Effects of Fe Addition on the Snoek-Type Damping Behavior of Surface-Oxidation-Treated Ti-Mo Alloys

Effects of Fe addition on the oxygen diffusion and the Snoek-type relaxation damping behavior of the Ti-15 wt pct Mo alloy were investigated in this study. After surface oxidation treatment, the Ti-15 wt pct Mo-1 wt pct Fe alloy exhibits a higher damping capacity compared to the Ti-15 wt pct Mo alloy. The dual-phase zone and the oxygen-enriched β-phase zone in the surface-oxidation-treated Ti-Mo alloys were determined by electron backscattered diffraction (EBSD) and hardness measurements. Based on the oxygen distributions in both alloys obtained through a diffusion model, the relative damping capacity of different zones contributing to the beam sample damping was estimated to be proportional to the thickness of the oxygen dissolved zones. On the other hand, the substitutional solute of Fe in the Ti-Mo-Fe alloy is considered to affect the oxygen distribution by lengthening the oxygen diffusion zone and increasing the oxygen concentration in this zone. As a result, the addition of Fe in Ti-Mo alloy improves the damping capacity of the surface-oxidation-treated alloys.     

Alloying mechanism of beta stabilizers in a TiAl alloy

The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti₅₂Al₄₈-xM (x=0, 1.0, 2.0, 4.0, or 6.0 at. pct and M=Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning of Fe, Cr, V, and Nb in the γ phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti₅₂Al₄₈-xM generally occur at the maximum solid solution of these elements in the γ phase, which is just before the formation of B2 phase. Ti₅₂Al₄₈-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti₅₂Al₄₈-1V, Ti₅₂Al₄₈-Cr, and Ti₅₂Al₄₈-2Nb alloys have about 1.1 to 1.3 pct elongation at room temperature. The increase of tensile strengths and creep resistance with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in the γ phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr]=Cr+Mn+3/5V+3/8Nb+3/2 (W+Mo)+3Fe=1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance at elevated temperatures in Ti₅₂Al₄₈-xM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in γ phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl.     

High-temperature oxidation of Ti3Al-based titanium aluminides in oxygen

The oxidation behavior of Ti-25Al, Ti-24Al-15Nb, and Ti-25Al-11Nb (at. pct) titanium aluminides was studied in dry oxygen at atmospheric pressure in the temperature range 1000 to 1300 K for 4 to 6 hours by thermogravimetry. The oxidation products were characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray (EDX) analysis. Although some departures from the parabolic rate law were found, the analysis of data revealed that the parabolic rate law was a more reliable basis for interpretation of results as compared to the empirical power law. Parabolic rate constants (k _p ) for Ti-24Al-15Nb and Ti-25Al-11Nb were almost the same at the same temperature. However, k _p values for Ti-25Al were 2 to 8 times larger than niobium-containing alloys except at 1000 K, where k _p values for all three alloys were approximately the same. The effective activation energy (Q _eff) was 289 kJ/mole for Ti-25Al in the range of 1000 to 1300 K, while in the case of Ti-24Al-15Nb and Ti-25Al-11Nb,Q _eff values were 329 and 330 kJ/mole, respectively, in the range of 1100 to 1300 K. In general, the scales on Ti-25Al were porous and exhibited significant spallation above 1100 K. The scales were thinner, compact, and adherent for niobium-containing alloys. The scales formed on these alloys were predominantly composed of TiO₂, while Al₂O₃ was also present as a minor constituent. The superior oxidation resistance of the niobium-containing alloys has been attributed to the doping effect of niobium in TiO₂.     

An investigation of the fracture and fatigue crack growth behavior of forged damage-tolerant niobium aluminide intermetallics

The results of a recent study of the effects of ternary alloying with Ti on the fatigue and fracture behavior of a new class of forged damage-tolerant niobium aluminide (Nb₃Al-xTi) intermetallics are presented in this article. The alloys studied have the following nominal compositions: Nb-15Al-10Ti (10Ti alloy), Nb-15Al-25Ti (25Ti alloy), and Nb-15Al-40Ti (40Ti alloy). All compositions are quoted in atomic percentages unless stated otherwise. The 10Ti and 25Ti alloys exhibit fracture toughness levels between 10 and 20 MPa√m at room temperature. Fracture in these alloys occurs by brittle cleavage fracture modes. In contrast, a ductile dimpled fracture mode is observed at room-temperature for the alloy containing 40 at. pct Ti. The 40Ti alloy also exhibits exceptional combinations of room-temperature strength (695 to 904 MPa), ductility (4 to 30 pct), fracture toughness (40 to 100 MPa√m), and fatigue crack growth resistance (comparable to Ti-6Al-4V, monolithic Nb, and inconnel 718). The implications of the results are discussed for potential structural applications of the 40Ti alloy in the intermediate-temperature (∼700 °C to 750 °C) regime.     

Effect of nitrogen on the oxidation behavior of Ti3Al-based intermetallic alloys

The effect of nitrogen on the oxidation behavior of Ti-25Al, T-24Al-15Nb, and Ti-25Al-11Nb (at. pct) has been examined in this study. The gases employed were nitrogen and oxygen-nitrogen and oxygen-argon mixtures in various proportions at a total pressure of 1 atmosphere. The experiments were carried out in the temperature range of 1100 to 1300 K by thermogravimetry. The suitability of employing the parabolic rate law as the basis of interpretation of weight gainvs time data has been discussed. Oxidation resistance of Nb-containing alloys was superior to that of binary Ti-25Al, irrespective of the gas composition employed. The nitridation rates of alloys, with or without Nb, were more than an order of magnitude lower than those for oxidation. The scales were characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray (EDX) analysis. The scales formed in all the conditions mostly consisted of TiO₂ and Al₂O₃. However, TiN was observed in scales of Nb-containing alloys in all nitrogen bearing gases and seemed to primarily account for improved oxidation resistance of the preceding in comparison to alloys without Nb. Nitrogen pretreatment was provided for some samples before oxidation for further elucidation of the role of nitrogen.     

An investigation of the effects of loading rate on resistance-curve behavior and toughening in cast lamellar gamma-based titanium aluminides

This article presents the results of a combined experimental and theoretical study of the effects of loading rate (1, 10, and 100 MPa√m · s⁻¹) on the resistance-curve behavior and toughening in cast lamellar gamma-based titanium aluminides (Ti-48Al-2Cr-2Nb, Ti-45Al-2Mn-2Nb + 0.8 vol pct TiB₂, and Ti-47Al-2Mn-2Nb + 0.8 vol pct TiB₂). Note that compositions are quoted in at. pct unless stated otherwise. The fracture-initiation toughness and resistance-curve behavior in Ti-48Al-2Cr-2Nb are shown to be similar at the three loading rates examined. In the case of the Mn-containing alloys, stronger resistance-curve behavior is observed as the loading rate increases from 1 to 10 MPa√m · s⁻¹. However, the fracture-initiation toughness and resistance-curve behavior of the Mn-containing alloys are similar at loading rates of 10 and 100 MPa√m · s⁻¹. The observed resistance-curve behavior is attributed largely to the role of ligament bridging and, to a lesser extent, to the effects of cracktip plasticity. Small- and large-scale bridging models are also shown to predict the measured resistance curves when the observed/measured bridging parameters and material properties are used in the micromechanical modeling of crack bridging. The implications of the results are also discussed for the design of damage-tolerant gamma alloys and microstructures.     

Intermetallic diffusion coatings for enhanced hot-salt oxidation resistance of nitrogen-containing austenitic stainless steels

This article presents the preparation, characterization, and hot-salt oxidation behavior of nitrogen-containing type 316L stainless steel (SS), surface modified with intermetallic coatings. Three different types of intermetallic coating systems, containing aluminum, titanium, and titanium/aluminum multilayers, were formed by diffusion annealing of type 316L austenitic SS containing 0.015, 0.1, 0.2, and 0.56 pct nitrogen. Analysis by using X-ray diffraction (XRD), scanning electron microscopy (SEM), and secondary ion mass spectroscopy (SIMS) confirmed the formation of various intermetallic phases such as AIN, Al₁₃Fe₄, FeAl₂, FeTi, Ti₂N, and Ti₃Al in the coatings. Hot salt oxidation behavior of the uncoated and surface-modified stainless steels was assessed by periodic monitoring of the weight changes of NaCl salt-applied alloys kept in an air furnace at 1023 K up to 250 hours. The oxide scales formed were examined by XRD and stereomicroscopy. Among the various surface modifications investigated in the present study, the results indicate that the titanium-modified alloys show the best hot-salt oxidation resistance with the formation of an adherent, protective, thin, and continuous oxide layer. Among the four N-containing alloys investigated, the titanium and Ti/Al multilayer modified 0.56 pct N alloy showed the best hot-salt oxidation resistance as compared to uncoated alloys. The slower corrosion kinetics and adherent scale morphology indicate that the surface-modified titanium intermetallic coatings could provide high-temperature service applications up to 1073 K, particularly in chloride containing atmospheres, for austenitic stainless steels.     

Part III. The tensile behavior of Ti-Al-Nb O+Bcc orthorhombic alloys

The tensile behavior of Ti-Al-Nb alloys with Al concentrations between 12 and 26 at. pct and Nb concentrations between 22 and 38 at. pct has been investigated for temperatures between 25 °C and 650 °C. Several microstructural features were evaluated in an attempt to identify microstructure-property relationships. In particular, the effects of the phase volume fraction, composition, morphology, and grain size were examined. In addition, the constitutive properties were evaluated using single-phase microstructures, and the results provided insight into the microstructure-property relationships of the two-phase orthorhombic (O)+body-centered-cubic (bcc) microstructures. The disordered fully-bcc (β) Ti-12Al-38Nb microstructure, produced through heat treatment above the β-transus, exhibited a room-temperature (RT) elongation of more than 27 pct and the lowest yield strength (YS-553 MPa) of all the alloys studied. The ordered fully-bcc (B2) microstructures, produced through supertransus heat treatment of near-Ti₂AlNb alloys, exhibited fracture strengths up to 672 MPa and low elongations-to-failure (ε _f≤0.6 pct). Thus, increasing the Al content, which favors ordering of the bcc structure, significantly reduces the ductility of the bcc phase. Similar to the ordered B2 microstructure, the ordered fully-O Ti₂AlNb microstructures exhibited intermediate RT strength (≤704 MPa) and ε _f (≤1 pct). The O+bcc microstructures tended to exhibit strengths greater than both the fully-O and fully-bcc microstructures, and this was attributed to the finer grain sizes in the two-phase microstructures compared to their single-phase counterparts. A RT of 1125 MPa was measured for the finest-grained two-phase microstructure. The O+bcc microstructures containing greater bcc-phase volume fractions tended to exhibit greater elongations yet poorer elevated-temperature strengths. A higher Al content typically resulted in larger elevated-temperature strengths. For the Ti-12Al-38Nb bcc-dominated microstructures, fine O platelets, which precipitated during aging, provided significant strengthening and a reduction in ε _f for the Ti-12Al-38Nb alloy. However, large RT elongations (ε _f>12 pct) were maintained for aged Ti-12Al-38Nb microstructures, which contained 28 vol pct O phase. Morphology did not appear to play a dominant role, as fully-lath and fully-equiaxed two-phase microstructures containing the same phase volume fractions exhibited similar RT tensile properties. The slip and cracking observations provided evidence for the ductile and brittle characteristics of the single-phase microstructures, and the slip compatibility exhibited between the two phases is an important part of why O+bcc microstructures achieve attractive strengths and elongations. The YS vs temperature behavior is discussed in light of other Ti-alloy systems.     

Effect of alloying and aging on morphological changes from lamellar to equiaxed microstructure of <Emphasis Type="Italic">α</Emphasis><Subscript>2</Subscript>+<Emphasis Type="Italic">γ</Emphasis> titanium aluminides

The stability of a lamellar structure consisting of α ₂ and γ phases in alloys Ti-48Al, Ti-48Al-2Mo, Ti-48Al-4Nb, and Ti-48Al-1Mo-4Nb has been studied as a function of aging time and temperature. The alloys were solution treated (1400 °C, 30 min, and air-cooled (AC)) and aged at 1000 °C and 1100 °C for 1, 4, and 16 hours, respectively. The results indicate that the kinetics of lamellae to equiaxed transformation depends on alloy chemistry, aging time, and temperature. The Nb decreases and Mo increases the kinetics of transformation. The combined effect of Nb and Mo results in the highest volume fraction of equiaxed microstructure at a given aging time and temperature. The results have been discussed in relation to microstructural features and have been compared with those reported in other α ₂+γ alloys.     

Chlorination treatment to improve the oxidation resistance of Nb-Mo-Si-B alloys

Recent studies have shown that the quaternary Nb-Mo-Si-B system is not oxidation resistant. The difference in oxidation resistance between Mo-Si-B and Nb-Mo-Si-B may be interpreted in terms of the volatility of the metal oxide that forms. MoO₃ evaporates from the surface scale at about 650 °C, leaving a porous borosilicate glassy scale. Nb₂O₅ persists as a rapidly growing condensed phase that overwhelms the ability of the borosilicate glass to form a protective layer. In the present work, a novel chlorination process was employed to selectively remove Nb₂O₅ from the scale of the quaternary alloy as volatile NbCl₅. A Nb-Mo-Si-B alloy was studied with a nominal composition of 63(Nb,Mo)-30Si-7B (at. pct) with Nb/Mo = 1:1. The alloy consisted of a three-phase microstructure of (Nb,Mo)₅Si₃B _x (T1)-(Nb,Mo)₅(Si,B)₃ (T2)-(Nb,Mo)₅Si₃B _x (D8₈). The oxidation behavior of these alloys in air was studied both before and after chlorination. Results showed that Nb₂O₅ can be selectively removed from the scale to leave a borosilicate-rich scale, which then forms a dense scale after heat treatment at 1100 °C in argon. The oxidation rate of the chlorinated alloy was about one-third that of the unchlorinated alloy under identical conditions. Alloy oxidation during heating to the test temperature was studied, and a plausible mechanism for the formation of porosity in the oxide scale has been offered. 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.     

Effect of Sn on microstructure and mechanical properties of Ti-base dendrite/ultrafine-structured multicomponent alloys

Ti_57−x Cu₁₅Ni₁₄Sn_4+x Nb₁₀ (x = 0, 5, or 10) alloys were prepared by copper mold casting. At Sn = 4 at. pct, a dendrite/ultrafine-structured multicomponent alloy was obtained, which exhibits 1271 MPa yield strength, 77 GPa Young’s modulus, and 2 pct plasticity at room temperature for 3-mm-diameter samples. The cooling rate significantly affects the as-cast microstructure and the mechanical properties. For 5-mm-diameter samples, the alloy exhibits 1226 MPa yield strength, 63 GPa Young’s modulus, and 2.5 pct plasticity. At Sn = 9 at. pct, Ti-, Sn-, and Nb-rich particles precipitate primarily. This near-hypereutectic alloy composition leads to the precipitation of intermetallics, which deteriorate the mechanical properties and result in the coexistence of ductile and brittle fracture mechanisms. At Sn = 14 at. pct, the alloy composition is completely in the intermetallic region, thus inducing the formation of Ti₂Cu, Ti₂Ni, and Ti₃Sn intermetallics. The alloy becomes very brittle because the intermetallic compounds dominate the fracture process.     

Micropyretic synthesis studies of Ni-, Al-, Ti-, and Nb-containing alloys

A series of new micropyretically synthesized complex intermetallics and intermetallic composites containing Ni, Al, Ti, and Nb have been studied. At the extremities of this series were the two alloys with the B₂ structure, namely, the NiAl and Nb₄₅Al₁₅Ti₄₀ intermetallic compounds, the former being brittle and the latter ductile. The alloys in the series were made by adding Ti and Nb to NiAl and processing them by micropyretic methods. In addition to the matrix B₂ phase, the synthesized alloys were found to contain other phases. Some of the alloys in the series were noted to possess high fracture toughness and oxidation resistance. A detailed microstructural characterization of the synthesized alloys was carried out. The mechanism of synthesis in the alloys has been established by microstructural examination of the partially reacted specimens by arresting the combustion process during the synthesis process.     

Embrittlement of titanium alloys by long time,high temperature exposure

α stabilized titanium alloys are known to exhibit embrittlement after long-time exposures above ∼800°F. The time-temperature dependency of this embrittlement phenomenon in the Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-lMo-0.25Si alloys was observed using a substandard fracture mechanics test. Room temperature slow-bend tests of fatigue precracked Charpy specimens were used to monitor toughness degradation after unstressed thermal exposures in the temperature range of 800° to 1100°F for times to 5000 hr. The activation energy for the embrittlement process was found to be ∼25 to 28 kcal per g mole, which approximates that for diffusion of aluminum or tin in α-Ti. The embrittlement is attributed to the Ti₃X (X = Al, Sn) phase with the rate controlling step that of diffusion controlled growth of the Ti₃X phase domains. The embrittlement process is reversible by heat treatment at temperatures above the α + Ti₃X two phase region.     

An investigation of the fatigue and fracture behavior of a Nb-12Al-44Ti-1.5Mo intermetallic alloy

This article presents the results of a study of the fatigue and fracture behavior of a damage-tolerant Nb-12Al-44Ti-1.5Mo alloy. This partially ordered B2 + orthorhombic intermetallic alloy is shown to have attractive combinations of room-temperature ductility (11 to 14 pct), fracture toughness (60 to 92 MPa√m), and comparable fatigue crack growth resistance to IN718, Ti-6Al-4V, and pure Nb at room temperature. The studies show that tensile deformation in the Nb-12Al-44Ti-1.5Mo alloy involves localized plastic deformation (microplasticity via slip-band formation) which initiates at stress levels that are significantly below the uniaxial yield stress (∼9.6 pct of the 0.2 pct offset yield strength (YS)). The onset of bulk yielding is shown to correspond to the spread of microplasticity completely across the gage sections of the tensile specimen. Fatigue crack initiation is also postulated to occur by the accumulation of microplasticity (coarsening of slip bands). Subsequent fatigue crack growth then occurs by the “unzipping” of cracks along slip bands that form ahead of the dominant crack tip. The proposed mechanism of fatigue crack growth is analogous to the unzipping crack growth mechanism that was suggested originally by Neumann for crack growth in single-crystal copper. Slower near-threshold fatigue crack growth rates at 750 °C are attributed to the shielding effects of oxide-induced crack closure. The fatigue and fracture behavior are also compared to those of pure Nb and emerging high-temperature niobium-based intermetallics.     

Deformation behavior of Zr3Al-Nb alloys II: Indentation creep studies

The high-temperature mechanical properties of Zr₃Al, Zr₃Al-3Nb, and Zr₃Al-10Nb alloys were assessed using the indentation hardness method. All three alloys showed negligible creep up to 500 K. Using Sargent and Ashby’s approach, different creep parameters such as stress exponent, activation energy, and activation area were estimated. Using the data generated in the present study and those available in the literature, a deformation creep curve was developed. The relationship between hardness and temperature in the high-temperature region can be expressed in terms of an Arrhenious equation. The activation energy estimated from this relationship was found to be in good agreement with that obtained from the indentation creep curve. On comparing the creep behavior of Zr₃Al-Nb alloys with some other intermetallics, it was observed that Zr₃Al-based intermetallics have better creep resistance compared to other intemetallics.