Mo Solid Solution Research Articles

Microstructure-based prediction of thermal aging strength reduction factors for grade 91 ferritic-martensitic steel

This study aimed to develop a microstructure-based mechanistic approach to address the long-term thermal aging effect on yield stress and ultimate tensile strength and to provide a physical basis for developing thermal aging factors for G91 for a design life of 60 years. Several heats of G91 steel were examined. Controlled aging experiments were conducted on two heats at temperatures of 550, 600, and 650 °C for times up to ~64,000 h. Specimens were also taken from archived creep-tested specimens of G91 and from a tube removed from the Kingston coal-fired power plant to obtain data that cover a wide range of temperatures and for aging times up to 155,000 h. Thermal aging caused significant subgrain recovery, coarsening of M23C6 carbides at subgrain boundaries and MX carbonitrides within subgrains. The intermetallic Laves phase forms during aging and grows rapidly. The growth rate of the subgrain width and MX mean size during thermal aging were described by kinetic models. Thermal aging results in the reductions in the yield stress and the ultimate tensile strength of G91. The effects of thermal aging on the reductions in yield stress and ultimate tensile strength were well described by the microstructure-strength model that considers three superimposed strengthening mechanisms, namely, sub-boundary strengthening, MX precipitation hardening, and Mo solid solution strengthening. The model was independently validated by the ASME Code values and is being used by the ASME to develop the aging-induced strength reduction factors for G91 steel for a design life of 60 years.

Dynamic observation of MoSiBTiC alloy phase transitions using in situ ultrahigh-temperature X-ray diffraction measurement

In situ ultrahigh-temperature XRD measurement of a MoSiBTiC alloy was carried out using a laser-heated diamond anvil cell and synchrotron radiation. This technique revealed the complicated dynamic behavior of phase transitions in the MoSiBTiC alloy on heating above 1950 °C. The MoSiBTiC alloy initially consisted of Mo solid solution (Moss), Mo5B2Si (T2), TiC, Mo2B, Mo3Si, and Mo2C phases. The Mo2B, Mo3Si, and Mo2C phases disappeared on heating at ~1647 °C. The Moss grains coarsened during heating and the Moss, TiC, and T2 phases were present >1950 °C. The obtained results verified the solidification pathway of the MoSiBTiC alloy.

Microstructural characteristics of laser surface melted MoSiBTiC alloy

The microstructural characteristics of a TiC-added Mo-Si-B (MoSiBTiC) alloy fabricated by laser surface melting were investigated. The microstructure inside a melt pool consisted of many cells, and each cell was composed of an ultrafine ternary eutectic lamellar structure with Mo solid solution (Moss), Mo5SiB2, and (Mo,Ti)2C phases. The lamellar structure completely disappeared in a heat affected zone (HAZ) formed by multi tracks test, and a fine round (Ti,Mo)C phase newly precipitated in the HAZ together with other phases. The chemical composition analyses identified the elemental supersaturation of Si and Ti in the Moss phase and of Ti in the T2 phase in the lamellar structure.

On the chemical and microstructural requirements for the pesting-resistance of Mo–Si–Ti alloys

In recent publications [Schliephake et al. in Intermetallics 104 (2019) 133–142 and Obert, Kauffmann & Heilmaier in Acta Materialia, 184 (2020) 132–142], an unexpected pesting-stability of fully eutectic and specific eutectic–eutectoid Mo–Si–Ti alloys was found. Several potential reasons were proposed: microstructural length scale being typically very fine due to the eutectic and eutectoid reactions, the phase distribution including the fraction of the eutectic and eutectoid regions, as well as the chemical composition of the individual phases. In the present study, we prove the Ti content to be decisive for the pesting-resistance in air at 800°C by investigating the microstructure and oxidation behaviour of Mo–Si–Ti alloys with systematically varying nominal Ti content. A critical Ti content of 43at% was identified. Due to the phase equilibrium, this corresponds to a local Ti content in Mo solid solution of 35at%. Other microstructural properties such as (i) lamellar size within eutectic and eutectoid regions and (ii) volume fraction of the eutectic and eutectoid regions were demonstrated to have an insignificant influence on the pesting-resistance of the alloys. Rather, the assessment of the oxidation behaviour of the monolithic phase MoSS, which was identified to be crucial for the oxidation behaviour of the Mo–Si–Ti alloys, confirmed an improvement in oxidation behaviour with increasing Ti content.

Characterization of high–valence Mo–doped PrBaCo2O5+ cathodes for IT–SOFCs

Abstract PrBaCo2–xMoxO5+δ (PrBCM–x, x = 0.00–0.07) layered perovskites were evaluated as cathodes for IT–SOFCs. The Mo solid solution limit for PrBCM–x was up to x = 0.05. Mo doping in the PrBCM–x decreased the electrical conductivity, which may mainly be due to the disturbed ···Co–O–Co··· electron transport channels with Mo doping. The thermal expansion coefficients decreased from 21.0 × 10−6 K−1 at x = 0.00 to 19.0 × 10−6 K−1 at x = 0.03 and then to 18.1 × 10−6 K−1 at x = 0.05. Although a reduction in electrochemical performance with high–valence Mo doping into PrBaCo2O5+δ (PrBCO) was demonstrated, the reduced TECs and improved electrochemical stability were more attractive for the application of SOFC cathode. Electrochemical dynamic processes determined by EIS fitting indicated that Mo doping contributed to the rate–determining role of ion charge transfer in the oxygen reduction reaction and thus resulted in reduced electrochemical performance. By comprehensive consideration, the PrBCM–0.03 was confirmed as the most suitable cathode candidate for IT–SOFCs. The polarization resistance was 0.067 Ω cm2 at 700 °C for the PrBCM–0.03 cathode on Sm0.2Ce0.8O1.9 (SDC) electrolyte. A single cell with PrBCM–0.03 cathode and 300 μm–thick SDC electrolyte showed a maximum power density of 339 mW cm−2 at 700 °C and almost no degradation in performance was detected after a 42 h testing. This study provides a preliminary basis for further research into high–valence ion doping in layered perovskite cathode materials.

Designing of a thin Mo diffusion barrier towards strong and reliable Ti/Cu dissimilar brazing

The dissimilar brazing of Ti and Cu has been recognized to be challenging owing to the affinity of forming brittle intermetallic compounds at joint interface. In the current investigation, it was demonstrated that such detrimental interfacial reaction products can be readily circumvented by employing a Mo diffusion barrier deposited on Ti substrate. Combining Mo interlayer and a Ag–Cu–Ti active filler alloy, intermetallic compounds free Ti/Cu brazed joints consisting of Ti–Mo solid solution, Mo interlayer and remnant braze can be obtained. As a result of the eradication of brittle phases, the resultant joints exhibit excellent structural integrity with bonding strength comparable to the parent material properties. Additionally, as a consequence of sluggish solid state diffusion at the Ti/Mo interface and limited dissolution of the Mo interlayer by filler alloy during brazing, the desirable joint microstructure can be reproduced within a wide range of bonding duration. Such results indicate that strong and reliable Ti/Cu dissimilar brazing can be achieved by the combination of a Mo diffusion barrier and Ag–Cu–Ti active filler alloy.

Dynamic Observation of MoSiBTiC Alloy Phase Transitions Using <i>In</i> Situ Ultrahigh-Temperature X-Ray Diffraction Measurement

In situ ultrahigh-temperature XRD measurement of a MoSiBTiC alloy was carried out using a laser-heated diamond anvil cell and synchrotron radiation. This technique revealed the complicated dynamic behavior of phase transitions in the MoSiBTiC alloy on heating above 1950 oC. The MoSiBTiC alloy initially consisted of Mo solid solution (Moss), Mo5B2Si (T2), , Mo2B, Mo3Si, and Mo2C phases. The Mo2, Mo3Si, and Mo2C phases disappeared on heating at ~1647 °C. The Moss grains coarsened during heating and the Moss, TiC, and T2 phases were present >1950 °C. The obtained results verified the solidification pathway of the MoSiBTiC alloy.

Effect of Cr addition on microstructure and oxidation resistance of a Ti5Si3-containing MoSiBTiC alloy

The effect of Cr addition on the oxidation behaviour of multiphase Mo–28Ti–14Si–6C–6B (at. %) alloy composed of Mo solid solution, Mo5SiB2, Ti5Si3, and TiC phases was investigated. Replacement of 10 at. % Cr for Mo hardly changed the microstructure of the alloy and significantly enhanced its oxidation resistance at 800 °C. At 1100 °C, the Cr-added MoSiBTiC alloy showed less weight loss than the Cr-free base alloy. However, Cr addition had little effect on the substrate reduction, particularly for longer oxidation periods, due to the high volatility of Cr oxide at higher temperatures.

Study of solidification pathway of a MoSiBTiC alloy by optical thermal analysis and in-situ observation with electromagnetic levitation

MoSiBTiC alloys are promising candidates for next-generation ultrahigh-temperature materials. However, the phase diagram of these alloys has been unknown. We have developed an ultrahigh-temperature thermal analyser based on blackbody radiation that can be used to analyse the melting and solidification of the alloy 67.5Mo–5Si–10B–8.75Ti–8.75 C (mol%). Furthermore, electromagnetic levitation (EML) was used for in-situ observation of solidification and microstructural study of the alloy. On the basis of the results, the following solidification pathway is proposed: Mo solid solution (Moss) begins to crystallize out as a primary phase at 1955 °C (2228 K) from a liquid state, which is followed by a (Moss+TiC) eutectic reaction starting at 1900 °C (2173 K). Molybdenum boride (Mo2B) phase precipitates from the liquid after the eutectic reaction; however, the Mo2B phase may react with the remaining liquid to form Moss and Mo5SiB2 (T2) as solidification proceeds. In addition, T2 also precipitates as a single phase from the liquid. The remaining liquid reaches the (Moss + T2 + TiC) ternary eutectic point at 1880 °C (2153 K), and the (Moss + T2 + Mo2C) eutectic reaction finally occurs at 1720 °C (1993 K). This completes the solidification of the MoSiBTiC alloy.

The potential of mechanical alloying to improve the strength and ductility of Mo–9Si–8B–1Zr alloys – experiments and simulation

Multi-phase Mo–Si–B alloys were intensively researched during the last decade as they are promising candidates for high temperature applications beyond the capabilities of single-crystalline Ni-based superalloys. Small additions of Zr improve the mechanical properties of the Mo solid solution phase (Moss), especially it lowers the brittle-to-ductile transition temperature and improves the fracture toughness. This work shows how mechanical alloying (MA) as the crucial step of powder metallurgical (PM) production will significantly influence the materials mechanical properties of Mo–9Si–8B–1Zr alloys in a wide temperature range from room temperature up to 1400 °C. The formation, continuity and length scale of the individual phases during PM processing is evaluated in detail using experimental methods and numerical studies. Furthermore, the mechanisms of deformation and fracture of the bulk materials are discussed with respect to the microstructural features. For comparison of the mechanical properties achieved by PM processing we produced the similar alloy composition by a conventional solidification process and characterized it by the same evaluation methods.

Effect of Heating Treatment on the Microstructure and Properties of Cr–Mo Duplex-Alloyed Coating Prepared by Double Glow Plasma Surface Alloying

In this study, Cr–Mo duplex-alloyed coating was prepared on carbon steel by double glow plasma surface alloying (DGPSA). The effect of annealing and quenching and tempering (Q&T) treatments on the microstructure and performance of the coating was investigated by X-ray diffraction (XRD), energy dispersive spectrometry (EDS), backscattering electron imaging (BSEI) and electron backscattering diffraction (EBSD) techniques. The results show that a gradient structured coating composed of an Fe–Cr–Mo solid solution (Fe–Cr–Mo SS) layer and an alloyed pearlite layer was obtained on the steel surface. The affected layer was adjacent to the coating. After annealing or Q&T, more carbides precipitated in the Fe–Cr–Mo SS layer and alloyed pearlite layer. Most of the C atoms in the subsurface were dragged into the coating to form carbides in the Fe–Cr–Mo SS and alloyed pearlite layers of the coating, transforming the affected layers into a carbon-poor zone. Annealing and Q&T hardly modified the thickness of the coating, but greatly changed the hardness and corrosion resistance of the coating. The Q&T treated samples had higher hardness and better corrosion resistance than the as-DGPSA treated and the annealed samples.

Effect of Fe addition and moist environment on the high temperature oxidation behavior of Mo76-xSi14B10Fex (x = 0, 0.5, 1 at.%) composites

The effect of Fe addition and the moist environment on the high temperature isothermal oxidation behaviors of multiphase Mo76-xSi14B10Fex (x = 0, 0.5, 1 at.%) composites in the range of 1000–1300 °C have been investigated. The microstructure of all the composites with α–Mo solid solution (SS) and eutectic mixture of Mo3Si + Mo5SiB2 phases, has been refined upon Fe addition, which improves the oxidation resistance and reduces the mass loss up to 55% in dry air and 31% in moist air for 24 h exposure. Whereas, the oxidation resistance in moist air improves due to the formation of a protective Fe-rich glassy borosilicate scale, which reduces the vaporization of MoO3 by forming Fe2(MoO4)3. The cross-sectional area of the residual alloy for x = 1 were estimated to be 97% in dry air and 85% in moist air that of unoxidized coupons. The bulk hardness of the as-cast composite is 1031 Hv for x = 1, which reduced only by 6.7% (962 Hv) upon oxidation at 1300 °C in both dry and moist air, showing superior stability of the silicide matrix composites for high temperature application. X-ray diffraction, scanning electron microscopy and energy dispersive x-ray spectroscopy studies were performed to identify the various products of oxidation and to explore the mechanism of protection.

Effect of moist environment on the oxidation behavior of Mo76-xSi14B10Fex (x = 0, 0.5, 1 at.%) ultrafine composites in the range of 700–800 °C

The effect of moist environment on the isothermal oxidation behavior of Mo76-xSi14B10Fex (x = 0, 0.5, 1 at.%) ultrafine composites containing proeutectic α–Mo solid solution phase and eutectic mixture of Mo3Si (A15) and Mo5SiB2 (T2) phases, have been investigated at temperatures in the range of 700–800 °C. The mass loss has been greatly improved by 99% in moist environment than that of dry air both at 700 °C and 800 °C. The addition of Fe further improves the oxidation resistance in both dry and moist air up to 24 h exposure. The x–ray diffraction and the scanning electron microscopic studies have been performed to reveal the evolution of different phases in the oxide scale and to explore the mechanism of oxidation. The cross-sectional area of the residue alloy in the oxidized coupons has been measured to be 98% in moist air for x = 1 and 50% in dry air for x = 0 than that of the initial alloy coupon pointing that the oxidation resistance of MoSiB can be greatly improved in presence of moist air as well as upon Fe addition.

Volume and size effects of intermetallic compounds on the high-temperature oxidation behavior of Mo-Si-B alloys

In this study, we investigated the high-temperature oxidation behavior of Mo-Si-B alloys with different volume fractions or sizes of intermetallic compound phases. Mo-Si-B alloys with uniformly dispersed intermetallic compound phases (Mo5SIB2 and Mo3Si) in Mo solid solution matrix phase were fabricated using a novel powder metallurgical route, as introduced in our previous study. An isothermal oxidation test was conducted at 1300 °C for up to 10 h. The high-temperature oxidation resistance of Mo-Si-B alloys improved by increasing the volume fraction of intermetallic compound phases; this was a result of the increased amount of protective oxidized layers, which protect the Moss phase from oxidation by covering the surface. In addition, Mo-Si-B alloy with smaller intermetallic compound phases pulverized by high-energy ball milling had better high-temperature oxidation resistance compared to Mo-Si-B alloy with as-synthesized intermetallic compound phases.

Examination of B in the Mo solid solution (Moss) in Moss + Mo5SiB2 + Mo2B alloys

During an examination of the Moss (Mo solid solution) + Mo5SiB2 + Mo2B three phase alloys a lattice constant expansion was observed for the Moss. To identify the origin of the expansion, high precision analysis of scanning transmission electron microscopy images was carried out to determine the solid solution type of B atoms. A quantitative analysis of the non-rigid registration image demonstrates that the B is located at the octahedral sites of the Mo lattice. This conclusion is consistent with the expansion of lattice constant and the small solid solubility limit of B in Moss.

First-principles investigation of activity and solubility of Si in Mo solid solution

The activity and solubility of Si in Mo solid solution were studied by the first-principles calculations and quasi-harmonic approximation. The lattice constants, bulk modulus, and formation enthalpies of Mo-Si intermetallics were investigated, and the thermodynamic properties of Mo were also calculated. The Si activity coefficient [Formula: see text] is approximately 10[Formula: see text] at 100 K, while it increases by about 15 orders of magnitude over the investigated temperature range (100–2000 K). It is clear that Si activity coefficient in Mo solid solution increased rapidly with the increasing temperature, which should be ascribed to the Si–Mo interaction being evidently stronger than Mo–Mo interaction. According to the calculated thermodynamic properties, the solubility curve of Si in Mo solid solution was also predicted. The solubility limit of Si in Mo solid solution is [Formula: see text]5.5 at.%, which agrees well with the other experiment and assessed results.

Powder fabrication and laser additive manufacturing of MoSiBTiC alloy

The MoSiBTiC alloy is one promising candidate for ultrahigh–temperature materials. However, it faces severe challenges in the machining of complex shapes because of its significant brittleness, high melting point, and stiffness. To overcome this challenge, we have successfully fabricated MoSiBTiC alloy parts via laser powder bed fusion (L-PBF). A combination of arc-melting and controllable high-energy ball milling (HEBM) was employed to prepare suitable MoSiBTiC powders for L-PBF. The evolution of powder morphology, constituent phases, and laser absorptivity, as well as particle size and distribution during HEBM, was investigated. Moreover, the effects of L-PBF parameters on the densification, microstructure, and mechanical performance of MoSiBTiC alloy builds were studied. A dense MoSiBTiC alloy, mainly consisting of a Mo solid-solution, Mo5SiB2, Mo2C, and TiC phases, was obtained at an energy density of 156 J mm−3 using the laser power of 70 W. Compared to the as-cast alloy, the L-PBF-processed MoSiBTiC alloy possessed more uniform and finer grain structures, while exhibiting a lower Vickers hardness due to the existence of internal microcracks. It was also proved that the small quantity of ZrO2 particles from HEBM processing was uniformly imbedded in L-PBF builds. This work may offer significant guidance for designing and producing complexly shaped refractory intermetallics with unique microstructures in ultrahigh–temperature applications.

Microstructure and improved mechanical properties of ultrafine-grained Mo-12Si-8.5B (at.%) alloys with addition of ZrB2

Multi-hierarchical Mo-12Si-8.5B-xZrB2 (x = 0, 0.5, 1.0, 1.5, 2.5 wt%) alloys consisting of three ultrafine-grained (UFG, 0.47–0.81 µm) phases of Mo5SiB2 (T2), Mo3Si and Mo solid solution (α-Mo) were prepared by mechanical alloying following hot pressing. Microstructure observations showed that the intermetallic phases (Mo3Si and T2) distributed dispersedly in the continuous α-Mo matrix associated with the homogeneously embedded nanoscaled particles (10–225 nm) in the grain interiors and at the grain boundaries. The Mo-12Si-8.5B-xZrB2 alloys exhibited monotonically increasing compressive strength to 3.13 GPa with increasing content of ZrB2, and the fracture toughness increased about 27% and reached at 11.5 MPa m1/2 at 1.0 wt% ZrB2, rendering the Mo-12Si-8.5B-1.0 wt% ZrB2 alloy possessing the best combined mechanical properties of high strength and high toughness. The underlying reason for the superior mechanical properties of the Mo-12Si-8.5B-xZrB2 alloys is that the dispersedly distributed nanosized particles in the UFG multi-phased-matrix can not only effectively block the dislocation motion to increase the strength but also store the dislocations to increase the strain hardening ability during mechanical deformation.

Alloying effects in Mo-5X (X=Zr, Ti,V) – Microstructural modifications and mechanical properties

The microstructures and mechanical properties of three different cast Mo-5X (X = Zr, Ti, V) alloys were investigated. The alloys Mo-5Ti and Mo-5V show a single-phase solid solution microstructure, whereas the Mo-5Zr alloy exhibits polycrystalline Mo2Zr phases embedded in the Mo(Zr) solid solution matrix. Microhardness measurements were carried out by the Vickers indentation method. Compared to the single-phase solid solution alloys the Mo2Zr precipitations in Mo-5Zr result in the highest microhardness. Based on a well-known solid solution model it was shown that Zr, Ti and V are effective solid solution strengtheners. Additionally, constant displacement tests in the compressive mode between room temperature and 1100 °C confirm these findings. However, the homogeneously distributed Mo2Zr phases offer an extraordinary potential to improve the high temperature strength of Mo-based alloys.

Interfacial microstructure and mechanical properties of TZM alloy and ZrC particle reinforced tungsten composite joint brazed using Ti-61Ni filler

TZM alloy and ZrC particle reinforced W composite (ZrCp-W) were vacuum brazed using Ti-61Ni eutectic brazing alloy. The typical interfacial microstructure of the brazed joint consisted of three zones: a Mo solid solution layer, a brazing seam consisting of TiNi phase, TiNi3 phase and (Ti, Zr)Ni2 phase, and a diffusion layer composed of W particles, (Ti, Zr)C particles and (Ti, Zr)Ni2 phase. The results revealed that brazing temperature affected the interfacial morphologies of the joint, the phase constituents of the brazing seam, the element contents of these phases and the mechanical properties of the brazed joint. The optimal shear strengths at room temperature and at 800 °C reached 124.8 MPa and 82.3 MP respectively when brazed at 1240 °C for 10 min. Meanwhile, the fracture paths located at the TZM substrate and the fracture surface exhibited inter-granular fracture characteristics.