Tensile creep of Mo–Si–B alloys

Multiphase Mo–Si–B alloys containing a Mo solid solution matrix and brittle Mo₃Si and Mo₅SiB₂ (T2) intermetallic phases are candidates for ultra-high-temperature applications_. The elevated temperature uniaxial tensile response at a nominal strain rate of 10^?4 s^–1 and the tensile creep response at constant load between 1000 °C and 1300 °C of a (i) single phase solid solution (Mo–3.0Si–1.3B in at.%), (ii) two-phase alloy containing ～35 vol.% T2 phase (Mo–6Si–8B in at.%) and (iii) three-phase alloy with ～50 vol.% T2 + Mo₃Si phases (Mo–8.6Si–8.7B in at.%) were evaluated. The results confirm that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. A Larson–Miller plot of the creep data showed improved creep resistance of the two- and three-phase alloys in comparison with Ni-based superalloys. The extent of Si dissolved in the solid solution phase varied in these three alloys and Si appeared to segregate to dislocations and grain boundaries. A stress exponent of ～5 for the solid solution alloy and ～7 at 1200 °C for the two multiphase alloys suggested dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation was observed and the precipitation kinetics appear to be affected by the test temperature and applied stress.     

Environmental Resistance of Mo–Si–B Alloys and Coatings

For high temperature application beyond the range of Ni-base superalloys, multiphase Mo–Si–B alloys with compositions, that yield the ternary intermetallic Mo₅SiB₂ (T₂) phase as a key microstructure constituent together with the Mo and Mo₃Si phases, offer an attractive balance of high melting temperature, oxidation resistance and mechanical properties. The investigation of reaction kinetics involving the T₂ phase enables the analysis of oxidation in terms of diffusion pathways and the design of effective coatings. From this basis kinetic biasing is used together with pack cementation to develop multilayered coatings and in situ diffusion barriers with self-healing characteristics for enhanced oxidation resistance. While a combustion environment contains water vapor that can accelerate attack of silica based coatings, the current pack cementation coatings provide oxidation resistance in water vapor up to at least 1500 °C. An exposure to hot ionized gas species generated in an arc jet confirms the robust coating performance in extreme environments.     

Influence of Mo in the structure of rapidly solidified Fe–Mo–Cr–Mn–Si–C alloy

Abstract

An Fe–Mo–Cr–Mn–Si–C alloy was prepared in an induction furnace and was cast into cylindrical rod in a copper mould in castmatic equipment (low pressure casting). A single phase non-equilibrium featureless (no visible microstructures after deep etching) phase was observed over a certain range of thickness of the rod. In this present work, the extent of the featureless phase was studied with different concentrations of Mo (5–25 wt-%) for 5·5 mm diameter of cylindrical rod at a cooling rate of 1100 K s^–1. Light optical microscopy, scanning electron Microscopy and Vickers hardness tests were used to analyse the samples. The amount of the featureless area varies as the Mo content changes and the maximum featureless area was obtained for 7 wt-% of Mo. This single phase featureless structure exhibits very high hardness (>1350 HV) which can be used in many interesting applications with or without suitable heat treatments.     

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.     

Enhanced Oxidation Resistance of Mo–Si–B–Ti Alloys by Pack Cementation

As new high-temperature structural materials, Mo–Si–B alloys satisfy several requirements such as oxidation and creep resistance. Recently, novel Ti-rich Mo–Si–B alloys have shown an increased creep resistance compared to Ti-free alloys. However, due to the formation of a duplex SiO₂–TiO₂ oxide layer, which allows for fast ingress of oxygen, the oxidation resistance is poor. To improve the oxidation resistance, a borosilicate-based coating was applied to a Mo–12.5Si–8.5B–27.5Ti (in at.%) alloy. After co-deposition of Si and B by pack cementation at 1000 °C in Ar, a conditioning anneal at 1400 °C is used to develop an outer borosilicate layer followed by an inner MoSi₂ and Mo₅Si₃ layer. During both isothermal and cyclic oxidation after an initial mass loss during the first hours of exposure, a steady state is reached for times up to 1000 h at temperatures ranging from 800 to 1200 °C, demonstrating a significantly enhanced oxidation resistance.     

Phase stability and structural defects in high-temperature Mo–Si–B alloys

For high-temperature application beyond the range of Ni-base superalloys, Mo–Si–B alloys with composition that yield the ternary intermetallic, Mo₅SiB₂, T₂, phase as a key microstructure constituent, offer an attractive property balance of high-melting temperature, oxidation resistance, and useful high-temperature mechanical properties. With the T₂ ternary phase as the focal point of the microstructure designs, the fundamental basis of the alloying behavior in T₂ including the mutual solid solution with a wide range of transition metals has been established in terms of the governing geometric and electronic factors. For non-stoichiometric compositions, constitutional defects such as vacancies for Mo-rich compositions and anti-site defects for Mo-lean compositions control the homogeneity range. Moreover, the aggregation of constitutional vacancies has been discovered to play a key role in the development of dislocation and precipitation reactions in the T₂ phase that directly impact high-temperature structural performance. The characteristically sluggish diffusion rates within the T₂ phase have also been quantified and applied to the materials processing strategies. The materials design based on the phase stability, diffusion and defect structure analysis in the Mo–Si–B system can also be applied to the design of new multiphase high-temperature alloys with balanced environmental and mechanical properties.     

Resistance of a Mo–Si–B-Based Coating to Environmental Salt-Based Hot Corrosion

Oxidation of Metals - During operation of gas turbine engines, different salts develop that lead to a hot-corrosion attack. While the hot corrosion associated with Na2SO4 and Na3VO4 has received...     

Influence of vibrational entropy on structural stability of Nb–Si and Mo–Si systems at elevated temperatures

The heats of formation of stable and metastable phases of the Nb–Si and Mo–Si systems were studied using density functional theory (DFT). The high-temperature behavior of the competing phases was studied by performing additional phonon calculations. Our theoretical results rationalize the major differences observed in the behavior of the Nb–Si and Mo–Si systems: Nb₃Si is only stable at temperatures above 2043 K, whereas Mo₃Si is always stable; Nb₅Si₃ and MoSi₂ undergo phase changes at elevated temperatures, in contrast to Mo₅Si₃ and NbSi₂. These differences are qualitatively explained by including the vibrational entropy to the free energies within the harmonic approximation. In particular, the softer shear moduli of the Nb₅Si₃ and MoSi₂ βphases cause their stabilities over the α phases at elevated temperature.     

Friction welding of Al–Mg–Si alloy to Ni–Cr–Mo low alloy steel

Abstract

It is difficult to weld the dissimilar material combination of aluminium alloys and low alloy steels using fusion welding processes, on account of the formation of a brittle interlayer composed of intermetallic compound phases and the significant difference in physical and mechanical properties. In the present work an attempt has been made to join these materials via the friction welding method, i.e. one of the solid phase joining processes. In particular, the present paper describes the optimisation of friction welding parameters so that the intermetallic layer is narrow and joints of acceptable quality can be produced for a dissimilar joint between Al-Mg-Si alloy (AA6061) and Ni-Cr-Mo low alloy steel, using a design of experiment method. The effect of post-weld heat treatment on the tensile strength of the joints was then clarified. It was concluded that the friction time strongly affected the joint tensile strength, the latter decreasing rapidly with increasing friction time. The highest strength was achieved using the shortest friction time. The highest joint strength was greater than that of the AA6061 substrate in the as welded condition. This is due to the narrow width of the brittle intermetallic layer generated, which progressed from the peripheral (outer surface) region to the centreline region of the joint with increasing friction time. The joints in the as welded condition could be bent without cracking in a bend test. The joint tensile strength in the as welded condition was increased by heat treatment at 423 K (150° C), and then it decreased when the heat treatment temperature exceeded 423 K. All joints fractured in the AA6061 substrate adjacent to the interface except for the joints heated at 773 K (500° C). The joints fractured at the interface because of the occurrence of a brittle intermetallic compound phase.     

Fracture and fatigue resistance of Mo–Si–B alloys for ultrahigh-temperature structural applications

Fracture and fatigue properties are examined for a series of Mo–Mo₃Si–Mo₅SiB₂ alloys, which utilize a continuous α-Mo matrix to achieve unprecedented room-temperature fracture resistance (>20 MPa√m). Mechanistically, these properties are explained in terms of toughening by crack trapping and crack bridging by the more ductile α-Mo phase.     

High temperature oxidation response of Al/Ce doped Mo–Si–B composites

A systematic investigation has been performed to study the oxidation response of Mo₇₆Si₁₄B₁₀ alloy doped with traceable amount of Ce and Al at temperature in the range of 900–1300 °C. The resistance to degradation of Mo₇₆Si₁₄B₁₀ ≥ 900 °C has increased due to the dissociation of Mo-oxides into Mo in protective glassy borosilicate layer. Addition of Ce aggrieves oxidation initially, but protection is achieved upon exposure within 3 h at all the different temperatures. No passivation has occurred in Al containing alloy due to the dissolution of Al₂O₃ in the borosilicate scale leading to the precipitation of aluminum-borate and mullite along with unreacted cristoballite.     

Deformation of a multiphase Mo–9.4Si–13.8B alloy at elevated temperatures

A 3-phase silicide alloy, Mo–9.4Si–13.8B (at.%), was prepared via powder metallurgy techniques. The tensile properties of the alloy at elevated temperatures were evaluated in vacuum at temperatures ranging from 1350 to 1550°C and strain rates ranging from 5.0×10⁻⁴ to 1×10⁻³ s⁻¹. The alloy was found to exhibit a stress exponent of about 2.8 and relatively a high activation energy 740 kJ/mol. Also, it displayed unusually large tensile ductility (>100%) at T>1400°C. The deformation mechanism as well as large ductility are discussed in the light of the microstructural observations. The alloy has a very good mechanical strength at elevated temperatures, comparable to some of the most advanced tungsten-based alloys.     

Liquidus projection for the Mo-rich portion of the Mo–Si–B ternary system

The solidification behavior in the Mo-rich portion of the Mo–Si–B ternary system has been examined based on the microstructures of arc-cast alloys. Several solidification characteristics in the Mo-rich portion of the system have been identified using XRD, SEM and TEM. The liquidus projection in the Mo-rich portion includes six primary solidification reactions; five reactions originating from the Mo–Si and Mo–B binary sections (Mo, Mo₂B, βMoB, Mo₃Si and Mo₅Si₃) and one from the ternary-based (Mo₅SiB₂) T₂ phase. The liquidus surface in general descends from the high melting temperature Mo–B binary side to the lower melting temperature Mo–Si binary side. The solidification path of alloys with compositions in the T₂ phase region is always preceded by the peritectic reaction of βMoB+L⇒T₂. In addition, four Class II reactions (four-phase reactions) and one Class I reaction (invariant ternary eutectic reaction) have been identified. The extent of solidification segregation in alloys with compositions in the Mo(ss)+T₂ two-phase field is discussed as it pertains to materials processing.     

Oxidation mechanisms in Mo-reinforced Mo5SiB2(T2)–Mo3Si alloys

Results of a systematic investigation of the oxidation in a Mo–Si–B alloy containing the three phases, Mo, Mo₃Si, and Mo₅SiB₂ (T2) are presented. The relative kinetics of B-containing silica-scale formation, permeation of MoO₃ through the viscous scale, the viscosity of the B-containing SiO₂ scale, volatilization of MoO₃ and B₂O₃ from the silica scale, are identified as key parameters that determine the kinetics of the oxidation of the alloy in the temperature range of 500–1300 °C. The oxidation is worst in the intermediate temperature range, 650–750 °C, where MoO₃ begins to volatilize but B₂O₃ does not, resulting in gaseous MoO₃ bubbling through a low viscosity borosilicate scale. In this temperature range, the scale provides insufficient protection suggesting that attempts to improve the oxidation resistance of this system must focus on this temperature range.     

Structural materials: metal–silicon–boron: On the melting behavior of Mo–Si–B alloys

Phase relations in the entire ternary system Mo–Si–B were studied at subsolidus temperatures on alloys prepared by arc-melting and employing Pirani–Althertum melting point data, differential thermal analysis, light and electron microscopy, X-ray diffraction, and electron probe microanalysis including the light element boron. All isothermal reaction temperatures measured were used to construct two vertical sections and a Schulz–Scheil flow diagram monitoring solidification (crystallization) in thermodynamic equilibrium over the whole concentration range. The ternary compound Mo₅SiB₂ exhibits a solubility range from 8 to 13 at.% Si at solidus temperatures. Stoichiometric Mo₅SiB₂ forms from a pseudobinary peritectic reaction with a maximum tie line at 2130 °C. The phase diagram is presented as a melting diagram projection.     

Microstructural characterization of Mo–12Si–8.5B alloy subjected to diffusion annealing treatment

Microstructural evolutions in the solidification and subsequent diffusion annealing treatment of Mo–12Si–8.5B alloys have been studied by using XRD, OM, SEM and TEM. Because the annealing temperature (1900 °C) is in the vicinity of four-phase equilibrium eutectic reaction (1950 °C) point of L = Mo₃Si + Mo₅SiB₂ (T₂)+Mo_ss (Mo solid solution), the dendrites and lamellar phase have almost been dissolved. Dislocations, Mo_ss precipitates and subgrain boundary have been observed in the T₂ phase while the other phases are not found. In addition, dislocations are from excess vacancy during the annealing treatment and play important roles for the Mo_ss precipitate. The dislocations can serve as the heterogeneous nucleation sites and compose of sub-boundary which refines the grain size to some extent. The results observed in this study can be used as a reference for future work on the relationships between microstructure and mechanical property of the Mo–Si–B alloys.     

Transient oxidation of Mo–Si–B alloys: Effect of the microstructure size scale

The composition and phase constituency of Mo–Si–B alloys are known to be important parameters in determining the oxidation response. For three phase Mo + T₂ + Mo₃Si alloys with constant composition and phase constituency, it is observed that a refined microstructure scale provides superior oxidation resistance. The transient stage of oxidation is shortened and the recession of the alloy is decreased with microstructural refinement. In order to identify the phase interaction during the transient stage, oxidation of each of the three alloy phases, Mo, Mo₃Si (A15) and Mo₅SiB₂ (T₂) has been investigated separately. Quantification of the separate phase size distributions by image analysis was coupled with the individual phase oxidation response to evaluate the overall oxidation behavior and phase interaction effects. A kinetic model for oxidation of Mo–Si–B alloys is proposed that incorporates the key role of microstructure scale on the transient stage and provides guidance for microstructure design.     

Effect of Zr Addition on the High-Temperature Oxidation Behaviour of Mo–Si–B Alloys

Mo–Si–B alloys are promising candidates for structural high-temperature applications due to their excellent high-temperature mechanical properties along with high melting temperatures and oxidation resistance. After an initial period with high weight loss rates as a consequence of the volatilization of Mo-oxide, a protective borosilica (glass) layer develops on the alloy surface and steady-state oxidation is achieved. Aiming at improved mechanical properties of Mo–Si–B alloys which exhibit a continuous Mo solid solution matrix as a consequence of a powder metallurgical production route, small amounts of Zr were added. The presence of oxygen in the alloy leads to the formation of thermodynamically very stable Zr-oxide precipitates in the bulk alloy causing an enhancement of its mechanical properties. It was observed that the addition of Zr (distributed in the alloy matrix) also has significant influence on the oxidation behaviour of Mo–Si–B alloys by reducing the period for the formation of the protective and stable silica scale. Furthermore, the weight loss due to vaporization of Mo-oxides is consequently reduced. Besides this beneficial effect, Zr is harmful for the oxidation resistance at temperatures beyond 1,200 °C. This is mainly due to the increased oxygen transport through defects in the silica scale.     

Formation,stability and crystal structure of the σ phase in Mo–Re–Si alloys

The formation, stability and crystal structure of the σ phase in Mo–Re–Si alloys were investigated. Guided by thermodynamic calculations, six critically selected alloys were arc melted and annealed at 1600 °C for 150 h. Their as-cast and annealed microstructures, including phase fractions and distributions, the compositions of the constituent phases and the crystal structure of the σ phase were analyzed by thermodynamic modeling coupled with experimental characterization by scanning electron microscopy, electron probe microanalysis, X-ray diffraction and transmission electron microscopy. Two key findings resulted from this work. One is the large homogeneity range of the σ phase region, extending from binary Mo–Re to ternary Mo–Re–Si. The other is the formation of a σ phase in Mo-rich alloys either through the peritectic reaction of liquid + Mo_ss → σ or primary solidification. These findings are important in understanding the effects of Re on the microstructure and providing guidance on the design of Mo–Re–Si alloys.     

Microstructure and oxidation resistance of Mo–Si–B coating on Nb based in situ composites

The development of robust high temperature oxidation resistant coatings for Nb–Si based alloy was evaluated for a Mo–Si–B coating system that was applied by a two step process. It is observed that the coating is composed of an outer layer of MoSi₂ containing boride dispersoids and an inner layer of unreacted Mo. The mass gain of substrate and Mo–Si–B coating is 190.08 and 1.28 mg cm⁻² after oxidation at 1250 °C in dry air for 100 h, respectively. The good oxidation resistance of the coating is attributed to the formation of a continuous borosilicate glass coverage.