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.     

Magnesium silicide intermetallic alloys

Methods of induction melting an ultra-low-density magnesium silicide (Mg₂Si) intermetallic and its alloys and the resulting microstructure and microhardness were studied. The highest quality ingots of Mg₂Si alloys were obtained by triple melting in a graphite crucible coated with boron nitride to eliminate reactivity, under overpressure of high-purity argon (1.3 X 10⁵ Pa), at a temperature close to but not exceeding 1105 °C ± 5 °C to avoid excessive evaporation of Mg. After establishing the proper induction-melting conditions, the Mg-Si binary alloys and several Mg₂Si alloys macroalloyed with 1 at. pct of Al, Ni, Co, Cu, Ag, Zn, Mn, Cr, and Fe were induction melted and, after solidification, investigated by optical microscopy and quantitative X-ray energy dispersive spectroscopy (EDS). Both the Mg-rich and Si-rich eutectic in the binary alloys exhibited a small but systematic increase in the Si content as the overall composition of the binary alloy moved closer toward the Mg₂Si line compound. The Vickers microhardness (VHN) of the as-solidified Mg-rich and Si-rich eutectics in the Mg-Si binary alloys decreased with increasing Mg (decreasing Si) content in the eutectic. This behavior persisted even after annealing for 75 hours at 0.89 pct of the respective eutectic temperature. The Mg-rich eutectic in the Mg₂Si + Al, Ni, Co, Cu, Ag, and Zn alloys contained sections exhibiting a different optical contrast and chemical composition than the rest of the eutectic. Some particles dispersed in the Mg₂Si matrix were found in the Mg₂Si + Cr, Mn, and Fe alloys. The EDS results are presented and discussed and compared with the VHN data. Formerly Formerly     

Heats of formation in transition intermetallic alloys

The heats of formation in intermetallic alloys are calculated within a tight-binding scheme for the d band. We show that the difference in bandwidth between the metals and the difference between their energy levels are two dominant effects in the determination of the formation energy of these alloys. The influence of charge transfer, determined by a self consistent way, on alloy formation is studied.     

Ambient- to elevated-temperature fracture and fatigue properties of Mo-Si-B alloys: Role of microstructure

Ambient- to elevated-temperature fracture and fatigue-crack growth results are presented for five Mo-Mo₃Si-Mo₅SiB₂-containing α-Mo matrix (17 to 49 vol pct) alloys, which are compared to results for intermetallic-matrix alloys with similar compositions. By increasing the α-Mo volume fraction, ductility, or microstructural coarseness, or by using a continuous α-Mo matrix, it was found that improved fracture and fatigue properties are achieved by promoting the active toughening mechanisms, specifically crack trapping and crack bridging by the α-Mo phase. Crack-initiation fracture toughness values increased from 5 to 12 MPa√m with increasing α-Mo content from 17 to 49 vol pct, and fracture toughness values rose with crack extension, ranging from 8.5 to 21 MPa√m at ambient temperatures. Fatigue thresholds benefited similarly from more α-Mo phase, and the fracture and fatigue resistance was improved for all alloys tested at 1300 °C, the latter effects being attributed to improved ductility of the α-Mo phase at elevated temperatures.     

Diffusion in ordered alloys and intermetallic compounds

Diffusion in binary ordered alloys has been treated using the pair-approximation of the Path Probability Method (PPM) based on an atomistic model. The effect of the atomic interaction on the ordering behavior and its influence on transport properties have been clarified. Compositional dependence of both the intrinsic diffusion and interdiffusion coefficients, correlation factor and thermodynamic factor agree very well with the Monte Carlo simulation. The calculated properties also show qualitative agreement with the experimental data on diffusivity, activity and creep in the NiAl system.     

Combustion synthesis of LiGa and LiAl intermetallic alloys

LiAl and LiGa intermetallic alloys have been synthesized using the simultaneous combustion mode of combustion synthesis. LiAl intermetallic is potentially suitable as a temper alloy for producing aluminum-lithium alloys and as an anodic material for high-energy batteries. LiGa can be used as a reduction alloy to recover valuable reactive metals from molten salt effluent in actinide recovery technology. The effects of particle size, preignition heating rate, and theoretical green density on the ignition and combustion temperatures have been studied in an effort to more precisely control the synthesis reaction of these intermetallics. A lithium particle size of -20/xm was found to be suitable when the combustion synthesis reaction was conducted at a high heating rate (>1.0 cC/s) and a moderate green density (55 to 65 pct theoretical). Preignition diffusion is suggested as the cause for low exothermic heat release at high green densities. A combustion temperature above the melting point of the LiGa intermetallic compound can be achieved under optimized conditions. However, the exothermicity and, therefore, the adiabatic temperature is too low for either LiAl or LiGa to be produced by the propagating mode of combustion synthesis. Formerly graduate student with the Kroll Institute for Extractive Metallurgy     

W-Mo-Ni-Fe重合金金属间化合物相析出机制研究

为了分析Mo元素含量对液相烧结W—Ni—Fe重合金显微组织的影响和探讨Mo元素含量与基地相、析出相等各相组成的关系，以及了解合金于炉冷后在固相与基地相接口析出的金属间化合物相之形成机制，本研究使用具有相同基地相组成但不同Mo元素含量(Mo的原子数分数为15％～59％)的试片，以SEM、EMPA观察显微组织的变化，并使用XRD对金属间化合物析出相进行结构分析与鉴定，最后再以DTA热分析仪器量测合金的相变化温度，以厘清金属间化合物相的析出机制。试验结果显示，当合金中含有高比例的Mo元素时，烧结后在基地相中固溶的Mo元素与W Mo元素总和将随之升高，除了造成基地相的凝固温度下降以外，也形成大量的(W，Mo)0.5-(Ni，Fe)0.5 x(z=0～0．04)金属间化合物析出相，而此金属间化合物相中，Mo元素所占的比例会随着添加的Mo元素比例增加而呈线性上升。热分析结果显示，金属间化合物相的析出温度大约是在1349～1355℃之间，而基地相的凝固温度则从1415℃降低到1336℃。根据这些现象可以了解，金属间化合物相的形成机制与合金成分中的MMo／(Mw MMo)比值有关，当比值高于0．66时，金属间化合物相的析出机制为偏晶反应(monotectic reaction)；当比值在0．5～0．66之间时，析出机制是属于共晶反应(eutectic reaction)；当比值在0．34～0．5之间时，析出机制可能是共析反应(eutectoid reaction)或者是包析反应(peritectoid reaction)；比值低于0．2时，则不会析出金属间化合物相。     

High-temperature environmental embrittlement of thermomechanically processed TiAl-based intermetallic alloys

Thermomechanically processed TiAl-based intermetallic alloys with various alloy compositions and microstructures were tensile tested in various environmental media, including air, water vapor, and a gas mixture of 5 vol pct, H₂ + Ar, as functions of temperature and strain rate. All the TiAl-based intermetallic alloys showed reduced tensile fracture stress (or elongation) in air, in water vapor, and in a gas mixture of 5 vol pct H₂ + Ar, not only at ambient temperature (RT ∼ 600 K), but also at high temperature, from 600 to 1000 K (and sometimes at temperatures higher than 1000 K). The high-temperature environmental embrittlement of TiAl-based intermetallic alloys depended upon the microstructure. The factors causing the high-temperature environmental embrittlement may include hydrogen atoms decomposed from water vapor (H₂O) or hydrogen gas (H₂), similar to those causing the low-temperature environmental embrittlement. Also, it is demonstrated that the oxidized scale is effective in reducing high-temperature environmental embrittlement.     

Ni_3Al金属间化合物的研究进展

Ni3Al金属间化合物因具有熔点高、抗蠕变强度大、密度低、耐腐蚀、耐氧化以及其它优异性能,已被广泛应用于航空、冶金、机械、电化学、环保工业等领域,并有进一步发展的潜力和扩大应用的需求.该文对Ni3Al金属间化合物的制备及性能的研究进展进行综合评述,并着重论述粉末冶金法制备Ni3Al金属间化合物及其产品的抗氧化与催化性能等的研究现状.     

NiAl金属间化合物的研究进展

对NiAl金属间化合物的国内外研究现状如改善NiAl合金力学性能和高温抗氧化性能等所 采用的合金化、制备多相合金、制备复合材料、定向凝固、机械合金化、热压及热等静压、燃烧合成、 微晶涂层等工艺以及NiAl合金的超塑性行为进行了系统综述,着重介绍并论述了合金化及定向 凝固等工艺。此外,还介绍了NiAl合金的固溶强化磁行为。     

Characterization of dispersed intermetallic phases in rapidly quenched Al-Ti-Ce alloys

The microstructures of Al-3Ti-lCe (wt pct) and Al-5Ti-5Ce alloys melt-spun under controlled He atmosphere have been characterized using analytical electron microscopy. The rapidly solidified microstructures comprise uniform, fine-scale dispersions of intermetallic phase in an aluminum matrix, and particular attention has been given to identification of the dispersed phases. In the Al-3Ti-lCe alloy, the dispersed particles are polycrystalline with a complex twinned substructure and a diamond cubic crystal structure(a _o =1.44 ±0.01 nm) and composition consistent with the ternary compound Al₂₀Ti₂Ce (Al₁₈Cr₂Mg₃ structure type, space group Fd3m). In the Al-5Ti-5Ce alloy, there is, in addition to the dispersed ternary phase, a separate uniform array of fine-scale particles of the binary compound Al₁₁Ce₃. The majority of such particles have the body-centered orthorhombic structure of the low-temperature polymorph, α-Al₁₁Ce₃, but there is evidence to suggest that at least some particles developvia initial formation of the high-temperature body-centered tetragonal phase, β-Al₁₁Ce₃. The accumulated evidence suggests that both binary and ternary particles formed as primary phases directly from the melt during rapid solidification, leaving only small concentrations of solute in aluminum matrix solid solution. Both phases are observed to be resistant to coarsening for up to 240 hours at 400 °C. Formerly Research Fellow, Department of Materials Engineering, Monash University.     

Gravity segregation of complex intermetallic compounds in liquid aluminum-silicon alloys

Primary crystals of intermetallics that are rich in iron, manganese, and chromium form at temperatures above the liquidus, and because their density is higher than that of liquid alumi-num, they cause gravity segregation in the melt. Segregation may occur either in the mold at slow cooling rates or in the bulk liquid in furnaces or ladles. The kinetics of settling of these intermetallic compounds in a melt of Al-12.5 pct Si having 1.2 pct Fe, 0.3 pct Mn, and 0.1 pct Cr has been studied. Sedimentation was investigated at 630 ‡C for settling times of 30, 90, and 180 minutes in an electric resistance furnace. The effect of settling time and height of melt on the volume percent, number, and size of intermetallic compounds was studied by image anal-ysis. The volume percent of intermetallics increases with distance from the melt surface. Both the number of particles and the average size increase during sedimentation. The rate of settling varies with position in the melt due to depletion of intermetallics near the surface and an increase near the bottom. The settling velocities obtained experimentally were compared with terminal velocities calculated by Stokes’ law. Good agreement was generally found. The settling speed of intermetallics reaches the terminal velocity at very short times and very close to the liquid surface. Stokes’ law is therefore applicable to virtually all locations within the melt.     

Mechanical properties of alloys of IrNb and other high-temperature intermetallic compounds

Starting from the results of earlier screening tests of binary high-temperature intermetallic compounds, alloys based on Al₃Nb, A1₈V₅, IrNb, Si₂V, and Si₃Y₅ have been studied by microhardness, elastic modulus, impact strength, and, in some cases, compressional stress-strain behavior. Significant improvements were observed only in the IrNb-based alloys, several of which, like IrNb itself, have good impact resistance and some plasticity at room temperature.     

Microstructural effects on moisture-induced embrittlement of isothermally forged TiAl-based intermetallic alloys

By using isothermally forged TiAl-based intermetallic alloys, various microstructures (of γ-grain, duplex, dual-phase, and fully lamellar microstructures) were prepared. These TiAl-based intermetallic alloys were tensile tested in vacuum and air as functions of strain rate and temperature to investigate microstructural effects on the moisture-induced embrittlement. All the intermetallic alloys with different microstructures showed different levels of reduced tensile stress (or elongation) in air at room temperature. The reduction in tensile stress (or elongation) due to testing in air diminishes as the testing temperature (or strain-rate) increases. From the fracture stress-temperature curves, it was found that the γ-grain microstructure was the most resistant to the moisture-induced embrittlement, and the dual-phase microstructure was the most susceptible to the moisture-induced embrittlement. Also, the moisture-induced embrittlement of the TiAl-based intermetallic alloys with a fully lamellar microstructure depends on the lamellar spacing and is reduced with decreasing lamellar spacing. The possible reasons for the observed microstructural effect on the moisture-induced embrittlement were discussed, in association with hydrogen behavior and properties in the constituent phases and at some interfaces.