Microstructural study of the titanium alloy Ti-15Mo-2.7Nb-3Al-0.2Si (TIMETAL 21S)

A relatively new titanium alloy, TIMETAL 21S (Ti-15Mo-2.7Nb-3Al-0.2Si-0.15O (in wt pct)), is a potential matrix material for advanced titanium matrix composites for elevated temperature use. In order to develop a perspective on the microstructural stability of this alloy, the influence of several commonly used heat treatments on the microstructure of TIMETAL 21S was studied using optical and transmission electron microscopy (TEM). Depending on the specific thermal treatment, a number of phases, includingα,ω- type, and silicide, can form in this alloy. It was found that both recrystallized and nonrecrystallized areas could be present in the microstructure of an annealed bulk alloy, but the microstructure of annealed sheet alloy was fully recrystallized. The mixed structure of the bulk alloy, developed as a result of inhomogeneous deformation, could not be removed by heat treatment alone at 900 °C. Athermalω-type phase formed in this alloy upon quenching from the solution treatment temperature (900 °C). Silicide precipitates were also found in the quenched sample. Thermal analysis was used to determine theβ transus and silicide solvus as close to 815 °C and 1025 °C, respectively. In solution-treated and quenched samples, a high-temperature aging at 600 °C resulted in the precipitation ofα phase. The precipitation reaction was slower in the recrystallized regions compared to the nonrecrystallized regions. During low-temperature aging (350 °C), the ellipsoidalω-type phase persisted in the recrystallized areas even after 100 hours, whereas a high density ofα precipitates developed in the nonrecrystallized areas within only 3 hours. The observed behavior in precipitation may be related to the influence of substructure in the nonrecrystallized areas, providing for an enhanced kinetics during aging. Theα precipitates (formed during continuous cooling from the solution treatment temperature, low-temperature aging, and high-temperature aging) always obeyed the Burgers orientation relationship. With respect to the microstructure, TIMETAL 21S is similar to other solute-lean, metastableβ titanium alloys.     

Metastable phase equilibria in the lead-tin alloy system: Part II. Thermodynamic modeling

The lattice stability of Sn is reassessed in view of additional pressure-temperature data since the earlier assessment by Kaufman. The metastable phase boundary of the α-Pb phase reported in Part I in connection with available thermochemical data is used to define more accurately the model paraaeters for the stable L, α, β, and metastable α₁-phases. Using these models, the stable phase diagram of Pb−Sn, the metastable extension of the stable phase diagram, and several possible metastable phase diagrams are calculated. Thermodynamic and kinetic criteria are used to discuss the formation of various phases during rapid solidification processes.     

Rapid growth of single crystals in the solid state utilizing a massive transformation

A method is described for rapidly growing single crystals in the solid state, utilizing the massive transformation. The technique involves moving a necked specimen through a thermal gradient and it has been shown to be successful in alloys of the Ag-Al system near 24.5 at. % Al. Single crystals of the hcp phase have been grown from the b c c phase at rates between 0.01 and 1.2 cm sec^–1. X-ray observations indicate the development of misorientations and a substructure with increasing crystal growth velocity. Metallographic observations on numerous crystals demonstrate that the occurrence of single crystal growth is closely related to the relationship between the undercooling at the transformation interface and the interface velocity. Favourable conditions require that the velocity of the transformation interface be near the maximum value possible under the imposed thermal conditions. Success in growing a single crystal decreases with increasing undercooling and no crystals could be grown when the undercooling exceeded about 20°C. Solid state crystal growth utilizing the massive transformation has many features in common with crystal growth resulting from undercooled liquids.     

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...     

Enhanced mechanical properties due to structural changes induced by devitrification in Fe–Co–B–Si–Nb bulk metallic glass

Fe₃₆Co₃₆B_19.2Si_4.8Nb₄ bulk glassy rods were synthesized by copper mould casting. The effects of annealing treatments on the microstructure, elastic and mechanical properties of this alloy are investigated. Annealing below the glass transition temperature induces the formation of atomic clusters with pseudo-tenfold symmetry with a close relationship to the Fe₂₃B₆ phase. Annealing at sufficiently high temperatures promotes the formation of stable Fe₂B and FeB phases and Fe(Co) solid solution. The as-cast alloy exhibits ultra-high hardness (H > 14 GPa), high reduced Young’s modulus (E_r > 200 GPa) and good wear resistance. These properties are further enhanced after thermal treatments (H > 18 GPa and E_r > 260 GPa are achieved in the fully crystallized sample). The mechanical hardening is accompanied with an increase of the elastic recovery and a decrease of the Poisson’s ratio. The different microstructural mechanisms responsible for these annealing-induced changes in mechanical and elastic properties are discussed.     

Interdiffusion in the Ni-Re System: Evaluation of Uncertainties

Diffusion couple experiments between Ni and Re at 1200 and 1350 °C were performed. These experiments established the limits of the two-phase FCC + HCP region. No intermediate phase was observed at these temperatures. Composition-dependent interdiffusion coefficients and associated uncertainties were estimated by three methods. The first employed fitting of the penetration curves in conjunction with the Sauer-Freise (SF) method. The second method employed a numerical solution of the Boltzmann-Matano ordinary differential equation for composition-dependent interdiffusion coefficient functions whose parameters were optimized by a least squares fitting to the data. Discrepancies between the results of these methods indicate typical uncertainties in experimental determination of diffusion coefficients. To further assess such discrepancies, a third method was employed to perform an uncertainty quantification of the diffusion coefficients via a statistical analysis based on the SF method.     

Application of Plasma Spraying as a Precursor in the Synthesis of Oxidation-Resistant Coatings

Thermal spray methods offer a versatile and flexible approach to the manufacture of coatings as a final product. A novel application of thermal spray coating is demonstrated by incorporating a plasma-sprayed Mo layer coating as a precursor step within an integrated costing design. The effectiveness of the two-step design is illustrated for aluminoborosilica coatings on SiC/C composites and W substrates based on the plasma-sprayed Mo precursor and subsequent codeposition of Si and B by a pack cementation method. Even with incomplete precursor coverage, an aluminoborosilica coating is developed because of the high initial fluidity of the as-pack coating. An effective oxidation resistance is observed following exposure at elevated temperatures (1373-1673 K) in ambient air and during torch testing at 1773 K, providing clear evidence that the plasma spraying of Mo is a viable precursor step in the formation of the oxidation-resistant Mo-Si-B-based coating.     

Synthesis, Thermodynamic Stability and Diffusion Mechanism of Al5Fe2-Based Coatings

Aluminide coating of steels enables more efficient power generation through higher operating temperatures. Low-temperature (T < 660 °C) pack cementation aluminide coatings form an Al₅Fe₂ phase which allows for the development of a large Al flux, but the mechanism is not clear. The coating structures and resultant oxides were examined in both austenitic and ferritic steels at 1,000 and 800 °C to evaluate the high temperature oxidation behavior in air. To understand the relatively fast Al diffusion, the stability of the Al₅Fe₂ phase and the defect structure have been examined by a cluster expansion method with density functional theory calculations. The Al₅Fe₂ phase has a low site occupancy and a high vacancy content that promotes rapid kinetics. The high vacancy concentration in the Al₅Fe₂ phase can be traced to the interaction between Al and vacancies along the [001] chains. The analysis offers useful guidance to enable an effective control of low temperature aluminizing.     

Strong,Ductile Magnesium-Zinc Nanocomposites

Incorporated SiC nanoparticles are demonstrated to influence the solidification of magnesium-zinc alloys resulting in strong, ductile, and castable materials. By ultrasonically dispersing a small amount (less than 2 vol pct) of SiC nanoparticles, both the strength and ductility exhibit marked enhancement in the final casting. This unusual ductility enhancement is the result of the nanoparticles altering the selection of intermetallic phases. Using transmission electron microscopy (TEM), the MgZn₂ phase was discovered among SiC nanoparticle clusters in hypoeutectic compositions. Differential thermal analysis showed that the MgZn₂ formation resulted in elimination of other intermetallics in the Mg-4Zn nanocomposite and reduced their formation in Mg-6Zn and Mg-8Zn nanocomposites.