Sulfidation Characteristics of an Advanced Superalloy and Comparison with Other Superalloys Intended for Gas Turbine Use

Sulfidation may occur even in an overall oxidizing environment beneath a corrosion product which assumes the role of a diffusion barrier allowing sulfur species transport at a faster rate when compared with that of oxygen species. The current paper presents sulfidation characteristics of an advanced single-crystal nickel-based superalloy (ANS) and compares performance with IN 792 and CMSX-4 superalloys. The results showed that all the superalloys were highly vulnerable to sulfidation and their lives were significantly reduced. Among them, the ANS was more susceptible to sulfidation and its life was reduced considerably. This is attributed to the changed chemistry of the advanced alloy. The results for ANS are compared with its oxidation data and the difference in its behavior is discussed. A degradation mechanism, which represents the deterioration of ANS under sulfidation conditions, is proposed based on the results obtained from different techniques. Finally, the necessity of protective coatings for shielding against high temperature sulfidation for potential application in enhanced efficiency of gas turbine engines is emphasized.     

Identification of a smart bond coating for gas turbine engine applications

A smart bond coating was successfully identified for thermal barrier coatings (TBCs) that promotes appropriate protective scales preferentially during service depending on the environmental conditions. It enhances the life of super alloy components life significantly, which is essential for increasing the efficiency of advanced gas turbine engines. It is expected to be a potential bond coating for advanced gas turbine engine blades of different types, i.e., aero, industrial, and marine for their protection against high temperature oxidation, type I, and type II hot corrosion.     

Electrodeposition of nanostructured coatings and their characterization—A review

Nanostructured materials have gained importance in recent years due to their significantly enhanced properties. In particular, electrochemistry has a special role in producing a variety of nanostructured materials. In the current review, we discuss the superiority of electrochemical deposition techniques in synthesizing various nanomaterials that exhibit improved characteristics compared with materials produced by conventional techniques, as well as their classification, synthesis routes, properties and applications. The superior properties of a nanostructured nickel coating produced by electrochemical deposition are outlined. The properties of various nanostructured coating materials produced by electrochemical techniques are also described. Finally, the importance of nanostructured coatings in industrial applications as well as their potential in future technologies is emphasized.     

Palladium and tantalum aluminide coatings for high-temperature oxidation resistance of titanium alloy IMI 834

The present article explains the efforts made in developing new protective coatings based on palladium, tantalum, and aluminum with considerably improved oxidation resistance for effective protection of titanium alloy IMI 834. Systematic characterization was carried out on as-prepared as well as oxidized coatings and these results are presented. The performance of new coatings was evaluated by generating weight-gain data as a function of time followed by detailed characterization in order to confirm the ability of the coatings to prevent oxidation and alpha-case formation. The results showed that tantalum aluminide and simple aluminide coatings exhibit improved oxidation resistance when compared to palladium aluminide. Finally, the advantages of developed new coatings and the necessity of their use in modern gas turbine engines that allow the alloy to be used safely at high temperatures, which in turn would enhance the efficiency of gas-turbine engine-compressor sections, will be stressed.     

Hot Corrosion Behavior of CM 247 LC Alloy in Na2SO4 and NaCl Environments

Hot corrosion studies of CM 247 LC alloy werecarried out in pure sodium sulfate, as well as sodiumchloride and sodium sulfate mixtures of differentconcentrations at various temperatures. A crucible test was employed to study the suitability of CM 247LC as a gas turbine blade material. It was observed thatbare CM 247 LC was severely corroded in just 4 hr, whileit was completely consumed in 70 hr when tested in 90% Na₂SO₄ +10% NaCl at 900°C. The results show that a chloridecontaining melt is more corrosive than pure sodiumsulfate. The weight loss is linearly related tot^1/2 (time) and temperature in the different environments studied. Thecorroded samples were characterized by EPMA, SEM, XRD,and metallographic techniques. The results show that hotcorrosion of CM 247 LC is an electrochemicalphenomenon.     

Characterisation of titanium alloy, IMI-834 for corrosion resistance under different environmental conditions

The current paper explains the corrosion characteristics of the titanium alloy, IMI-834 in three different environments which simulates acidic, marine and industrial environments at various temperatures. The titanium alloy forms a protective oxide scale under different environmental conditions at lower temperatures. However, they do not form a protective oxide scale at higher temperatures. The corrosion rate in different environments and at different temperatures increases by about five times in acidic and industrial environments when the temperature increases by a factor of 2. While in marine environments, the corrosion rate was found to increase by two times when the temperature increases by two times. The pitting corrosion studies in different environments revealed that the alloy is resistant to pitting and crevice corrosion at lower temperatures but is susceptible at higher temperatures. The corrosion morphologies were correlated and the degradation mechanism that is leading the titanium alloy to fail under various environmental conditions was discussed. Finally, based on the results obtained with different techniques, the alloy was recommended to fabricate components intended to use in a variety of environmental conditions.     

Effects of niobia and vanadia additions on the mechanical properties of alumina

The effect of niobia and vanadia additions on the mechanical properties of alumina has been studied extensively and reported. It was observed that sintered density values of niobia doped samples increase with an increase in temperature. The optimum density has been obtained at 1500°C, 500 min soaking with 0·5 wt% dopant concentration. The modulus of rupture values have been determined and correlated with a densification parameter.

Vanadia doped samples have shown a different tendency in their sintered density values. In this case, the sintered density increases with an increase in temperature, but decrease with an increase in dopant concentration.     

Factors governing breakaway oxidation of FeCrAl‐based alloys

At temperatures above around 1100 °C the life time of FeCrAl based alloy components can be limited by oxidation. Growth and spalling of the protective alumina scale leads after long exposure times to a depletion of aluminium in the alloys, eventually resulting in breakaway oxidation. This life time limit can be predicted using a recently developed model, taking into account scale growth rate (characterized by the parameters k and n), initial alloy Al content (C_o), critical Al content for protective alumina formation (C_B), oxide adherence and component geometry. Based on the evaluation of long term oxidation data for a number of commercial and model FeCrAl alloys it is shown that the life time can substantially be increased by decreasing the oxide growth rate and/or increasing C_o, whereby application of the latter factor is in most practical cases limited due to restrictions imparted by the alloys' mechanical properties. For typical commercial ODS materials C_B is around 1 wt‐%, however, this value is strongly affected by the exact alloy composition, especially Cr‐content. C_B seems to be higher for dispersion strengthened alloys than for conventional wrought materials. The adherence of the oxide scale not only depends on type and exact amount of reactive element (oxide) addition but also on other common minor alloying additions, such as Ti. Indications were found, that oxide adherence is also affected by the mechanical strength of a material and/or component.     

Grain boundary and microstructure engineering of Inconel 690 cladding on stainless-steel 316L using electron-beam powder bed fusion additive manufacturing

This research explores the prospect of fabricating a face-centered cubic(fcc) Ni-base alloy cladding(Inconel 690) on an fcc Fe-base alloy(316 L stainless-steel) having improved mechanical properties and reduced sensitivity to corrosion through grain boundary and microstructure engineering concepts enabled by additive manufacturing(AM) utilizing electron-beam powder bed fusion(EPBF). The unique solidification and associated constitutional supercooling phenomena characteristic of EPBF promotes[100] textured and extended columnar grains having lower energy grain boundaries as opposed to random, high-angle grain boundaries, but no coherent {111} twin boundaries characteristic of conventional thermo-mechanically processed fcc metals and alloys, including Inconel 690 and 316 L stainless-steel.In addition to [100] textured grains, columnar grains were produced by EPBF fabrication of Inconel 690 claddings on 316 L stainless-steel substrates. Also, irregular 2–3 μm diameter, low energy subgrains were formed along with dislocation densities varying from 108 to 109 cm~2, and a homogeneous distribution of Cr_(23)C_6 precipitates. Precipitates were formed within the grains(with ~3 μm interparticle spacing),but not in the subgrain or columnar grain boundaries. These inclusive, hierarchical microstructures produced a tensile yield strength of 0.527 GPa, elongation of 21%, and Vickers microindentation hardness of 2.33 GPa for the Inconel 690 cladding in contrast to a tensile yield strength of 0.327 GPa, elongation of 53%, and Vickers microindentation hardness of 1.78 GPa, respectively for the wrought 316 L stainlesssteel substrate. Aging of both the Inconel 690 cladding and the 316 L stainless-steel substrate at 685?C for50 h precipitated Cr_(23)C_6 carbides in the Inconel 690 columnar grain boundaries, but not in the low-angle(and low energy) subgrain boundaries. In contrast, Cr_(23)C_6 carbides precipitated in the 316 L stainless-steel grain boundaries, but not in the low energy coherent {111} twin boundaries. Consequently, the Inconel690 subgrain boundaries essentially serve as surrogates for coherent twin boundaries with regard to avoiding carbide precipitation and corrosion sensitization.     

Macro to nanoscale deformation of transformation-induced plasticity steels: Impact of aluminum on the microstructure and deformation behavior

This work aims to elucidate the impact of aluminum-content on microstructure and deformation mechanisms of transformation-induced plasticity(TRIP) steels through macroscale and nanoscale deformation experiments combined with post-mortem electron microscopy of the deformed region.The solid-state transformation-induced mechanical deformation varied with the Al contents,and influenced tensile strength-ductility combination.Steels with 2–4 wt% Al were characterized by TRIP effect.In contrast to 2 Al-TRIP and 4 Al-TRIP steels,twinning-induced plasticity(TWIP) was also observed in conjunction with strain-induced martensite in 6 Al-TRIP steel.This behavior is attributed to the increase in stacking fault energy with the increase of Al content and stability of austenite,which depends on the local chemical variation.The study addresses the knowledge gap with regard to the effect of Al content on austenite stability in medium-Mn TRIP steels.This combination is expected to potentially enable cost-effective alloy design with high strength-high ductility condition.