Mo Solid Solution Research Articles

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 (MoSS), 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.

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.

Phase Equilibria, Microstructure, and High-Temperature Strength of TiC-Added Mo-Si-B Alloys

TiC was added to Mo-Si-B alloys using a conventional Ar arc-melting technique, and the phase equilibria, microstructure evolution, and high-temperature strength at 1673 K (1400 °C) were investigated. The primary phase changed to Mo solid solution (Moss), Mo5SiB2 (T2), or TiC depending on the composition. Following the primary phase solidification, a Moss + TiC, Moss + T2, or Moss + T2 + TiC + Mo2C eutectic reaction took place as the secondary solidification step. In some alloys, Moss + T2 + TiC and Moss + T2 + Mo2C eutectic reactions were present as higher-order solidification steps. After annealing at 2073 K (1800 °C) for 24 hours, Moss, T2, TiC, and Mo2C coexisted stably with microstructural coarsening. The coarsening rate was much faster in an alloy with no TiC dispersion, suggesting that TiC has a strong pinning effect on the grain boundary and interface migration. Compression tests conducted at 1673 K (1400 °C) revealed strength properties of almost all the alloys that were better than those of the Mo-Hf-C alloy (MHC). Alloy densities were 9 g/cm3 or less, which is lighter than pure Mo and MHC (≥10 g/cm3) and competitive with Ni-base superalloys. TiC-added Mo-Si-B alloys are promising candidates for ultrahigh-temperature materials beyond Ni-base superalloys.

Study of the synergistic interaction between nickel, gold and molybdenum in novel modified NiO/GDC cermets, possible anode materials for CH4 fueled SOFCs

The present study concerns the structural and physicochemical characterization of novel Au–Mo–NiO/GDC cermets, alongside with measurements of their carbon tolerance. This is an investigation on whether commercially available NiO/GDC powder that is modified with Au and/or Mo can yield possible electrocatalysts as anodes for CH4 fueled SOFCs. Specifically, the performed preparation methods of deposition-precipitation and deposition-coprecipitation permitted the modification of a commercial electrocatalyst and lead to cermets that exhibit different reaction kinetics for the catalytic dissociation of CH4 and different performance for CH4 steam reforming. The most interesting structural findings were observed in the reduced form of the prepared powders, where a bimetallic Ni–Au solid solution is formed in the Au-modified sample. In the ternary composite material a trimetallic Ni–Au–Mo solid solution is formed, which induces a controversial synergistic effect on the sample‘s catalytic performance. On the other hand, the presence of Au compared to Mo shows the best combination of results in terms of structural properties, carbon tolerance and selectivity to the desirable products of H2 and CO, for the reaction of CH4 steam reforming.

Oxidation and Mechanical Behaviors of Two Phase (Mo + T2(Mo5SiB2)) and Three Phase (Mo + T2(Mo5SiB2) + Mo3Si) Alloys at 1 073 K and 1 373 K

Abstract The oxidation behaviors of Mo rich alloys have been investigated in the Mo-Si-B ternary system. Seven selected alloy systems composed of two phase alloys (Mo (solid solution) + T2 (Mo5SiB2) and three phase alloys (Mo (solid solution) + T2 (Mo5SiB2) + Mo3Si) have been fabricated by arc melting elemental components. Mechanical behaviors of the selected alloy systems have been investigated in order to identify the effect of composition on the ternary system. Also, oxidation tests have been performed in order to identify the oxidation behaviors at 1 073 and 1 373 K. When the alloy compositions move towards the Mo side, the oxidation reactions were accelerated for the two phase alloys due to the presence of Mo (s. s.), while the tendency of oxidation was not fast for the three phase alloys. The oxidation behaviors at above and below 1 273 K have been discussed in terms of microstructural observations during oxidation tests.

A Study on Effect of Reactive and Rare Earth Element Additions on the Oxidation Behavior of Mo–Si–B System

Mo–9Si–8B–1Ti, Mo–9Si–8B–1.8Ti, Mo–9Si–8B–0.2La and Mo–9Si–8B–0.4La2O3 (at.%) alloys were prepared using mechanical alloying followed by hot isostatic pressing and field assisted sintering. XRD, SEM and EBSD analysis confirmed the formation of Mo solid solution, A15 and T2 phases in the alloys. Isothermal oxidation behavior of the specimens was studied in the temperature range from 750 to 1,300 °C for up to 100 h. Both the Ti and La containing alloys showed superior oxidation behavior compared to unalloyed Mo–Si–B at 900 °C at the initial periods of oxidation. Ti-added alloys suffered higher rate of weight loss at higher temperatures (1,000–1,300 °C) due to the formation of non-protective low viscosity SiO2-TiO2-B2O3 scale. La-alloyed Mo–Si–B showed superior oxidation resistance at intermediate temperatures (900 °C) as well as at higher temperatures. Enrichment of La at the oxide/alloy interface was found to be the reason for improved oxidation behavior of La-alloyed Mo–Si–B. Amongst the four materials studied, the La2O3 containing alloy showed the best oxidation resistance at 900 °C.

Re effects on phase stability and mechanical properties of MoSS+Mo3Si+Mo5SiB2 alloys

In this paper, we investigate the effects of Re additions on the microstructure and mechanical properties of a ternary alloy with the composition Mo-12.5Si-8.5B (at.%). This alloy has a three-phase microstructure consisting of Mo solid-solution (MoSS), Mo3Si, and Mo5SiB2 and our results show that up to 8.4at.% Re can be added to it without changing its microstructure or forming any brittle σ phase at 1600°C. Three-point bend tests using chevron-notched specimens showed that Re did not improve fracture toughness of the three-phase alloy. Nanoindentation performed on the MoSS phase in the three-phase alloy showed that Re increases Young’s modulus, but does not lower hardness as in some Mo solid solution alloys. Based on our thermodynamic calculations and microstructural analyses, the lack of a Re softening effect is attributed to the increased Si levels in the Re-containing MoSS phase since Si is known to increase its hardness. This lack of softening is possibly why there is no Re-induced improvement in fracture toughness.

Measurement of an Iso‐Curie Temperature Line of a CoCrMo Solid Solution by Magnetic Force Microscopy Imaging on a Diffusion Multiple

AbstractThe 24 °C iso‐Curie temperature line of a CoCrMo fcc solid solution is obtained by performing magnetic force microscopy (MFM) imaging on solid solution compositions created in a diffusion multiple. The MFM imaging clearly reveals the boundary that separates the paramagnetic region without magnetic domains from the ferromagnetic region with domains. Compositional analysis along the boundary yields a constant Curie temperature (24 °C) composition line. Such a measurement is more efficient than one‐alloy‐at‐a‐time tests and can be used to screen new ferromagnetic materials.

Nanocrystalline structure and soft magnetic properties of nickel–molybdenum alloy thin films electrodeposited from acidic and alkaline aqueous solutions

Nanocrystalline nickel–molybdenum (Ni–Mo) alloy thin films were electrochemically synthesized in acidic and alkaline aqueous solutions. Transmission electron microscope bright-field images and electron diffraction patterns of the electrodeposits made it obvious that pure Ni consists of a submicron crystalline phase with the grain diameter of several hundred nanometers, while Ni–20 %Mo alloy was composed of a nanocrystalline phase with the grain diameter of a few nanometers. It was estimated that the nanocrystalline phase of electrodeposited Ni–Mo alloy thin films was introduced by the formation of supersaturated Ni–Mo solid solution phase with Mo content in the deposit more than 20 %. Submicron crystalline pure Ni thin films were hardly magnetized in perpendicular direction to the film plane while the nanocrystalline Ni–20 %Mo alloy thin films were isotropically magnetized. It was suggested that the isotropical magnetization behavior was caused by decreasing the demagnetizing field and the magneto crystalline anisotropy with a decrease in the magnetic moment and the average crystal grain size. Coercive force of a submicron crystalline pure Ni thin film electrodeposited from an acidic aqueous solution was ca. 100 Oe while that of a nanocrystalline Ni–20 %Mo alloy thin film electrodeposited from an alkaline aqueous solution was only 1∼2 Oe. Soft magnetic properties of Ni–Mo alloy thin films electrodeposited from an alkaline aqueous solution were better than that from an acidic aqueous solution and it was improved with an increase in Mo content in the deposit. It was estimated that the electrodeposited Ni–Mo alloy catalysts could be easily recovered with magnetic field less than 1 kOe.

Compositional regions of single phases at 1800 °C in Mo-rich Mo–Si–B ternary system

Single-phase regions in Mo-rich Mo–Si–B alloys at 1800°C were experimentally determined using a field-emission (FE) gun-type electron probe micro-analyzer (EPMA) in this study. A quantitative analysis by FE-EPMA was conducted with the calibration method we applied in our previous study to improve the accuracy in the B measurements. The compositional ranges of the determined Mo solid solution, silicide and boride phases were in good agreement with those of Mo–Si and Mo–B binary phase diagrams. On the other hand, the determined solubility of B in Mo solid solution, Mo3Si and T1 (Mo5Si3) were quite different from that indicated in previously reported ternary phase diagrams. The compositional region of the T2 (Mo5SiB2) single-phase ranges from 9.7 to 13.3at% for Si and from 23.5 to 26.8at% for B. The following two points were made clear by the ternary phase diagram as determined in this study. One is that no stoichiometric T2 composition is in the T2 single-phase region, confirming no T2 single-phase material can be obtained at the stoichiometric composition at 1800°C. The another is that the T2 region of this study expands to a Si-rich area from the stoichiometric composition.

Structural study of nanocrystalline solid solution of Cu–Mo obtained by mechanical alloying

Abstract This work studied the structural evolution of Cu– x Mo ( x = 5 and 8 wt.%) alloys processed by mechanical alloying using x-ray diffraction profiles, scanning electron microscopy, differential scanning calorimetric and microhardness. X-ray diffraction analysis was done using the modified Williamson–Hall and Warren–Averbach methods. These were used to determine structural properties, such as crystallite size, stacking fault probability and energy, dislocation density of metallic powder as a function of the amount of Mo and milling time. The main results obtained for both alloys were higher dislocation density and Vickers microhardness values were measured and crystallites sizes of around 10 nm were measured for both systems at 50 h of milling. Lattice defects increase the free energy and the free energy curves shift upwards, therefore the solubility limits change and Cu–Mo solid solution is formed.

Multiphase Mo-Si-B alloys processed by directional solidification

ABSTRACTMultiphase Mo-Si-B alloys are potential candidates for applications in the aerospace and power generation industry due to their enhanced creep and oxidation resistance at ultra-high temperatures. It is documented that the microstructure and the resulting properties of Mo-based alloys are heavily influenced by their fabrication procedure. In this study we investigate different multiphase Mo-Si-B alloys processed by zone melting (ZM) starting from cold pressed elemental powders. Microstructural characterization of zone melted alloys based on SEM investigations shows elongated arrangements of phases parallel to the growing direction as well as homogeneously distributed phases in the cross-section for some of the alloys investigated. First compression creep tests were performed at about 1100°C. In comparison to the creep resistance of powder metallurgically (PM) processed alloys the behaviour of ZM materials was found to be substantially improved. Hence, targeted application temperatures of around 1200°C to 1300°C may become feasible. Furthermore, the oxidation behaviour was found to be influenced by the volume fraction of the Mo solid solution phase since the volatilization of the Mo solid solution phase leads to a mass loss of the compound.

Tensile properties of multiphase Mo–Si–B refractory alloys at elevated temperatures

Abstract Multiphase refractory alloys with nominal composition Mo–9Si–8B–3Hf and Mo–10Si–14B–3Hf (at.%) were produced by a powder metallurgy (PM) technique using elemental powders. These alloys have fine grain sizes and 3-phase, consisting of α-Mo (Mo solid solution) and the intermetallic phases Mo 3 Si and Mo 5 SiB 2 . The tensile properties of these alloys at elevated temperatures were evaluated in vacuum. The Mo–10Si–14B–3Hf alloy has a very good mechanical strength at elevated temperature and displays large tensile ductility. This alloy exhibits a tensile strength of 560 MPa at 1400 °C and a strain rate of 2 × 10 −2 s −1 . The Mo–9Si–8B–3Hf alloy displays extensive plasticity or superplasticity at temperatures ranging from 1400 to 1560 °C. The tensile elongation of 410% was measured at 1560 °C. Grain boundary sliding is the main mechanism of plastic deformation for these alloys.

Superplasticity of a multiphase fine-grained Mo–Si–B alloy

Mo–9Si–8B–3Hf alloy consisting of a Mo solid solution and intermetallic phases Mo 3Si and Mo 5SiB 2 was fabricated by hot pressing sintering to yield a fine microstructure with all three phases being in the size range of micrometer. The tensile properties of this alloy were evaluated in vacuum at elevated temperatures. This alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400 °C to 1560 °C with strain rate of 3 × 10 − 4 s − 1 . The tensile elongation of 410% was measured at 1560 °C. Grain boundary sliding was the main mechanism of plastic deformation for this alloy.

Microstructural Evolution of Mo-Si-B Ternary Alloys through Heat Treatment at 1800°C

First of all, the as-cast microstructures of Mo-rich Mo-Si-B ternary alloys were investigated around the triple junction point of the primary Mo solid solution, Mo5SiB2 and Mo2B in this work, based on the liquidus projections of the Mo-Si-B system which have been reported in earlier studies. Subsequently, their microstructural evolution through heat treatment was investigated. Since Mo2B crystallizes out during solidification into a primary or secondary phase even though the alloy composition lies in the triangle of Mo-Mo5SiB2-Mo3Si in the Mo-Si-B equilibrium phase diagram, the as-cast microstructures include the non-equilibrated Mo2B in wide compositional ranges. However, Mo2B was completely decomposed during heat treatment at 1800 °C for 24 h and this contributed to the development of homogeneous, fine microstructures. On the other hand, since Mo2B was not decomposed perfectly during 24 h of 1600 °C heat treatment, as-cast microstructures largely remained. Therefore, it is realized that the heat treatment at 1800 °C is necessary to obtain well-developed microstructures of Mo-Si-B alloys.

Deformation Behavior of a Multiphase Fine-Grained Mo-Si-B Alloy

Mo-9Si-8B-3Hf alloy consisting of a Mo solid solution and intermetallic phases Mo3Si and Mo5SiB2 was fabricated by hot pressing sintering to yield a fine microstructure with all three phases being in the size range of micrometer. The tensile properties of this alloy at elevated temperature were evaluated in vacuum at elevated temperatures. This alloy displayed extensive plasticity or superplasticity at temperatures ranging from 1400 °C to 1560 °C with strain rate of 3×10-4 s-1. The tensile elongation of 410% is measured at 1560 °C. Grain boundary sliding is the main mechanism of plastic deformation for this alloy.

Synthesis of Mo5SiB2 based nanocomposites by mechanical alloying and subsequent heat treatment

In this study, systematic investigations were conducted on the synthesis of Mo5SiB2-based alloy by mechanical alloying and subsequent heat treatment. In this regard, Mo–12.5mol% Si–25mol% B powder mixture was milled for different times. Then, the mechanically alloyed powders were heat treated at 1373K for 1h. The phase transitions and microstructural evolutions of powder particles during mechanical alloying and heat treatment were studied by X-ray diffractometry and scanning electron microscopy. The results showed that the phase evolutions during mechanical alloying and subsequent heat treatment are strongly dependent on milling time. After 10h of milling, a Mo solid solution was formed, but, no intermetallic phases were detected at this stage. However, an α-Mo–Mo5SiB2 nanocomposite was formed after 20h of milling. After heat treatment of 5h mechanically alloyed powders, small amounts of MoB and Mo2B were detected and α-Mo–MoB–Mo2B composite was produced. On the other hand, heat treatment of 10h and 20h mechanically alloyed powders led to the formation of an α-Mo–Mo5SiB2–MoSi2–Mo3Si composite. At this point, there is a critical milling time (10h) for the formation of Mo5SiB2 phase after heat treatment wherein below that time, boride phase and after that time, Mo5SiB2 phase are formed. In the case of 20h mechanically alloyed powders, by increasing heat treatment time, not only the quantity of α-Mo was reduced and the quantity of Mo5SiB2 was increased, but also new boride phases were formed. Finally, after 5h heat treatment, the Mo phase completely disappeared and a Mo5SiB2-based composite was completely formed.

Oxidation Response and Coatings for Mo-Si-B Alloys

ABSTRACTMo-Si-B alloys respond to high temperature oxidation in two distinct stages. First, there is a transient stage with an initial high recession rate that corresponds to the evaporation of volatile MoO3 due to the oxidation of the molybdenum rich phases. The steady state stage of the oxidation begins when a borosilica layer that initiated in the transient period becomes continuous and protects the alloy from further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to improve the oxidation performance of the Mo-Si-B alloys, it is necessary to minimize the transient stage. The three phases, Mo (solid solution), Mo3Si (A15) and Mo5SiB2 (T2), composing the Mo-Si-B alloys play different roles in the transient stage. The interaction of the three phases with a reduced microstructure scale can reduce considerably the transient oxidation stage. As a further approach to inhibit the transient stage, a kinetic biasing strategy has been developed to capitalize on the reactions between different phases to develop useful reaction products and alloy compositions that evolve toward a steady state of a compatible system. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to apply diffusion coatings. Two areas are highlighted for successful coating applications on Mo-Si-B alloys and robust high temperature oxidation resistance: development of metal-rich silicide + borosilicide high-temperature coating and in-situ thermal-barrier + borosilica coatings.

Microstructural and mechanical properties of ternary Mo-Si-B alloys resulting from different processing routes

ABSTRACTMo-base silicide alloys take advantage of their outstanding intrinsic properties, notably the high melting point and, thus, their excellent mechanical and creep strength. We demonstrate how the processing route influences the microstructure and consequently the mechanical and oxidation behaviour. Therefore two fabrication routes, a powder metallurgical (PM) and a zone melting (ZM) process, both starting from elemental powders, were used to prepare several Mo-Si-B alloys with varying chemical compositions. While PM processing leads to an ultrafine microstructure with a continuous Mo solid solution (“α-Mo”) matrix and embedded particles of the two intermetallic compounds Mo3Si and Mo5SiB2, the directionally solidified (ZM) materials possess a coarse grained structure composed of an intermetallic matrix with dendritic islands of α-Mo. A comparative assessment of the mechanical behaviour of the alloys utilizing both the Vickers indentation fracture (VIF) technique and three-point bending tests emphasizes the beneficial effect of a continuous Mo matrix resulting in increased room temperature fracture toughness and a reduction of the brittle-to-ductile-transition-temperature (BDTT). Likewise, the positive effect of the fine grained and homogeneous microstructure on oxidation performance is shown by the evaluation of mass change during heat treatment at 1100°C.

NiTiAl Intermetallic Alloys Strengthened by Mo Replacement

NiTiAl based alloys have attracted attention as potential high temperature structural materials. Alloying is an effective way to improve their mechanical properties. The microstructures and mechanical properties of Ni50Ti44−xAl6Mox (x = 0, 0.5, 1, 1.5, 3) alloys have been investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and compressive tests. It is found that Ni50Ti44−xAl6Mox alloy is composed of NiTi (B2) and Ti2Ni (FCC) phases when Mo content is no more than 1 at%. The effect of Mo solid solution hardening at room temperature has been indicated by the rise of yield strength and the fall of plasticity with increasing Mo content. For Ni50Ti42.5Al6Mo1.5 and Ni50Ti41Al6Mo3 alloys, a Mo solid solution appears and increases the yield strength by precipitation strengthening. The maximum yield strength at 600 °C and elevated temperatures is presented in Ni50Ti43Al6Mo1 alloy not in alloys with a higher Mo content, which is possibly due to the softness of Mo-Ti-Ni solid solution phase.