping + powder metallurgy. X-ray diffraction experiment, hardness test, three-point bending test and high-temperature tensile test were carried out to characterize the samples. The XRD pattern of a typical sample shows that the sintered Mo was mainly composed of Mo, La₂O₃ and Mo₅Si₃. Mo₅Si₃ was probably formed through the reaction between MoSi₂ and the Mo matrix. Densities and fracture toughnesses of both doped Mo and pure Mo were measured and contrasted. Sintered Mo with the addition of 0.2 wt% La₂O₃/MoSi₂ has the highest toughness, while more addition of La₂O₃/MoSi₂ has smaller effect on improving toughness or even embrittles Mo. The results of three-point bending test and high-temperature tensile test show that the bending strength and high-temperature tensile strength of doped Mo are both higher than those of pure Mo. The formation of Mo₅Si₃ improves the high-temperature strength. The La₂O₃/Mo₅Si₃ dispersed in the Mo matrix refined the grains, and thus strengthened the Mo matrix by dispersion strengthening and grain refinement.     

Effect of MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum

MoSi₂/La₂O₃ and MoSi₂/Y₂O₃ composite particles were prepared by mechanical milling and doped into molybdenum by solid-solid method, respectively. Rods with a diameter of 17 mm were made by pressing and sintering. The effects of different composite particles on microstructures and strength of the as-sintered molybdenum were investigated. Results show that the MoSi₂/La₂O₃ and MoSi₂/Y₂O₃ composite particles transformed to La₂O₃/Mo₅Si₃ and Y₂O₃/Mo₅Si₃ composite particles due to the in situ reaction between Mo and MoSi₂ during sintering process. Mo₅Si₃/La₂O₃ and Mo₅Si₃/Y₂O₃ composite particles can reduce the grain size and improve both strength and toughness of sintered molybdenum significantly. Mo₅Si₃/Y₂O₃ composite particles contribute more to the strength, while the effect of Mo₅Si₃/La₂O₃ on toughness is greater than that of Mo₅Si₃/Y₂O₃.     

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.     

不同(Mo+B)/(Ni+Cr)质量比的原位合成MoB/NiCr涂层的组织结构与性能

采用超音速火焰喷涂技术沉积含3种不同(Mo+B)/(Ni+Cr)质量比（1:1,2:1和3:1）的Mo-B-Ni-Cr球磨复合粉末以原位反应制备获得MoB/NiCr涂层。采用扫描电子显微镜（SEM）和X射线衍射仪（XRD）分析了MoB/NiCr涂层的组织结构和物相。同时讨论了不同(Mo+B)/(Ni+Cr)质量比对涂层的组织结构、硬度、结合强度和耐腐蚀性能的影响。研究结果表明,(Mo+B)/(Ni+Cr)质量比为1：1的MoB/NiCr涂层孔隙率最低及涂层厚度最大。在3种涂层中均原位反应生成了Mo₂NiB₂三元硼化物,且随着(Mo+B)/(Ni+Cr)质量比的增加,涂层中三元硼化物含量随之增加,涂层的硬度值增加,结合强度反而随之降低;由于涂层中三元硼化物的原位生成,MoB/NiCr涂层的硬度值均高于316L不锈钢基体。通过能谱和XRD分析发现,经过360 h熔融锌腐蚀试验后,涂层表层中没有发现锌元素及其金属间化合物,然而随着(Mo+B)/(Ni+Cr)质量比的增加,涂层的孔隙率增加及厚度降低。最后,综合分析可得,相比其他涂层,(Mo+B)/(Ni+Cr)质量比为1:1的MoB/NiCr涂层具有更好的耐熔融锌腐蚀能力。     

Mo5Si3-B and MoSi2 deposits fabricated by radio frequency induction plasma spraying

Induction plasma-spray processing was used to produce free-standing parts of Mo₅Si₃-B composite and MoSi₂ materials. The oxidation resistance, up to 1210 °C, of the Mo₅Si₃-B composite was compared with MoSi₂, which is known to be resistant to high-temperature oxidation. The deposits were oxidized isothermally in air at atmospheric pressure. The structural performance of these materials under high-temperature oxidation conditions was found to depend on the boron content in the specimens. In particular, the composite containing 2 wt.% boron exhibited excellent resistance to oxidation, as indicated by the specimen mass change, which was found to be near zero after the 24 h oxidation test.     

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。     

High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate

The two-layer MoSi₂/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200 °C for 100 h and at 1300 °C for 80 h were 0.270 and 0.499 mg/cm², respectively. Compared with the monolithic MoSi₂ coatings, the transformation of MoSi₂ phase in the MoSi₂/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo₅Si₃ layer in the composite coatings was two orders of magnitude lower than that of the Mo₅Si₃ layer in the monolithic coatings at 1300 °C. The microstructural degradation of MoSi₂ in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi₂ coatings at high temperatures.     

Enhanced sintering, microstructure evolution and mechanical properties of 316L stainless steel with MoSi2 addition

Sintering 316L stainless steel to near full density with an appropriate sintering additive can ensure high mechanical properties and corrosion resistance. We present here a sintering approach which exploits the dissociation of ceramics in steels at high temperatures to activate sintering densification to achieve near full dense 316L stainless steel materials. MoSi₂ ceramic powder was used as a sintering additive for pre-alloyed 316L stainless steel powder. Sintering behavior and microstructure evolution were investigated at various sintering temperatures and content of MoSi₂ as sintering additive. The results showed that the sintering densification was enhanced with temperature and MoSi₂ content. The distribution of MoSi₂ was characterized by XMAPs. It was found that MoSi₂ dissociated during sintering and Mo and Si segregated at the grain boundaries. Excess Mo and Si were appeared as separate phases in the microstructure. Above 98% of theoretical density was achieved when the specimens were sintered at 1300 °C for 60 min with 5 wt.% MoSi₂ content. The stainless steel sintered with 5 wt.% MoSi₂ exhibited very attractive mechanical properties.     

Growth Kinetics and Microstructure Evolution of Intermediate Phases in MoSi<Subscript>2</Subscript>-Si<Subscript>3</Subscript>N<Subscript>4</Subscript>-WSi<Subscript>2</Subscript>/Mo Diffusion Couples

The growth kinetics and silicon diffusion coefficients of intermediate silicide phases in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple prepared by spark plasma sintering were investigated in temperatures ranging from 1200 to 1500 °C. The intermediate silicide phases were characterized by x-ray diffraction. The microstructures and components of the MoSi₂-Si₃N₄-WSi₂/Mo composites were investigated using scanning electron microscope with energy-dispersive spectroscopy. A special microstructure with MoSi₂ core surrounded by a thin layer of (Mo,W)Si₂ ring was found in the MoSi₂-Si₃N₄-WSi₂ composites. The intermediate layers of Mo₅Si₃ and (Mo,W)₅Si₃ in the MoSi₂-Si₃N₄-WSi₂/Mo diffusion couples were formed at different diffusion stages, which grew parabolically. Activation energy of the growth of intermediate layers in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo diffusion couple was calculated to be 316 ± 23 kJ/mol. Besides, the hindering effect of WSi₂ addition on the growth of intermediate layers was illustrated by comparing the silicon diffusion coefficients in MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂/Mo and MoSi₂-3.5 vol.% Si₃N₄/Mo diffusion couples. MoSi₂-3.5 vol.% Si₃N₄-5.0 vol.% WSi₂ coating on Mo substrate exhibited a better high-temperature oxidation resistance in air than that of MoSi₂-3.5 vol.% Si₃N₄ coating.     

Synthesis of dense MoSi2 by high-frequency induction heated combustion and its mechanical properties

Dense MoSi₂ compound was synthesized with the high-frequency induction heated combustion synthesis method in one step from elemental powders of Mo and Si within 2 min. Simultaneous combustion synthesis and densification were carried out under the combined effects of induced current and mechanical pressure. A highly dense MoSi₂ with a relative density of up to 98% was produced with simultaneous application of 60 MPa pressure and induced current. The percentages of the total shrinkage occurring before and during the synthesis reaction were 16% and 53%, respectively. The average grain size was about 15 μm and a slight amount of Mo₅Si₃ was observed at the boundaries of the MoSi₂ grains. The fracture toughness and hardness values obtained were 3.5 MPa·m^1/2 and 1050 kg/mm², respectively. These values were similar to those of commercial ones.     

Microstructure ofin situ MoSi2/SiC nanocomposite coating formed on Mo substrate by displacement reaction

The microstructure of an in-situ Mosi₂/β-SiC nanocomposite coating formed by the solid-state displacement reactions of Si deposited by chemical vapor deposition (CVD) with Mo-carbide layers at 1100°C, which had previously been formed on the surface of a Mo substrate by a CVD process, was investigated. The Mo-carbide layers formed by the simultaneous CVD of Mo and carbon at 900°C for 5 h using a gas mixture of C₂H₄−MoCl₅−Ar consisted of two layers, an inner layer of Mo₂C and an outer layer of MoC. While the monolithic MoSi₂ coating showed a typical colummar microstructure perpendicular to the Mo substrate, the MoSi₂/β-SiC nanocomposite coating formed by the solid-state displacement reactions between the Mo-carbide layers and Si was composed of equiaxed MoSi₂ grains with an average size of 150–500 nm and β-SiC particles with an average size of 80–105 nm. The β-SiC particles exhibited an oblate-spheroidal shape and were located mostly at the grain boundaries of MoSi₂. The volume percentage of β-SiC particles ranged from 18.5 to 29.2% with respect to the carbon concentration in Mo-carbide layers.     

Effects of ternary additions on the microstructure and thermal stability of directionally-solidified MoSi2/Mo5Si3 eutectic composites

Effects of ternary additions on the microstructure and thermal stability of directionally-solidified MoSi₂/Mo₅Si₃ eutectic composites have been studied for twelve different elements (Ti, V, Cr, Fe, Co, Ni, Nb, Ta, W, Ir, B and C) paying special attention to the variation of lattice misfits and interface segregation behavior with ternary additions. Among six elements (type-1: Ti, V, Cr, Nb, Ta and W) with a relatively high solubility in MoSi₂ and Mo₅Si₃, Ta and W are found to be beneficial to microstructure refinement. All other six ternary elements (type-2: Fe, Co, Ni, Ir, B and C) with a negligibly low solubility in MoSi₂ and Mo₅Si₃ exhibit a strong tendency to segregate on MoSi₂/Mo₅Si₃ interfaces, resulting in both microstructure refinement and the modification of the interface morphology.     

Simultaneous synthesis and densification of MoSi2-20vol.%Nb composite and MoSi2-20vol.%ZrO2 composite by field-activated and pressure-assisted combustion

A method to simultaneously synthesize and consolidate MoSi₂-20vol.%Nb composite and MoSi₂-20vol.% ZrO₂ composite from powders of Mo, Si, Nb and ZrO₂ was investigated. Combustion synthesis was carded out under combined electric field and mechanical pressure. Highly dense MoSi₂-20vol.%Nb and MoSi₂-20vol.%ZrO₂ composites up to 96% of theoretical density were produced from powders of Mo, Si, Nb and ZrO₂ with 60MPa of pressure and 3000A of current on the reactant.     

Reaction diffusion of MoSi2 and Mo5SiB2

To establish quantitative basis for oxidation-protective coating of Mo–Si–B ternary alloys by MoSi₂, phase transformations in MoSi₂ vs. Mo₅SiB₂ diffusion couples have been studied. Two layers are formed on reaction diffusion at temperatures between 1400 and 1600 °C: a single-phase layer of Mo₅Si₃ and a two-phase layer consisting of Mo₅Si₃ and MoB. The growth obeys the parabolic low for both the layers, and the rate constants of the two layers are found to be approximately equal. The interdiffusion coefficient in the T₁ layer has also been evaluated. The microstructural evolution in the diffusion zone is modeled in terms of mass conservation, as well as that of a Mo–9Si–18B two-phase alloy coated with MoSi₂ reported previously [Intermetallics 12 (2004) 407].     

Effect of Spraying Powders Size on the Microstructure,Bonding Strength,and Microhardness of MoSi2 Coating Prepared by Air Plasma Spraying

Molybdenum disilicide (MoSi₂) coatings were deposited on carbon steel by air plasma spraying technology with different feedstock powder sizes (i.e., powder A: ?15 + 2.5 μm, powder B: ?30 + 15 μm, powder C: ?54 + 30 μm, powder D: ?74 + 54 μm and powder E: ?106 + 74 μm). Phase composition and microstructure of coatings were characterized by x-ray diffraction (XRD) and scanning electron microscope. The bonding strength and microhardness of coatings were also evaluated. The XRD results show that there exists mutual transformation between T-MoSi₂ and H-MoSi₂ phase and part of Mo-rich phases are formed because of oxidization during the spraying process. With the increase of spraying powders size, the content of Mo-rich phases (Mo or Mo₅Si₃) and molybdenum oxide (MoO₃) in coatings decreases, and that of disilicide-rich phase (MoSi₂) in coatings increases. The oxidation degree of MoSi₂ particle gradually decreases during the spraying process with the increase of spraying powders size. The MoSi₂ is the main phase of the as-sprayed coatings when the spraying powders size is beyond 30 μm. With the increase of spraying powders size, the porosity of the as-sprayed coating increases, and the bonding strength of the coating gradually decreases. The hardness of coatings first increases and then decreases with the increase of spraying powders size.     

Interdiffusion kinetics in the Mo5SiB2 (T2) phase

Diffusion couples based on Mo₂B and Mo₅Si₃ were used to determine the diffusion kinetics of T₂ phase development and the relative diffusivities controlling the kinetics. Annealing the Mo₂B/Mo₅Si₃ diffusion couple above 1600 °C yielded an initial diffusion path sequence of Mo₂B/T₂/Mo₃Si/Mo₅Si₃, which was subsequently transformed to Mo₂B/T₂/(Mo₃Si+Mo₅Si₃)/Mo₅Si₃ by shrinkage of the Mo₃Si phase upon long-term annealing. The T₂ phase developed from Mo₂B and during the growth of the T₂ phase, Si and B atom movements were driven by the Si concentration gradient. The activation energy for Si interdiffusion in the T₂ phase was evaluated to be 355 kJ/mole compared with about 300 kJ/mole for Mo₅Si₃ in both Mo/Si and MoSi₂/T₂ diffusion couples. The larger activation energy is mainly responsible for the 10³ lower diffusivity for Si in the T₂ phase compared with Mo₅Si₃. The relatively slow diffusion in the T₂ phase is consistent with the enhanced creep resistance exhibited by T₂ phase microstructures. This article was presented at the Multicomponent-Multiphase Diffusion Symposium in Honor of Mysore A. Dayananda, which was held during TMS 2006, 135th Annual Meeting and Exhibition, March 12–16, 2006, in San Antonio, TX. The symposium was organized by Yongho Sohn of University of Central Florida, Carelyn E. Campbell of National Institute of Standards and Technology, Richard D. Sisson, Jr., of Worcester Polytechnic Institute, and John E. Morral of Ohio State University.     

MoSi2 oxidation resistance coatings for Mo5Si3/MoSi2 composites

In order to improve the oxidation resistance properties of 30 at.% Mo₅Si₃/MoSi₂ composite at high temperature in air, a molybdenum disilicide coating was prepared on its surface by a molten salt technology. XRD and SEM analysis showed that only tetragonal MoSi₂ phase existed in the coating after being siliconized for 5 h at 900°C. The oxidation film formed on the uncoated sample was not dense, so that oxygen diffused easily through it. The volatilization of MoO₃ resulted in the oxidation film separating from the substrate. The MoSi₂ coating was proved to be an effective method to prevent 30 at.% Mo₅Si₃/MoSi₂ composites from being oxidized at 1200°C. A dense glassy SiO₂ film was formed on the MoSi₂ coating surface, which acted as a barrier layer for the diffusion of oxygen atoms to the substrate. The 30at.% Mo₅Si₃/MoSi₂ composites with a MoSi₂ coating showed much better oxidation resistance at high temperature.     

Microstructure evolution during reactive plasma spraying of MoSi2 with methane

The mechanisms that govern microstructure evolution during reactive plasma spraying of MoSiz using 100% methane were investigated, with particular emphasis on the thermodynamics and kinetics of the relevant phase transformations and chemical reactions. The reactive plasma-sprayed M0Si₂ exhibited a dense, multilayered microstructure. In addition to the M0Si₂ matrix, significant amounts of M0₅Si₃ and elemental carbon were observed, along with a small amount of SiC. Thermodynamic and kinetic analysis predicted a large volume fraction of M0₅Si₃ and a small amount of SiC in the as-deposited reactive plasma-sprayed MoSi₂, in agreement with the experimental observations.     

Effect of Mo/B atomic ratio on the microstructure and mechanical properties of Mo2FeB2 based cermets

Four series of Mo₂FeB₂ based cermets with the Mo/B atomic ratio in the range from 0.8 to 1.1 were prepared by reaction sintering process. The microstructure and crystalline phases were studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX) and X-ray diffractometry (XRD). The results indicate that the transverse rupture strength (TRS) of the cermets increases with an increase of Mo/B atomic ratio and shows a maximum value of 1.9 GPa at a Mo/B atomic ratio of 0.9. At a higher atomic ratio the TRS decreases. The hardness of the cermets decreases monotonically from 90.8 HRA to 88.8 HRA with an increase of Mo/B atomic ratio. In Mo-rich cermets with an atomic ratio of Mo/B above 1.0, a new M₂₃B₆ type phase (M₂₃B₆, where M represents a metal) is found. This phase has a lattice parameter a = 1.09 nm containing Mo, Fe, Cr and B with an atomic ratio close to 16:6:1:6 and is precipitated at the interface of Mo₂FeB₂ grains or at the Mo₂FeB₂ grain boundaries.