Mo5SiB2 Phases Research Articles

Alloy designs for high temperature Mo-base systems

For high temperature applications Mo base alloy requirements include both superior structural performance and environmental resistance. To address these requirements alloys in the Mo-Si-B system and refractory multi-principal element alloys (RMPEA) are being developed that exhibit a promising potential, but also have some remaining challenges to improve ductility, lower density and enhance environmental resistance. In the Mo-Si-B system microstructures with a Mo solid solution (Moss) Mo3Si and Mo5SiB2 (T2) phases have been the focus of attention. However, the Si solubility in the Moss phase diminishes the ductility and toughness. In order to address this issue a new design based upon Moss, Mo2B and T2 phases lowers the Si solubility in the Moss to improve ductility while the T2 phase maintains the oxidation performance. Selected additions of Al and Ti enable a density reduction to below 8 g/cm3. The RMPEA designs for Mo-rich alloys provide for excellent structural performance, but the complex oxidation products provide no protection. In this case a new coating design has been introduced that provides the required environmental resistance.

New insight into the structural stability, ductility and melting point of Mo5SiB2 under high-pressure environment

Mo5SiB2 is an attractive high-temperature ceramic because of the excellent mechanical and thermodynamic properties. However, the role of high pressure on the structural stability and the related properties of Mo5SiB2 is entirely unclear. In this work, we apply the first-principles calculations to study the influence of pressure on the structural stability, mechanical properties and melting point of Mo5SiB2. Here, three possible Mo5SiB2 phases: D8l-Mo5SiB2, D8m-Mo5SiB2 and Cmcm-Mo5SiB2 are considered. The results show that all Mo5SiB2 are thermodynamic stability. In particular, it is found that the thermodynamic stability of Mo5SiB2 follows the order of D8l-Mo5SiB2>cmcm-Mo5SiB2> D8m-Mo5SiB2. The calculated bulk modulus and Young's modulus of D81-Mo5SiB2 under zero pressure are bigger than the cmcm-Mo5SiB2 and D8m-Mo5SiB2. Importantly, the pressure significantly improves the ductility of all Mo5SiB2 because the pressure can adjust the localized hybridization between Mo, Si and B atoms. In addition, it is found that the calculated melting point of all Mo5SiB2 increases with increasing the pressure. The melting point of the Cmcm-Mo5SiB2 is higher than the D8l and D8m phases under high pressure.

Selective laser melting of MoSiBTiC alloy with plasma-spheroidized powder: Microstructure and mechanical property

Spherical MoSiBTiC alloy powder with little internal gas pores and cracks was successfully fabricated using plasma-spheroidization method. This powder fabrication process significantly decreased the oxygen content of the raw crushed powder. Then, MoSiBTiC alloy samples were fabricated via selective laser melting (SLM) with the plasma-spheroidized powder, and the microstructural characteristics and mechanical property were investigated. The as-built MoSiBTiC alloy exhibited a bimodal microstructure; the microstructure inside the melt pool (MP) was different from that at a melt pool boundary (MPB). The microstructure inside the MP consisted of primary Mo solid solution (Moss) dendrites, and other constituent phases, such as Mo5SiB2 (T2) phase, were formed at the inter-dendritic regions. On the other hand, (Ti,Mo)C precipitates around the Moss dendrites were preferentially coarsened at the MPB, forming a heat affected zone (HAZ) at the underside within the MPB. At the upside within the MPB, the microstructure of the previous layer was partially remelted, and ternary eutectic lamellar structure (Moss/T2/(Mo,Ti)2C) was newly formed. The as-built samples were subsequently subjected to heat treatment at different temperatures of 1600, 1700, and 1800 °C for 4 h. With increasing in the heat-treatment temperature, the Moss dendritic microstructure decomposed, and other constituent phases were coarsened at the inter-dendritic regions. Through a detail microstructural characterization of both the as-built and heat-treated samples, it was confirmed that any large oxide was not formed in this study. Rapid solidification via SLM resulted in the refinement of microstructure and the formation of a supersaturated Moss phase, which hardened the as-built sample but simultaneously decreased the crack resistance of the Moss phase.

Effect of hot isostatic pressing on the microstructure and fracture toughness of laser additive-manufactured MoSiBTiC multiphase alloy

MoSiB-based alloys are extremely promising ultrahigh–temperature materials, owing to their high melting points, good thermal stability, and excellent elevated-temperature mechanical properties. However, their fabrication is limited to conventional manufacturing techniques. In this work, a MoSiBTiC multiphase alloy, mainly consisting of Mo solid solution (Moss), Mo5SiB2 (T2), and TiC phases, was fabricated by laser powder bed fusion (L-PBF) using ball-milled alloy powders; subsequently, the effect of hot isostatic pressing (HIP) on its microstructure and fracture toughness was investigated. Due to the non-equilibrium metallurgical characteristics of L-PBF, the as-built alloy included uniform and fine-grained structures, as well as internal microcracks. The length of microcracks was effectively reduced with the assistance of the TiC bridging effect during HIP. As revealed by a high-resolution transmission electron microscope, the TiC was uniformly dispersed and was in close contact with the Moss or T2 phase. Consequently, the MoSiBTiC multiphase alloy exhibited a high fracture toughness of 9.0 MPa(m)1/2 after HIP at 1973 K. This work could be an important step towards the fabrication of high-performance refractory components with advanced properties in ultrahigh–temperature use.

Effects of Zirconium and Yttrium Oxide on Mechanical and Oxidation Properties of Mo–3Si–1B–1Zr–1Y2O3 (wt.%) Alloy

Mo–3Si–1B alloys with zirconium (1 wt.%) and yttrium oxide (1 wt.%) additives were fabricated by vibrating sintering techniques. The doped Mo–3Si–1B alloys consisted mainly of α-Mo, Mo3Si, and Mo5SiB2 (T2) phases. It was found that the grains were reduced, and the intermetallics particles were dispersed more homogeneously after the addition of Zr and Y2O3. The optimization in microstructure induced corresponding improvements in both fracture toughness and oxidation resistance. The predominant strengthening mechanisms were fine-grain strengthening and particle dispersion strengthening. In addition, fracture toughness test showed that the additions could improve the toughness of Mo–3Si–1B alloys, for which the toughening mechanism involved a crack trapping by α-Mo phases and extensive small second phase particles in the alloys. What should be paid attention to is the satisfactory oxidation resistance, both at medium-low temperature (800 °C) and high temperature (1200 °C) with doped additives.

Significant improvement of the oxidation resistance of MoSiBTiC-based multiphase alloys by Ti enrichment

The effects of the Ti content on the microstructure and oxidation behaviour of Mo–yTi–14Si–6C–6B (atom %) alloys were investigated. Enrichment with Ti of up to 54% did not change the four-phase constitution of the Moss, Mo5SiB2, Ti5Si3, or TiC phase, and improved the oxidation resistance of the alloy. The Mo–50Ti–14Si–6C–6B alloy exhibited the best oxidation resistance at 800–1200 °C through formation of a dense layer of TiO2. This improvement can be described in terms of a modified Pilling–Bedworth ratio, which indicated that the densest TiO2 layer formed on Moss grains.

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.

Orientation relationship between bcc precipitates and Ti5Si3 matrix in Mo–Si–B–Ti–C quinary alloys

In this study, the crystallographic orientation relationship between bcc precipitates and a Ti5Si3 matrix in Mo–50Ti–14Si–6C–6B (at.%) alloy was established. The alloy microstructure consisted of the bcc solid solution, Ti5Si3, Mo5SiB2, and TiC phases. The metal-rich Ti5Si3 phase was formed during a solidification stage, followed by the precipitation of the bcc phase within the Ti5Si3 matrix in the subsequent annealing process. The orientation relationship between the two phases was described by the expression: 112bcc//011‾0Ti5Si3 and 3‾11bcc//0001Ti5Si3. The formation of a low-energy interface between the two phases was explained by the edge-to-edge matching of their corresponding lattice planes at the faceted interface.

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.

Experimental study on the feasibility of using liquid-assisted processing in fabrication of Mo-Si-B alloys

In this work, we used a sessile drop experiment to assess a feasibility of novel processing design engaging liquid-phase assisted fabrication of multi-phase materials from Mo-Si-B system. For this purpose, binary eutectic silicon-boron alloy (Si-3.2B wt%) was subjected to contact heating with polycrystalline molybdenum substrate at temperature up to 1385 °C. It was in-situ observed that molten Si-3.2B alloy wets and rapidly spreads over the Mo surface. After the high temperature test, the solidified couple was subjected to structural characterization by scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy and X-ray diffraction methods. The obtained results revealed that the direct interaction between examined materials under applied testing conditions results in the reactively formed product layer having a thickness of ∼80 µm. The following structural features were recognized starting from the surface side: (I) the main product layer composed of columnar-like MoSi2 phase and Mo5Si3 phase; (II) an intermediate layer made of MoB and Mo5SiB2 phases. Although these results give the first indication for designing a novel liquid-assisted fabrication method of Mo-Si-B materials, an existence of porosity in the product layer clearly suggests that a further optimization of the process parameters is necessary.

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.

Variation of phase composition of Mo-Si-B alloys induced by boron and their mechanical properties and oxidation resistance

Mo-12Si-xB (x = 5, 8.5, 17 at%) alloys, doped with 0.9 wt% nanometer La2O3, were fabricated via a powder metallurgical process. The distribution of the α-Mo, Mo3Si and Mo5SiB2 phases existed in these alloys was dependent on the volume fractions of the individual phases. The Mo-12Si-5B (~52 vol% of α-Mo) and Mo-12Si-8.5B (~46 vol% of α-Mo) alloys possessed a continuous α-Mo matrix where Mo3Si/Mo5SiB2 particles distributed dispersedly and the former exhibited the highest fracture toughness. Our theoretical analysis demonstrated that the crucial contribution of crack trapping mechanism to the toughness and the α-Mo volume fraction increasing effectively promoted this mechanism. Besides, the α-Mo/intermetallic interface debonding and the formation of intragranular microcracks induced by La2O3 provided extra contributions to toughness. The Mo-12Si-17B alloy (~65 vol% of intermetallic) formed an intermetallic matrix with embedded semi-continuous α-Mo colonies and other isolated α-Mo islands, having the highest compression strengths at 800–1300 °C. Especially at 1200–1300 °C, the intermetallic matrix with low plastic deformation played a significant role in reinforcing alloy. Besides, the intragranular La2O3 particles strengthened alloy by pinning down and accumulating dislocations in the grain interior. The Mo-12Si-17B was also found to have the excellent oxidation resistance at 1000 °C owing to exhibiting the shortest duration and minimal mass loss in the transient oxidation stage. Moreover, higher B content decreased the viscosity of the borosilicate and thereby formed a dense and protective glassy borosilicate scale quickly, which substantially reduced the growth of inner oxide layers in the steady oxidation stage. In addition, the La2O3 as a blocking phase could impede the permeation of MoO3 bubbles/clusters through the flowing SiO2 glass scale.

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.

Synthesis and stabilization of a new phase regime in a Mo-Si-B based alloy by laser-based additive manufacturing

Mo-Si-B alloys are potential creep resistant materials for accessing harsh loading scenarios beyond Ni-based superalloys due to their excellent mechanical performance at ultra-high temperatures (> 1200 °C). Here, we report on the fabrication through laser additive manufacturing of a Mo rich Mo-Si-B alloy with and without dispersion of oxide (La2O3) particles. The major phase in the solidified material is dendritic α-Mo. The inter-dendritic regions contain a mixture of the Mo5Si3 (T1) + Mo5SiB2 (T2) phases, and not the expected equilibrium Mo3Si + Mo5SiB2 (T2) phases. This combination of phases is shown to yield improved high temperature creep resistance but was only accessible through by addition of Nb, W or Ti that substitute Mo in the intermetallic phases. Whereas here it is attributed to the large undercooling in the small melt pool produced during laser processing. We show that this phase mixture, upon annealing, is stable at 1200 °C for 200 h. We also demonstrate successful dispersion of oxide particles mainly in the inter-dendritic regions leading to a high indentation fracture toughness of ∼18 MPa√m at room temperature. Toughening originates from crack trapping in the ductile α-Mo and the formation of micro-cracks and crack deflection in the vicinity of oxide particles.

Enhanced oxidation resistance of Mo-12Si-8.5B alloys with ZrB2 addition at 1300 °C

Mo-12Si-8.5B and Mo-12Si-8.5B-1.0wt%ZrB2 alloys were fabricated using mechanical alloying, followed by hot-pressing. Both alloys exhibited uniform microstructure, with the Mo3Si and Mo5SiB2 phases distributing dispersedly in the α-Mo matrix. Mo-12Si-8.5B-1.0wt%ZrB2 showed a finer-grained microstructure than Mo-12Si-8.5B alloy owing to the addition of ZrB2. The results of isothermal oxidation tests at 1300°C in air revealed that Mo-12Si-8.5B and Mo-12Si-8.5B-1.0wt%ZrB2 alloys initially suffered a transient stage with high mass loss due to the volatilization of MoO3, and then achieved a steady stage owing to the formation of a protective borosilicate scale on the alloy surface. Especially, the transient stage of Mo-12Si-8.5B-1.0wt%ZrB2 alloy was shortened to be less than 300s, and the mass loss of this stage was reduced by at least 88% compared with that of Mo-12Si-8.5B alloy, indicating a significant improvement in the oxidation resistance. The addition of ZrB2 not only resulted in a continuous borosilicate scale quickly covering the entire base alloy during the transient stage, but also improved the protectiveness of the borosilicate scale of the steady stage by bringing out a large number of ZrO2/ZrSiO4 particles embedded discontinuously in the borosilicate scale, which effectively restricted the inward diffusion of oxygen by acting as diffusion barriers and decreased the thickness of inner oxide layers in particular.

Microstructures and Room Temperature Mechanical Properties of Mo-12Si-8.5B-8Cr Alloy

Microstructures and room temperature mechanical properties of Mo-12Si-8.5B and Mo-12Si-8.5B-8Cr alloys were investigated. The alloys, which were prepared by spark plasma sintering at various temperatures, were analyzed using scanning electron microscopy, x-ray diffraction, vicker hardness test, and three-point bending test. Moss matrix with uniformly distributed intermetallic phases of Mo3Si and Mo5SiB2 were observed in the microstructures of both Mo-12Si-8.5B and Mo-12Si-8.5B-8Cr alloys. The solution of Cr in solid phases of Mo, Mo3Si and Mo5SiB2 phases was detected. Hardness and room temperature flexure strength of two alloys increased with the increase in sintering temperature. Under the same sintering conditions, the addition of Cr obviously decreased the porosity of Mo-12Si-8.5B sintered body, and significantly increased the hardness, room temperature flexure strength and fracture toughness.

Polymer derived oxidation barrier coatings for Mo-Si-B alloys

Thermodynamic analysis was carried out to predict the phase composition of a perhydridopolysilazane-type polymer derived ceramic coating on Mo5SiB2 matrix particles after heat treatment. The most probable chemical reactions between these constituents and resulting phases were calculated. The feasibility of PHPS/Mo5SiB2 chemical reactions was proved experimentally. An amorphous SiOxNy phase and free Si were found by X-ray diffraction analysis and Raman spectroscopy. The presence of these two phases explain the improvement in oxidation resistance of the Mo5SiB2 particles, which was found to be as twice as high at 800°C and 1100°C in air, compared to the unprotected plain Mo5SiB2 phase. The oxidation of the free silicon provided by the PHPS conversion was addressed as an oxygen trap.

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

Addition of Fe refines the microstructure of Mo76-xSi14B10Fex (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.

Prediction of phase distribution pattern in phase field simulations on Mo5SiB2-primary areas in near eutectic Mo-Si-B alloy

A Mo-10.9Si-20.3B (if not stated otherwise all compositions are given in at.%) alloy was modeled using the phase field method with linearized phase diagrams and thermodynamic data. The simulation results showed that there are two specific microstructural constituents. According to the simulations and experimental microstructural investigations the primary Mo5SiB2 phase observed in this study was combined by the Moss-Mo3Si-Mo5SiB2 eutectics. The overall composition of the both primary and eutectic area was probed and the phase evolution along with the concentration change of the core areas were explored and visualized in the solidification domain. To verify the accuracy of the simulation results, they were comprised with the experimental achievements. In terms of the fraction of phases, the portion of the primary phase and the eutectic constituent and the phase distribution pattern our results were in good agreement with the experimental observations.