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The oxidation behavior of alloy 625 and 120 foils was studied at 650, 700 and 800 $$^\circ$$ C in dry air and flowing air + 10% H $$_2$$ O up to 10,000 h (alloy 625) or 30,000 h (alloy 120). The effect of water vapor on Cr loss was investigated. Manganese and iron in the 120 foil induced faster Cr depletion in the foil and breakdown of the Cr $$_2$$ O $$_3$$ scale into Fe and Cr-rich oxide compared to the alloy 625 foil. In the latter case, the presence of Nb led to the formation of Nb and Cr-rich oxide after breakdown of the Cr $$_2$$ O $$_3$$ scale. Simultaneous Cr loss due to oxidation and volatilization of the Cr $$_2$$ O $$_3$$ oxide scale was predicted and compared to experimental Cr loss measurements for exposures up to 30,000 h.

Oxide dispersion strengthened (ODS) FeCrAl alloy (Fe-12Cr-5Al wt%) was neutron irradiated to 1.8 displacements per atom (dpa) at 215, 357, and 557 °C to investigate (Y,Al,O) nano-oxide stability and Cr-rich α′ precipitate formation using atom probe tomography. The nano-oxide sizes remain unchanged after irradiation, but the compositions are dependent on irradiation temperature due to competition between ballistic dissolution and precipitate reformation. Cr-rich clusters observed after 357 °C irradiation have lower Cr contents (61 at.%) than equilibrium α′, with a lower number density than wrought FeCrAl alloys. This suggests a suppression of α′ precipitation due to the high sink strength of ODS FeCrAl.

Extremely fast cooling rates during laser powder-bed fusion (LPBF) can result in materials with unique microstructures. For LPBF 316L stainless steel, the formation of sub-grain cellular structures with high dislocation density has been linked to superior tensile properties at room temperature. This cellular structure offers also a new route for the development of high temperature LPBF steels with the nucleation of nano-size strengthening carbides in the cell walls. HK30Nb steel (Fe-25Cr-20Ni-Nb-C) was, therefore, fabricated by LPBF to evaluate its potential for high temperature applications. Optimization of the fabrication parameters yielded material with density greater than 99.7%, with nano Nb-rich precipitates in the cell walls. Annealing at 800 °C for 5h resulted in the nucleation and growth of additional precipitates mainly in the cell wall and at grain boundaries. The high dislocation density led to yield strength at 20-900 C two to three times higher than yield strength for cast HK30Nb and the nano carbides in the cell walls significantly improved the cellular structure stability at 800 °C.

Additive manufacturing processes supplement traditional material processing routes with unique capabilities which can have profound impacts on component production. Physical prototyping is accelerated and the fabrication of complex components, difficult or impossible to produce conventionally, is realized. Metals research is often focused on identifying process windows to avoid defects which thereby yield desirable properties. In electron beam melting fusion processes, however, precise spatial control of the heat source allows for detailed microstructure manipulation. Design is therefore extended to the microstructure scale offering greater overall flexibility towards engineering high performance components. In this work the role of geometry and beam path sequencing in a powder bed electron beam melting process is investigated. It is observed that by carefully engineering the melting sequence the morphology and texture at the mesoscale can be controlled. Solidification in the build direction, which usually prefers [001] directions, is tilted by control of the heat flux vector which yields large columnar crystals with a strong [011] build direction preference. This newly developed conduction control strategy is demonstrated for producing alternating mesoscale structures in bulk samples. Furthermore, a new scanning strategy is demonstrated which may be suitable for promoting a randomized crystallographic texture during the additive manufacturing process.

The oxidation and hot corrosion performance of NiCoCrAlY coatings fabricated via electrolytic codeposition was investigated. The cyclic oxidation behavior of NiCoCrAlY coatings on Ni-based superalloys was evaluated at 1000 and 1100 °C. Preliminary Type I hot corrosion testing was conducted to assess the hot corrosion resistance of these coatings in a Dean rig with flowing O2 + 0.1% SO2 gas. NiAl coatings synthesized by a non-contact pack cementation method were included in the oxidation and corrosion tests for comparison. The NiAl coatings exhibited excellent oxidation resistance; however, they showed very poor hot corrosion performance at 900 °C. The electro-codeposited NiCoCrAlY coatings demonstrated good overall resistance to both cyclic oxidation and hot corrosion and also showed better chemical/microstructural compatibility with the superalloy substrates.

Surface reliefs due to phase transformations in a 56.8Ni-5.6Pt-37.6Al at. pct alloy were characterized in situ using digital holographic imaging during thermal cycling from room temperature up to 405 K (132 °C). The 3D images of the surface revealed that the austenite plates formed during heating are exactly the same for each cycle, which is not the case for the martensite plates formed during cooling. The martensite start temperature was found to vary by up to ~ 20 K from one grain to another within the same specimen. The absence of Ni3Al γ′ precipitates, due to the relatively high Al content, results in the propagation of the martensitic transformation over grains up to a millimeter in size. Bright-field optical imaging showed the formation of large martensite plates in some grains, with cracks perpendicular to these plates, upon cycling. Cracks were also observed at grain boundaries and could be related to the height variations across the grain boundaries.

Low-Cr oxide dispersion strengthened (ODS) FeCrAl alloys were developed as accident tolerant fuel cladding because of their excellent oxidation resistance at very high temperature, high strength and improved radiation tolerance. Fe-12Cr-5Al wt.% gas atomized powder was ball milled with Y2O3+FeO, Y2O3+ZrO2 or Y2O3+TiO2, and the resulting powders were extruded at 950 °C. The resulting fine grain structure, particularly for the Ti and Zr containing alloys, led to very high strength but limited ductility. Comparison with variants of commercial PM2000 (Fe-20Cr-5Al) highlighted the significant impact of the powder consolidation step on the alloy grain size and, therefore, on the alloy mechanical properties at T

Additive manufacturing technologies have emerged as potentially disruptive processes whose possible impacts range across supply chain logistics, prototyping, and novel materials synthesis. Numerous works illustrate the ability to control microstructure in fusion based processes and a few recent authors have even produced single crystals. However, a number of questions remain open regarding the process window which enables printing of single crystals. Furthermore, it has been observed that these additively manufactured single crystals exhibit a preferred 〈 011 〉 secondary orientation parallel to the scanning direction. In this work we investigate the fabrication conditions that enable printing of single crystals via electron beam melting. A space filling design of experiments is utilized to efficiently explore the fabrication space. Single crystals are successfully obtained using both commercially available powders and custom melt alloys. Microstructures obtained via these exploratory experiments exhibited a continuum of columnar structures ranging from weakly textured polycrystals, near single crystal, and fully single crystalline material. Complex geometry experiments are performed to study the grain selection mechanism. We find that the grain selection mechanism is independent of the bulk scale geometry and must therefore be driven by local heat transfer and solidification dynamics. Furthermore, grain selection is shown to be driven by competing driving forces; one which prefers epitaxial growth and another which is driven by the imposed processing conditions.

Increasing peak cylinder pressures and operating temperatures of turbo-charged internal combustion engines (ICEs) engines present new challenges for existing materials employed in engine exhaust valves. Oxidation induced degradation of current Ni based alloys for these components will be a life-limiting mechanism while strict cost margins in the automotive transportation industry further limit the choice of suitable candidate materials. Metallic diffusion aluminide coatings can provide a cost-effective way to improve the oxidation resistance of underlying materials. In this study, the high temperature oxidation behavior of diffusion nickel aluminide coatings on two Ni based alloys (commercially available alloy 31V and a developmental alloy) during cyclic oxidation behavior in air+10% H2O at 900 °C was investigated. A complete depletion of the beneficial Al-rich β-(NiFe)Al phase was observed in the coating on alloy 31V while a significant fraction of this phase was retained in the coating on the second alloy. A coupled thermodynamic-kinetic model showed that the disappearance of the Al-rich phase in the coating on 31V was mainly due to the combined effect of steeper chemical potential gradients of Al and Ti between the coating and the substrate and their faster diffusivities compared to the coated developmental alloy. A higher content of Fe was shown to support the retention of β-(NiFe)Al phase in the coating on the developmental alloy while the presence of Ti in the alloy was shown to be detrimental for long-term coating performance.

A new test rig was developed to conduct creep evaluation in steam. The isothermal creep performance for ferritic 9Cr and ferritic/martensitic Gr.91 steels was superior at 650 °C in steam than in air. Microstructure characterization revealed the presence of an adherent and continuous (Fe,Cr)-rich inner oxide layer because of crack healing by re-oxidation. Stress calculation based on a composite structure showed that the longer lifetimes in steam were likely due a transfer of load from the alloy to the inner scale. However, a significant decrease of the Gr91 creep lifetime was observed in steam at 650 °C when the specimens were thermally cycled every 10 h. This could be explained by localized cracking of the load-bearing inner layer.

For the next-generation of turbine-based combined heat & power (CHP) systems, materials development can enable lower costs and higher efficiency, particularly for alloy foils used in the recuperator. To quantify the potential benefits, lifetime models are being developed for current chromia-forming alloys such as 625, 120, 310 and 709 (also called 2025Nb) as well as new alumina-forming austenitic (AFA) steels, which are much more oxidation resistant in exhaust gas. A combination of long-term laboratory (up to 30 kh) and field exposure data (up to 100 kh) are being used to develop and validate the model for operation at 650°–800°C.

High temperature material solutions for future power generation applications require longer component lifetimes at higher temperatures to improve durability and efficiency. From a research and development point of view, the lifetimes are too long for experimental verification. Thus, accurate lifetime models are needed. Examples are provided where modeling is being applied to lifetime prediction of thin-walled NiCr wrought alloy components. Specific laboratory experiments have been conducted for alloy X at 900°–950°C in dry air and air with water vapor with cycle times from 1 to 500 h. Along with microstructure characterization, comparisons are made between experiment and model predictions. The eventual goal is to predict both oxidation and mechanical behavior.

s Burst behavior of austenitic and ferritic Fe-based alloy tubes has been examined under a simulated large break loss of coolant accident. Specifically, type 304 stainless steel (304SS) and oxidation resistant FeCrAl tubes were studied alongside Zircaloy-2 and Zircaloy-4 that are considered reference fuel cladding materials. Following the burst test, characterization of the cladding materials was carried out to gain insights regarding the integral burst behavior. Given the widespread availability of a comprehensive set of thermo-mechanical data at elevated temperatures for 304SS, a modeling framework was implemented to simulate the various processes that affect burst behavior in this Fe-based alloy. The most important conclusion is that cladding ballooning due to creep is negligible for Fe-based alloys. Thus, unlike Zr-based alloys, cladding cross-sectional area remains largely unchanged up to the point of burst. Therefore, for a given rod internal pressure, the temperature onset of burst in Fe-based alloys appears to be simply a function of the alloy's ultimate tensile strength, particularly at high rod internal pressures.

The microstructures of modified CF8C-Plus (Fe-19Cr-12Ni-0.4W-3.8Mn-0.2Mo-0.6Nb-0.5Si-0.9C) with W and Cu (CF8CPWCu) and CF8CPWCu enhanced with 21Cr + 15Ni or 22Cr + 17.5Ni were characterized in the as-cast condition and after creep testing. When imaged at lower magnifications, the as-cast microstructure was similar among all three alloys as they all contained a Nb-rich interdendritic phase and Mn-based inclusions. Transmission electron microscopy (TEM) analysis showed the presence of nanoscale Cu-rich nanoprecipitates distributed uniformly throughout the matrix of CF8CPWCu, whereas in CF8CPWCu22/17, Cu precipitates were found primarily at the grain boundaries. The presence of these nanoscale Cu-rich particles, in addition to W-rich Cr23C6, nanoscale Nb carbides, and Z-phase (Nb2Cr2N2), improved the creep strength of the CF8CPWCu steel. Modification of CF8CPWCu with Cr and Ni contents slightly decreased the creep strength but significantly improved the oxidation behavior at 1073 K (800 °C). In particular, the addition of 22Cr and 17.5Ni strongly enhanced the oxidation resistance of the stainless steel resulting in a 100 degrees or greater temperature improvement, and this composition provided the best balance between improving both mechanical properties and oxidation resistance.

316L stainless steel fabricated by laser powder-bed fusion (LPBF) has attracted significant attention due to a unique combination of strength and ductility at room temperature. Understanding of high temperature tensile properties of LPBF fabricated 316L is, however, limited. In the present investigation, tensile testing was conducted at 20–700oC on LPBF 316L in as printed condition and after annealing for 5 h at 500–800oC. Room temperature data confirmed the excellent ductility of the LPBF-processed 316L steel due to the cellular structure with high dislocation density. However, a significant decrease in ductility was observed at temperatures above 200oC. These results are consistent with a change of deformation mechanisms observed in wrought 316L, with twinning playing a key role at room temperature. Microstructure characterization and tensile testing revealed that the cellular structure is stable up to 500oC, but a decrease of yield strength was observed at temperatures above 600oC likely due to a decrease in dislocation density via annealing.

The effects of surface roughness on the high-temperature corrosion of nickel-based superalloy X produced through electron-beam melting (EBM) additive manufacturing were studied. Surface area of samples in the rough, as-fabricated condition was measured using a 2D cross-section image analysis technique and a 3D image analysis technique. To verify the accuracy of surface area measurements, oxidation testing of samples in the as-fabricated and polished conditions was performed; polished sample oxidation curves were then compared to the as-fabricated sample oxidation curves before and after being adjusted using obtained surface area measurements. It was found that the 2D technique accurately measured surface area and accounted for the overlapping features present on EBM sample surfaces, whereas the 3D technique did not. Further, it was also observed that oxidation kinetics were moderately faster on the alloy X samples in the as-fabricated condition than for the polished condition.

We have previously reported (Boatner et al., J. Alloys Compd. 691 (2017) 666–671) on the discovery, formation, processing, and application of a new decorative ternary steel. alloy and on its use in a wide range of practical applications including: custom and commercial knives, art objects, jewelry, electronics, and furniture. This decorative property results when polished single crystals of the 70 wt%Fe-15 wt%Ni-15 wt% Cr austenitic alloy are cryo-quenched into the martensitic phase resulting in the formation of a three-dimensional raised pattern of mixed austenitic/martensitic laths. These laths propagate across large dimensions of the material due to the absence of grain boundaries in the single crystal. The macroscopic optically reflective 3-D decorative pattern reproduces the structural symmetry inherent in the single-crystal orientation. This pattern results solely from the metallurgical phase properties of the ternary alloy and is completely distinct from the properties of Damascus steels or more modern so-called pattern-welded steels. Here we use X-ray diffraction, nanoindentation, hardness measurements/scratch testing, tribological measurements, and alloy resistivity and magnetization methods to obtain a more fundamental and comprehensive study of the effects of the austenitic/martensitic phase transition on the structural and physical properties of the 70 wt%Fe-15 wt%Ni-15 wt% Cr alloy. These results reveal new insight into irreversible phenomena that are associated with the inhibited phase transition on heating of the two-phase, mixed austenitic/martensitic structure that is formed subsequent to cryo-quenching the alloy crystal to 77 K.

The microstructure and oxidation behavior of Laser Beam Melted, Electron Beam Melted and wrought Hastelloy X were studied at 950 °C in dry air, 50 h cycles, up to 1000 h. The variability of processing parameters and powder compositions strongly impacted the oxidation and spallation behavior. The alloys with high Mn and Si were associated with higher oxidation rates, finer Cr2O3 grain microstructure and increased spallation was observed for the alloys with large elongated grain microstructures (aspect ratio of 10 ± 3).

Oxide dispersion strengthened (ODS) FeCrAl alloys are promising candidate materials for advanced nuclear reactor applications requiring high-temperature strength, corrosion resistance, and irradiation tolerance. As these alloys have increased in compositional complexity through attempts to use highly reactive elements such as Zr to refine particle sizes and optimize nanoprecipitate dispersion characteristics, much debate has ensued as to the effects of these alloying element additions on alloy properties. In an attempt to reconcile differences in nanoprecipitate distributions reported in the literature over the past decade, a detailed investigation of a recently developed ODS FeCrAl alloy with nominal composition Fe–10Cr-6.1Al-0.3Zr+0.3Y2O3 is presented using a combination of atom probe tomography (APT), scanning/transmission electron microscopy (S/TEM), and computational thermodynamics modeling. It is illustrated that based on the amount of Zr available in the lattice, Zr competes with Al and Cr to form carbides and nitrides as opposed to oxygen-rich precipitates. This alloy system has a high number density (>1023 m−3) of ∼2–4 nm diameter (Y,Al,O)-rich nanoprecipitates, but it is shown that due to the compositional spread and unknown partitioning of Al between the matrix and precipitates, significant challenges still exist for quantifying the exact compositions of these precipitates using APT. However, the noted compositional spread is supported by identified complex oxides yttrium aluminum monoclinic (YAM) and yttrium aluminum garnet (YAG) using S/TEM. As a result of these findings, researchers developing ODS FeCrAl with reactive element additions must pay careful attention to C and N impurities when optimizing reactive element additions.

This investigation reports an unusual effect of hold time (up to 10seconds) on retardation in the growth of creep-fatigue small cracks at 550 degrees C in Grade 91 steel. The observed phenomenon was interpreted by elucidating multiple processes that are active in the plastic zone at the crack tip. To this effect, microstructural and mechanical responses of the crack tip plastic zone were compared with the mechanical and microstructural responses during low cycle fatigue/creep-fatigue. It is proposed that the stress relaxation that occurs during the hold time can reduce the stress intensity in the plastic zone of crack tip thus, contributing to retardation in the small crack growth rate. Aside from stress relaxation, stress intensity in the crack tip can be further reduced by the phenomenon of cyclic softening that occurs because of plasticity-induced microstructural coarsening. Separately, the contribution of oxidation-induced crack tip shielding is also considered to explain the observed effects of hold time on crack growth rates. Taken together, a combination of stress relaxation, enhanced rate of cyclic softening, and higher degree of oxidation with the introduction of a hold time is demonstrated to be responsible for reduction in the crack growth under creep-fatigue conditions.

A major challenge in the design of oxide dispersion strengthened (ODS) FeCrAl alloys is the optimization of the fine-scale particle size distribution that provides both beneficial mechanical properties and irradiation resistance. To address this obstacle, the nucleation, growth, and coarsening of the fine-scale (Y,Al,O) nanoprecipitates within an ODS FeCrAl powder was studied using atom probe tomography (APT) and small-angle neutron scattering (SANS). Mechanically alloyed Fe 10Cr-6.1Al-0.3Zr + Y2O3 wt.% (CrAZY) powders were heated in-situ from 20 to 1000 °C to capture the nucleation and growth of the nanoprecipitates using SANS. Furthermore, CrAZY powders were annealed at 1000 °C, 1050 °C, and 1100 °C for ageing times from 15 min to 500 h followed by either APT or magnetic SANS to study the structure, composition, and coarsening kinetics of the nanoprecipitates at high temperature. In-situ SANS results indicate nanoprecipitate nucleation and growth at low temperatures (200–600 °C). APT results revealed compositions corresponding to the cubic Y3Al5O12 garnet (YAG) stoichiometry with a possible transition towards the perovskite YAlO3 (YAP) phase for larger precipitates after sufficient thermal ageing. However, magnetic SANS results suggest a defective structure for the nanoprecipitates indicated by deviations of the calculated A-ratio from stoichiometric (Y,Al,O) phases. Particle coarsening kinetics follow n = 6 power law kinetics with respect to particle size, but the mechanism cannot be explained through the dislocation pipe diffusion mechanism. The potential effect of precipitate coarsening during pre- and post-consolidation heat treatments on the irradiation resistance of ODS FeCrAl alloys is discussed with respect to sink strength maximization.

Oxide dispersion strengthened (ODS) ferritic steels typically contain a fine dispersion of nanoscopic Y(Al, Ti) oxides, leading to an improvement in mechanical and physical properties. A rapid prototyping technique, selective laser melting (SLM), was successfully applied to consolidate as-mechanically alloyed ODS-PM2000 (Fe–19Cr–5.5Al–0.5Ti–0.5Y2O3; all wt.%) powder to fabricate solid and thin-walled builds of different thickness. This work is intended to act as a first study to investigate the tensile response of such configurations at room temperature, using miniaturized test specimens along and perpendicular to the growth direction. The 0.2% offset yield strength of as-grown wall builds was inferior to conventional PM2000 alloy (recrystallized), but could be significantly increased by conducting post-build heat treatments. Young’s modulus and yield strength showed anisotropy and were enhanced when testing perpendicular to the build growth direction. Electron backscatter diffraction revealed a strong [0 0 1] fibre texture along the growth direction, which explains the anisotropic behaviour. Additionally, studies on the morphology of the individual fracture surfaces, the grain structure of the cross-section near this region and the size distribution of ODS particles in such builds were conducted. A fine dispersion of precipitates was retained in all SLM builds, and findings suggest that a certain amount of Y is probably still in atomic solution in the as-grown condition and forms new small nanoscopic dispersoids during annealing, which lead to enhanced strengthening.

The electron beam melting (EBM) process is an attractive additive manufacturing technology that can be applied to a number of different materials. However, fabrication of each material requires tweaking and identifying optimal processing parameters. In this study, process parameters were successively varied to identify a processing regime to fabricate the nickel-base (Ni-base) superalloy Haynes 282. Key parameters to minimize porosity and mitigate the potential for cracking were beam velocity, beam current, hatch spacing, line order, and beam focus. Overall, the EBM process window produced a combination of promising microstructure and 99.5% dense material with no observable cracking. Electron backscatter diffraction revealed a crystallographic texture along the [001] direction of the cube orientation aligned with the build direction. Within the grain interiors of the as-fabricated material were observed uniformly distributed γ′ precipitates with a size distribution ranging from ~80 nm spherical to ~190 nm cuboidal particles with an average particle size of ~128 nm. Grain boundary carbides were observed in two morphologies: blocky and thin film–like. Hardness and tensile testing of the as-fabricated EBM material indicated a 10% higher hardness and slightly lower tensile strength compared with the as-annealed wrought form of the Haynes 282 alloy. The EBM alloy exhibited pronounced ductility except at T > 600 °C perpendicular to the build direction. Annealing based on standard wrought heat treatments showed that γ′ precipitates measuring 20–30 nm in average size can be achieved to improve the alloy's high-temperature performance.

Increasing the efficiency of natural gas reciprocating engines will require materials with better mechanical and corrosion resistance at high temperatures. One solution to increase the lifetime of exhaust valves is to apply an aluminide coating to prevent corrosion assisted fatigue cracking, but the impact of the coating on the valve material mechanical properties needs to be assessed. In addition to cyclic oxidation testing in dry and humid air at 800°C, creep and high cycle fatigue (HCF) testing were conducted at 816°C on bare and slurry or pack-coated 31V alloy. The coated and bare creep specimens exhibited very similar creep rupture lives, as long as the specimens were annealed according to the 31V standard heat treatment before testing. The HCF behavior of the pack-coated alloy was close to the behavior of the bare alloy, but fatigue lifetimes of slurry-coated 31V specimens had higher variability. Aluminide coatings have the potential to improve the valve performance at high temperature, but the coating deposition process needs to be tailored for the substrate standard heat treatment.

Oxide dispersion strengthened (ODS) FeCrAl alloys with 12–15% Cr are being evaluated for improved compatibility with Pb-Li for a fusion energy application and with high temperature steam for a more accident-tolerant light water reactor fuel cladding application. A 12% Cr content alloy showed low mass losses in static Pb-Li at 700°C, where a LiAlO2 surface oxide formed and inhibited dissolution into the liquid metal. All the evaluated compositions formed a protective scale in steam at 1200°C, which is not possible with ODS FeCr alloys. However, most of the compositions were not protective at 1400°C, which is a general and somewhat surprising problem with ODS FeCrAl alloys that is still being studied. More work is needed to optimize the alloy composition, microstructure and oxide dispersion, but initial promising tensile and creep results have been obtained with mixed oxide additions, i.e. Y2O3 with ZrO2, HfO2 or TiO2.

The creep behavior of various pack cementation aluminide coatings on Grade 91 ferritic–martensitic steel was investigated at 650 °C in laboratory air. The coatings were fabricated in two temperature regimes, i.e., 650 or 700 °C (low temperature) and 1050 °C (high temperature), and consisted of a range of Al levels and thicknesses. For comparison, uncoated specimens heat-treated at 1050 °C to simulate the high temperature coating cycle also were included in the creep test. All coated specimens showed a reduction in creep resistance, with 16–51% decrease in rupture life compared to the as-received bare substrate alloy. However, the specimens heat-treated at 1050 °C exhibited the lowest creep resistance among all tested samples, with a surprisingly short rupture time of

A new, cast austenitic stainless steel, CF8C-Plus (CF8C-P), has been developed for a wide range of high temperature applications, including diesel exhaust components, turbine casings and turbocharger housings. CF8C-P offers significant improvements in creep rupture life and creep rupture strength over standard CF8C steel. However, at higher temperatures and in aggressive environments such as those containing significant water vapor, an oxidation-resistant protective coating will be necessary to extend service life. The oxidation behavior of alloys CF8C and CF8C-P with various aluminide coatings were compared at 800 °C in air plus 10 vol% water vapor. Due to their affordability, slurry aluminides were the primary coating system of interest, although chemical vapor deposition and pack cementation coatings were also compared. Additionally, a preliminary study of the low-cycle fatigue (LCF) behavior of aluminized CF8C-P was conducted at 800 °C. Each type of coating provided substantial improvements in oxidation behavior, with simple slurry aluminides exhibiting very good oxidation resistance after 3,000 h testing in water vapor. Preliminary LCF results indicated that thicker aluminide coatings degraded high temperature fatigue properties of CF8C-P, whereas thinner coatings did not. Results suggest that appropriately designed slurry aluminide coatings are a viable option for economical, long-term oxidation protection of austenitic stainless steels in water vapor.

The evolving 3D printer technology is now at the point where some turbine components could be additive manufactured (AM) for both development and production purposes. However, this will require a significant evaluation program to qualify the process and components to meet current design and quality standards. The goal of the project was to begin characterization of the microstructure and mechanical properties of Nickel Alloy X (Ni-22Cr-18Fe-9Mo) test bars fabricated by powder bed fusion (PBF) AM processes that use either an electron beam (EB) or laser beam (LB) power source. The AM materials produced with the EB and LB processes displayed significant differences in microstructure and resultant mechanical properties. Accordingly, during the design analysis of AM turbine components, the specific mechanical behavior of the material produced with the selected AM process should be considered. Comparison of the mechanical properties of both the EB and LB materials to those of conventionally processed Nickel Alloy X materials indicates the subject AM materials are viable alternatives for manufacture of some turbine components.

Although expensive, Ni-based superalloys are of interest for the ultrasupercritical steam program because of their good creep and oxidation resistance at temperature above 700 C. However, the effect of steam oxidation on the alloy mechanical properties is unknown, and creep specimens of alloy CCA617, 740 and 230 were pre-oxidized for 2000 and 4000h in steam at 800 C before testing in air at the same temperature. Exposure in steam decreased the creep properties of alloy CCA617 compared with as fabricated material, had less of an effect on alloy 740, and did not affect alloy 230. Testing of a specimen repolished after steam exposure as well as microstructure observation indicate that the oxidation affected zone at the specimen surface is not responsible for the properties degradation. Surprisingly, a similar time anneal in an inert environment resulted in a drastic decrease of creep rupture life and an increase in the creep rate and elongation at rupture. TEM analysis revealed that the mechanical properties decrease for alloy CCA617 is related to the absence of precipitates in the grain.

The surface scale formed on specimens of a commercial oxide dispersion strengthened (ODS) FeCrAl alloy (PM2000) exposed for 1 and 500 h at 1050°C in dry O2, Air+10%H2O and Ar+10%H2O consisted of a two-layer α-Al2O3 structure with a columnar grain inner layer and a finer grain outer layer. The alumina scales formed in Air+10%H2O and Ar+10%H2O were slightly more than half of the thickness of the scale formed in dry O2. The same two-layer structure was also observed after exposure for 500 h at 1100°C in dry O2 and 50%CO2+50%H2O. The alumina scales formed in both atmospheres were similar in thickness. Oxides rich in Y and Ti at the gas – scale interface grew in size and number with time in each case. Using analytical transmission electron microscopy, alumina grain boundary segregation of both Y and Ti was evident near the gas interface but only Y segregation was detected near the metal interface. This difference was attributed to Ti depletion in the adjacent metal and the rapid outward flux of the sma...

Creep tests were performed at 875 °C and 1050 °C on commercially available FeCrAl foils (∼50 μm, 2 mil thickness) over a wide range of stress and duration. The oxide scales formed on the creep specimens were analyzed and compared to those that formed on unstressed specimens to assess the effect of stress and strain on oxide growth behavior. Below a specific stress threshold, the creep rate becomes moderately dependent on the applied load, the lifetime independent of that load, and rupture occurs due to the onset of breakaway oxidation. A creep rate model based on the strengthening of the FeCrAl foils due to load-bearing by the thermally grown alumina scale was observed to be in good agreement with the experimental results.

The ductility of various coatings deposited by chemical vapor deposition, pack cementation and slurry processes on Fe- and Ni-based alloys was characterized by indentation at room temperature. A hot indentation apparatus has also been developed to more rapidly determine the ductile to brittle transition temperature of coated specimens. Creep testing has been conducted on bare and coated alloy 230 (NiCrW) specimens at 800 °C with a significant decrease in creep life observed. Based on the observed failure of coated 230 specimens, the impact of coating ductility on substrate creep properties is discussed.

Short cracks, extending to the interdiffusion zone, have been observed in platinum-modified nickel-aluminide diffusion coatings on single-crystal CMSX-4 superalloys after these have been subject to short (10 min) rapid thermal cycles. The cracks were revealed after removing the thermally grown oxide formed during cycling on the coating surfaces. Cracking was observed for coatings subject to thermal cycling between 1150 and 1050 °C, with and without an applied compressive stress, as well as on thermal cycling between 1150 °C and room temperature.

Thermal cycling of a platinum modified, nickel aluminide (Ni,Pt)Al coated single crystal superalloy, between 1000 and 1150 °C with 10 min holds at each temperature, and subject to a compressive uniaxial stress is reported. There are two major effects of the superimposed compressive stress not observed in the absence of the stress. One is that the rumpling pattern exhibits an asymmetry with an increase of the bond coat surface roughness perpendicular to the applied loading axis. The other is the formation of cracks in the thermally grown oxide aligned parallel to the stress axis.

The evolution of the surface roughness of an initially flat platinum-modified nickel aluminide (NiPtAl) coating on a single crystal superalloy with cyclic oxidation has been measured by optical profilometry and correlated with the aluminide microstructure. The roughness evolution is dominated by the relative motion of individual grains on thermal cycling, with the larger sized grains moving up and the smaller grains moving down with respect to the average surface. Detailed crystallographic analysis indicates that the grains in the coating are randomly distributed and have no crystallographic relationship with the underlying single crystal superalloy. Furthermore, there is a correlation between the number of sides a grain has and the propensity for its center to move relative to the average surface plane: grains with six or more sides bow up, whereas the surfaces of those grains with fewer than six sides bow down. It is proposed that rumpling initiates due to the vertical displacement of the smaller grains having fewer than six sides. Once initiated, rumpling proceeds driven by the strain energy in the thermally grown oxide.

The alumina scale formed on undoped iron aluminides (Fe–40Al and Fe–28Al–2Cr) was studied using scanning electron microscopy (SEM) and atomic force microscopy (AFM). For cast alloys, a convoluted scale was observed after exposures at 950° and 1000 °C. However, oxide dispersed Fe–28Al–2Cr formed a flat oxide after similar exposures. To clarify the role of the substrate, the high temperature mechanical properties of these materials were studied in tension and/or compression. The largest scale convolutions were observed on the material with the lowest 1000 °C yield stress, wrought Fe–28Al–2Cr, suggesting that substrate mechanical properties can explain the observed differences in scale morphology.

The evolution in surface morphology of platinum modified nickel aluminide (Ni,Pt)Al oxidation coatings during thermo-mechanical testing has been evaluated. One type of test consisted of cyclic oxidation between an upper temperature of 1150°C and a lower temperature varying from room temperature to 1050°C. The other type of test was cycling between 1000°C/1150°C under an applied compressive stress. Profilometry using optical interferometry was used to quantify the surface “rumpling”. First and second-order statistical parameters including RMS roughness and the auto-correlation function were calculated from the profilometry measurements. The results indicate that the grain structure of the aluminide coating plays a major role in the early stages of rumpling and set its wavelength. Also, the superimposed compressive stress during thermal cycling leads to an asymmetry in the rumpling pattern with respect with the loading axis as well as cracking along the applied stress direction.

Cast stainless steel CF8C-Plus (19wt%Cr/12%Ni) has excellent creep properties, but limited oxidation resistance above 700°C in environments containing H2O. One strategy to improve the alloy oxidation performance is to increase the Cr and Ni concentration. Two new alloys, with, respectively, 21wt%Cr–15wt%Ni and 22wt%Cr–17.5wt%Ni were therefore developed and their long-term oxidation behaviors in humid air were compared with the oxidation behavior of five other cast alloys. At 650°C and 700°C, all the alloys formed internal Cr-rich nodules, and outer nodules or layers rich in Fe and Ni, but they grew a protective Cr-rich inner layer over time. At 750°C, the lower alloyed steels such as CF8C-Plus showed large metal losses, but the two new alloys still exhibited a protective oxidation behavior. The 21Cr–15Ni alloy was severely oxidized in locations at 800°C, but that was not the case for the 22Cr–17.5Ni alloy. Therefore, the two new modified alloys represent a potential operating temperature gain of, respectively, 50°C and 100°C in aggressive environments compared with the CF8C-Plus alloy.