Oxidation behavior of multiphase Nb-Mo-Si-B intermetallics

The mixed substitution of Nb and Mo in the ternary systems Mo-Si-B and Nb-Si-B was studied with the goal of balancing oxidation resistance with mechanical behavior. The microstructure and oxidation behavior of six compositions in the Nb-Mo-Si-B system were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy, electron probe microanalysis (EPMA), and thermogravimetric analysis. Proper selection of the total metal content and the Nb/Mo ratio results in the co-existence of a T1 phase, as (Nb, Mo)₅Si₃B_x, and a solid solution (Nb,Mo) metal phase. At 800 °C, all compositions exhibited catastrophic oxidation, while changing to a quasi-steady-state mass gain at 1200 °C. The high rate constants at 1200 °C indicate that the scales formed were not passivating. A complex scale consisting of four layers formed that was about 350-to 450-μm thick after oxidation for 50 hours at 1200 °C. Borosilicate glass did form within the scale, but the significant prevalence of Nb₂O₅ within the glass, and the resulting inability of the glass to seal pores formed by the evaporation of MoO₃, contributed to the overall poor oxidation resistance compared to the ternary Mo-Si-B system. The Nb and Mo content of the alloy must be further studied and optimized before these alloys may be considered for further development for hightemperature applications. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Development of Mo(Si,Al)<Subscript>2</Subscript>-base oxidation-resistant coating on Nb-base structural materials

Mo(Si,Al)₂-base oxidation-resistant coatings for Nb-base structural materials have been studied. The coating is composed of a Mo(Si,Al)₂-base Al reservoir and Al₂O₃ interlayer to suppress interface reactions between the Al reservoir and the substrate. To develop a suitable Al-reservoir material, some Mo(Si_0.6,Al_0.4)₂-HfB₂ composites were prepared. Their oxidation resistance and coefficients of thermal expansion were investigated, in addition to their chemical reactivity with the Nb substrate at high temperatures. As a result, Mo(Si_0.6,Al_0.4)₂-20 vol pct HfB₂ was selected as one of the satisfactory Al reservoirs. The introduction of a stable Al₂O₃ interlayer was attempted using a novel powder metallurgical process to overlay the Nb substrates with the Al reservoir, where the Nb substrates were subjected to a slight surface oxidation prior to the coating process. The Nb specimens, which are thoroughly coated with the Al reservoir and Al₂O₃ interlayer, can be successfully fabricated by this method. The coated Nb specimens are not damaged at all after prolonged exposure in flowing Ar-20 pct O₂ at 1673 K for 120 hours. Furthermore, the Al₂O₃ interlayer is very effective and no reactions occur at the interface. Thus, this Mo(Si,Al)₂-base oxidation-resistant coating is applicable to Nb. The utility of the coating system is also confirmed for a Nb_SS/Nb₅Si₃ composite. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

A Study on the Oxidation Behavior of Nb Alloy (Nb-1 pct Zr-0.1 pct C) and Silicide-Coated Nb Alloys

In the current work, silicide coatings were produced on the Nb alloy (Nb-1 pct Zr-0.1 pct C) using the halide activated pack cementation (HAPC) technique. Coating parameters (temperature and time) were optimized to produce a two-layer (Nb₅Si₃ and NbSi₂) coating on the Nb alloy. Subsequently, the oxidation behavior of the Nb alloy (Nb-1 pct Zr-0.1 pct C) and silicide-coated Nb alloy was studied using thermogravimetric analysis (TGA) and isothermal weight gain oxidation experiments. Phase identification and morphological examinations were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. TGA showed that the Nb alloy started undergoing accelerated oxidation at and above 773 K (500 °C). Isothermal weight gain experiments carried out on the Nb alloy under air environment at 873 K (600 °C) up to a time period of 16 hours exhibited a linear growth rate law of oxidation. In the case of silicide-based coatings, TGA showed that oxidation resistance of silicide coatings was retained up to 1473 K (1200 °C). Isothermal weight gain experiments on the silicide coatings carried out at 1273 K (1000 °C) in air showed that initially up to 8 hours, the weight of the sample increased, and beyond 8 hours the weight of the sample remained constant. The oxide phases formed on the bare samples and on the coated samples during oxidation were found to be Nb₂O₅ and a mixture of SiO₂ and Nb₂O₅ phases, respectively. SEM showed the formation of nonprotective oxide layer on the bare Nb alloy and a protective (adherent, nonporous) oxide layer on silicide-coated samples. The formation of protective SiO₂ layer on the silicide-coated samples greatly improved the oxidation resistance at higher temperatures.     

Microstructural properties of Nb-Si alloys investigated using EBSD at large and small scale

Texture and crystallographic orientation relationships in arc-melted hypoeutectic and hypereutectic binary Nb-Si alloys are investigated. Electron backscattered diffraction (EBSD) is used here in conventional conditions, i.e., at relatively high spatial resolution (<1 μm) for ∼400×400 μm fields, as well as on very large fields (1.1×1.1 mm), at lower resolution, to get a statistical overview of the microstructure. In as-cast Nb-16Si and Nb-22Si alloys (compositions are in at. pct), [001]_Nb3Si is found parallel to the local thermal gradient, with Nb₃Si + Nb eutectic cells, giving rise to a microstructure similar to that obtained by directional solidification. In Nb-22Si alloy, the following orientation relationships between poles of metallic and silicide phases have been found: (111)_Nb//(111)_Nb3Si (as cast), (011)_Nb//(011) _α-Nb5Si3, and (111)_Nb//(100) _α-Nb5Si3 (heat treated at 1500 °C, 75 hours). This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Toughening of MoSi2 with a ductile (niobium) reinforcement

The effect of interfacial reactions and Y₂O₃ coatings on toughening of MoSi₂ by ductile phase Nb reinforcements has been investigated. In the absence of coating the interfacial reaction layer exhibits parabolic growth with Mo₅Si₃, (Mo, Nb)₅Si₃, (Nb, Mo)₅Si₃ and Nb₅Si₃ phases forming. In precracked laminates subjected to tensile loads the ductile phase deformation is partially constrained, with debonding occurring within the interfacial reaction zone. Dense Y₂O₃ coating inhibits interdiffusion and results in more extensive debonding. In either case, significant toughening is expected with measured work of rupture values χ ≈ 5.7 to 6.3. Bulk composite MoSi₂ reinforced with 20 vol.% Nb particles subjected to a chevron-notched three point flexure test had a work of rupture almost five times larger than the unreinforced MoSi₂ matrix.     

Effect of Nb and Nb5Si3 Powder Size on Microstructure and Fracture Behavior of an Nb-16Si Alloy Fabricated by Spark Plasma Sintering

The influence of Nb and Nb₅Si₃ powder sizes (Nb: 83.8, 37.4, 4.9 μm; Nb₅Si₃: 86.4, 43.6, 2.9 μm) on microstructure and fracture behavior of an Nb-16Si alloy fabricated by spark plasma sintering (SPS) was investigated. The results revealed that all as-sintered samples consisted of Nb and Nb₅Si₃ phases, with no new phases formed during SPS. The morphologies of the Nb and Nb₅Si₃ phases in the as-sintered samples depended on the original Nb and Nb₅Si₃ powder sizes. Large Nb₅Si₃ powders 86.4 and 43.6 μm in combination with Nb powder of described sizes resulted in an Nb matrix plus Nb₅Si₃ island biphase microstructure, which supplied excellent fracture toughness. The microstructure made from the finest Nb powder (4.9 μm) and the largest Nb₅Si₃ powder (86.4 μm) had a high fracture toughness of 12.4 MPa m^1/2 through a mixed fracture mechanism of dimple, tear, and cleavage from the small Nb grains. When the Nb₅Si₃ powder size was decreased to 2.9 μm, the sample tended to form an Nb₅Si₃ matrix plus Nb island biphase microstructure, which exhibited a poor fracture toughness value of between 5.6 and 8.0 MPa m^1/2 due to the crack propagation mainly within the brittle Nb₅Si₃ phase.     

Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

Microstructure of a complex Nb–Si-based alloy and its behavior during high-temperature oxidation

A in-situ composite Nb–Si–Ti–Hf–Cr–Mo–Al composite material alloyed with yttrium and zirconium is studied. The evolution of the structure–phase state of the alloy during oxidation under dynamic and isothermal conditions is considered on samples prepared by vacuum remelting and directional solidification. The phase composition and the microstructure of the alloy are examined by the methods of physico-chemical analysis, and the distribution of alloying elements in initial samples and the products of oxidation is estimated. Thermogravimetric experiments are performed on powders and compacted samples during continuous (in the range 25–1400°C) and isothermal (at 900 and 1100°C) heating in air. The directional solidification of an Nb–Si–Ti–Al–Hf–Cr–Mo–Zr–Y is found to cause the formation of an ultradispersed eutectic consisting of α-Nb_ss and γ-Nb₅Si_3ss cells. The as-cast sample prepared by vacuum remelting has a dendritic structure and contains Nb₃Si apart from these phases. Oxidation leads to the formation of a double oxide layer and an inner oxidation zone, which retain the two-phase microstructure and the ratio of alloying elements that are characteristic of the initial alloy. Diffusion redistribution is only detected for molybdenum. The cyclicity of heating at the initial stage of oxidation weakly influences the oxidation resistance of the alloy.     

The effect of Nb and W alloying additions to the thermal expansion anisotropy and elastic properties of Mo<Subscript>5</Subscript>Si<Subscript>3</Subscript>

The directional thermal expansion and elastic properties of Mo₅Si₃, (Mo_0.8Nb_0.2)₅Si₃, and (Mo_0.85W_0.15)₅Si₃ have been studied as a function of temperature through the use of single crystals. Thermal expansion anisotropy was reduced by Nb and W alloying. The decrease in thermal expansion anisotropy by Nb alloying was only found to occur at low temperatures, and thermal expansion anisotropy of (Mo_0.8Nb_0.2)₅Si₃ was similar to that for the other two compounds at 800 °C. Values for the polycrystalline Young’s, bulk, and shear moduli calculated from the measured single-crystal elastic constants are reduced by Nb alloying, and increased by W alloying at all temperatures studied. The elastic modulus E was calculated for the orientations between [100]-[001] and [100]-[010]. In contrast to the effects of Nb on thermal expansion anisotropy, Nb alloying increased the E _[001]/E _[100] elastic anisotropy. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 3. Stability of Silicide Coatings on Niobium Heated in Air at 1500-1800°C

The kinetics of phase redistribution in the (Mo, W)Si₂ Nb system at 1500-1800°C was investigated. The kinetic parameters for growth of the lower silicides (Mo, W, Nb) ₅Si₃ + Nb₅Si₃ and decrease in the layer thickness of the higher silicide (Mo, W)Si₂ as function of the oxidation temperature were determined. It was established that the stability of the multiphase and multicomponent system was more than twice that of the system MoSi₂ Nb, and 15-18 times that of MoSi₂ Mo.     

Processing,microstructure, and mechanical properties of (Nb)/Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript> two-phase alloys

It has been shown that a fine lamellar structure composed of Nb solid solution, (Nb), and Nb₅Si₃ is formed through eutectoid decomposition in the Nb-Si binary system and its ternary derivatives. Such alloys would exhibit a high strength at over 1400 K, yet showing room-temperature toughness of over 10 to 20 MPa m^1/2 if a proper lamellar spacing is chosen. In the present work, effects of processing on the microstructure evolution and mechanical properties are investigated on the Nb-18 at. pct Si alloys prepared by hot pressing (HP) and spark-plasma sintering (SPS). The powders used in the present work are of pure Nb and Nb₅Si₃ in order for the fabrication to become possible at temperatures higher than the melting point of Si and to reduce the formation of SiO₂. The results show that the SPS yields more uniform two-phase microstructure but the alloy fabricated through HP tends to provide higher elevated temperature strength. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Effects of Zr on the eutectoid decomposition behavior of Nb<Subscript>3</Subscript>Si into (Nb)/Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>

The effect of Zr on the formation of Nb/Nb₅Si₃ lamellar microstructure by eutectoid decomposition reaction of Nb₃Si is investigated. It has been shown that the kinetics of the eutectoid decomposition of high-temperature Nb₃Si phase into Nb and Nb₅Si₃ phases are sluggish in the binary Nb-Si system and that they are enhanced by Zr additions. The time-temperature-transformation (TTT) diagram for the decomposition is experimentally determined and the acceleration of the reaction by small Zr addition of 1.5 at. pct is confirmed by comparison with the reported TTT curves of binary and ternary alloys containing Ti. The role of the ternary element on the decomposition kinetics is discussed in terms of crystallographic orientation relationships (ORs) and Zr distribution in the parent Nb₃Si phase during solidification. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Fabrication and evaluation of Nb/Nb5Si3 microlaminate foils

Alloys of Nb and Nb₅Si₃, and in particular Nb/Nb₅Si₃ microlaminates, have potential as high-temperature materials. In this study, microlaminates of Nb and amorphous Nb-37.5 at. pct Si are magnetron sputter deposited from elemental Nb and polycrystalline Nb₅Si₃ targets. The microlaminates are heat treated at high temperatures to produce crystalline layers of Nb and Nb₅Si₃ that are flat, distinct, and stable for at least 3 hours at 1200 °C. The layers consist of textured Nb grains and equiaxed submicron Nb₅Si₃ grains. Initial room-temperature tensile tests indicate that the microlaminates have strengths similar to cast and extruded alloys of Nb and Nb₅Si₃. The fracture mode of the Nb layers is dependent on the Nb layer thickness, with thin layers failing in a ductile manner and thick layers failing by cleavage. The Nb layers bridge periodic cracks in the Nb₅Si₃ layers, and using a shear lag analysis, the tensile strength of Nb₅Si₃ is estimated. The results indicate that microstructurally stable and mechanically robust microlaminates of Nb and Nb₅Si₃ can be fabricated by sputter deposition with a high-temperature heat treatment. The processing, microstructure, and mechanical properties of these microlaminates are discussed.     

The effect of nb morphology on the cyclic oxidation resistance of MoSi2/20 vol pct Nb composites

The effect of Nb morphology on the 1200 °C cyclic oxidation resistance of MoSi₂/20 vol Pct Nb composites was investigated. Niobium was incorporated into MoSi₂ as particles, random short fibers, and continuous aligned fibers. After oxidation, it was found that all the composites had lost weight and essentially disintegrated. This was attributed to spalling of both the Nb₂O₅ scale and the MoSi₂ matrix. The spalling of the matrix was a result of cracks originating in the oxidized Nb and propagating through the MoSi₂ matrix. These cracks arose from two sources: (1) the volume expansion associated with the transformation of Nb to Nb₂O₃ and (2) the difference in thermal expansion between Nb₂O₅ and MoSi₂. However, it was found that the addition of smaller diameter Nb reinforcements tended to retard the disintegration of the composites. This was attributed to the effect of reinforcement size on CTE mismatch cracking. D.E. Alman, Formerly Graduate Student, Materials Engineering Department, Rensselaer Polytechnic Institute     

The stability of Nb/Nb5Si3 microlaminates at high temperatures

The microstructural, phase, and chemical stability of Nb/Nb₅Si₃ microlaminates was investigated at temperatures ranging from 1200 °C to 1600 °C. Freestanding Nb/Nb₅Si₃ microlaminates were prepared by sputter deposition and their stability was investigated by annealing either in vacuum or in an Ar atmosphere. The microlaminates were generally structurally stable, with no evidence of layer pinchoff, even after annealing at 1600 °C. However, a small volume fraction (<2 pct) of voids formed in the silicide layers at 1500 °C and 1600 °C, which are attributed either to the Kirkendall diffusion of Si or to the growth of silicide grains. In terms of phase stability, there was no discernible dissolution of the Nb₅Si₃ layers and no silicide precipitates in the Nb layers following anneals at 1400 °C. Annealing at higher temperatures, though, resulted in the formation of non-equilibrium Nb₃Si on the Nb/Nb₅Si₃ interfaces. This phase is thought to precipitate from the supersaturated Nb-Si solid solution on cooling, and is stabilized by the development of tensile stresses in the Nb layers. The most pervasive observed high-temperature breakdown mechanism was chemical in nature, namely, the loss of Si via sublimation to the environment. The Si loss was partially suppressed either by annealing in a Si-rich atmosphere or by annealing in Ar.     

Toughness and strength characteristics of Nb-W-Si ternary alloys prepared by Arc melting

This article describes the room-temperature and high-temperature mechanical properties and failure modes of series Nb-W-Si alloys—Nb-10W, Nb-10Si, Nb-10Si-5W, Nb-10W-5Si, and Nb-10W-10Si—prepared by arc melting. For the Nb-10W alloy, the microstructure was a monolithic Nb solid solution (Nb _ss ) with a grain size up to a few hundred microns, while the other four alloys consisted of primary Nb _ss and a eutectic of Nb _ss /Nb₅Si₃ (5-3 silicide) as a result of replacing Nb with Si. Among all alloys, the Nb-10W showed the highest fracture toughness of about 15.3 MPa√m^1/2 and the lowest 0.2 pct yield compressive strength of 90 MPa at 1670 K. Conversely, the Nb-10Si-10W had the highest 0.2 pct yield strength of about 330 MPa at 1670 K and the lowest fracture toughness of 8.2 MPa√m^1/2. It is suggested that toughness is supplied by the metallic Nb _ss phase, while high-temperature strength is mainly provided by the brittle silicide phase. For the Nb-10W alloy with the monolithic Nb _ss , intergranular cleavagelike crack propagation is the fracture mode at room temperature, and dislocation movement within the grains and grain-boundary sliding are the dominant modes of high-temperature failure. With two-phase Nb _ss /Nb₅Si₃ microstructures, the compressive damage of all four alloys at high temperature was dominated by debonding of the interfaces between the Nb _ss and the silicide; however, the fracture mode at room temperature is transgranular, controlled by the primary Nb _ss cleavage.     

Mechanical properties of As-cast and directionally solidified Nb-Mo-W-Ti-Si <Emphasis Type="Italic">in-situ</Emphasis> composites at high temperatures

To improve the high-temperature strength of Nb-Mo-Ti-Si in-situ composites, alloying with W and a directional solidification technique were employed. The alloy composition of Nb-xMo-10Ti-18Si (x=10 or 20) was used as the base, and Nb was further replaced by 0, 5, 10 and 15 mol pct W. For samples without W, the as-cast microstructure was a eutectic mixture of fine Nb solid solution (Nb _SS ) and (Nb, Me)₅ Si₃ silicide (Me = Mo, W, or Ti), while large primary Nb _SS particles appeared besides the eutectic mixture as a result of replacing Nb by W. The directionally solidified samples consisted of coarse Nb _SS and (Nb,Me)₅ Si₃ silicides, and the microstructure showed a slight orientation in the direction of growth. The maximum compressive ductility (ɛ _max) at room temperature decreased with increasing W content and was in the range of 0.8 to 2.3 pct, in contrast to the Vickers hardness (HV), which increased with W content. The 0.2 pct yield compressive strength (σ _0.2) and the specific 0.2 pct yield compressive strength (σ _0.2S ) (σ _0.2 divided by the density of sample) at elevated temperatures were markedly improved by the W addition. The directionally solidified samples always showed higher σ _0.2 and σ _0.2S values than the as-cast samples. At elevated temperatures, the directionally solidified sample with 10 mol pct Mo and 15 mol pct W had the highest σ _0.2 and σ _0.2S values; even at 1770 K, σ _0.2 was as high as 650 MPa. The directionally solidified materials alloyed with W exhibited excellent compressive creep performance. The sample with 10 mol pct Mo and 15 mol pct W had a minimum creep rate of 1.4×10⁻⁷s⁻¹ and retained steady creep deformation at 1670 K and an initial stress of 200 MPa. Under compression, the damage and failure of these in-situ composites were dominated by decohesion of interfaces between the Nb _SS and silicide matrix.     

Role of Ti on Phase Evolution,Oxidation and Nitridation of Co–30Ni–10Al–8Cr–5Mo–2Nb–(0, 2 & 4) Ti Cobalt Base Superalloys at Elevated Temperature

Titanium is an important alloying addition to γ/γ′ cobalt-based superalloys that enhances the high temperature microstructural stability and make the alloys lighter. In this work, we probe the role of Ti composition on the phase stability and oxidation behavior of Co–30Ni–10Al–8Cr–5Mo–2Nb superalloys. With Ti addition, the γ′-solvus temperature is enhanced and the γ′-precipitate shape changes from spherical to rounded cuboids. Addition of 4 at. pct Ti to the alloy promotes topologically-close-packed (TCP) phase formation that are rich in Co, Cr, and Mo. During oxidation at 900 °C, Ti was found to facilitate the early formation of passivating oxide layers (spinel CoCr₂O₄/CoAl₂O₄) on the exposed surfaces, however, it was not effective in reducing the oxidation-induced mass gain. Microstructural analysis reveals that Ti delays the Al₂O₃ layer formation eventually leading to faster oxidation kinetics. Additionally, we also found formation of (Ti,Nb)N in the γ′ denuded zones near the alloy-oxide interface.

     

Microstructural Evolution and Mechanical Behaviors of an Nb-16Si-22Ti-2Al-2Hf Alloy with 2 and 17 at. pct Cr Additions at Room and/or High Temperatures

X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy were used to investigate the microstructures and orientation relationships (ORs) of Nb-16Si-22Ti-2Al-2Hf-(2,17)Cr alloys (hereafter referred to as 2Cr and 17Cr alloys, respectively). The mechanical properties of the two alloys at room and/or high temperatures were compared. The 2Cr alloy comprised Nb_SS and (α + β)-Nb₅Si₃ phases, while the 17Cr alloy consisted of Nb_SS, (α + β)-Nb₅Si₃ and Laves Cr₂Nb phases with a C15 structure. The β-Nb₅Si₃ and Laves Cr₂Nb phases exhibited variable ORs with respect to the Nb_SS phase. The Laves Cr₂Nb phase was found to play a negative role on the fracture toughness at room temperature and on the compressive strength at temperatures from 1523 K to 1623 K (1250 °C to 1350 °C). The fracture toughness and the compressive yield strength at 1623 K (1350 °C) both decreased from 14.4 to 10.3 MPa m^1/2 and from 300 to 85 MPa, respectively, when the nominal Cr content increased from 2 to 17 at. pct. Finally, the fracture modes of these typical Nb_SS/Nb₅Si₃ and Nb_SS/Nb₅Si₃/Cr₂Nb microstructures under bending and compression conditions at room and high temperatures were investigated and discussed.     

Atomic diffusion and phase equilibria at the interfaces of the CoAl/Ir multilayer on Nb<Subscript>5</Subscript>Si<Subscript>3</Subscript>-base alloys

Atomic diffusion and phase equilibria have been investigated at the interfaces of Ir/CoAl and Ir/Nb₅Si₃ to evaluate the suitability of a diffusion-barrier layer of Ir between an oxidation-resistant layer of B2-CoAl and a base material Nb₅Si₃. Diffusion couples were prepared by hot pressing and annealed at 1573 K for up to 178 hours. Diffusion layers of (Ir, Co) solid solution and B2-(Ir, Co)Al were formed at the Ir/CoAl interface. The concentration of Al dramatically dropped at the interface, which indicates that the Ir layer effectively works as the diffusion barrier against the inward diffusion of Al. To quantitatively evaluate the potential of Ir as a diffusion barrier, the Boltzmann-Matano analysis was employed to determine the diffusion coefficient of Al using Ir-8 at. pct Al/Ir diffusion couples annealed at temperatures of 1573, 1673, and 1773 K. For instance, an extremely low value of 7.0×10⁻¹⁹ m²/s is evaluated for Ir-4 at. pct Al at 1573 K. At the Ir/Nb₅Si₃ interface, the intermetallic phases Ir₃Si and Ir₃Nb are formed on the Ir side and the Nb₅Si₃ side, respectively. The formation of Ir₃Si is controlled by the diffusion of Si through Ir₃Nb in which the solubility of Si is limited quite small. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.