Laves phases are intermetallic phases well known for their excellent strength at high temperatures but also for their pronounced brittleness at low temperatures. Especially in high-alloyed steels, Laves phases were long time regarded as detrimental phases as they were found to embrittle the material. Perusing the more recent literature, it seems the negative opinion about the Laves phases has changed during the last years. It is reported that, if the precipitation morphology is properly controlled, transition metal-based Laves phases can act as effective strengthening phases in heat resistant steels without causing embrittlement. For a targeted materials development, the mechanical properties of pure Laves phases should be known. However, the basic knowledge and understanding of the mechanical behavior of Laves phases is very limited. Here we present an overview of experimental results obtained by micromechanical testing of single-crystalline NbCo 2 Laves phase samples with varying crystal structure, orientation, and composition. For this purpose, diffusion layers with concentration gradients covering the complete homogeneity ranges of the hexagonal C14, cubic C15 and hexagonal C36 NbCo 2 Laves phases were grown by the diffusion couple technique. The hardness and Young's modulus of NbCo 2 were probed by nanoindentation scans along the concentration gradient. Single-phase and single crystalline microcantilevers and micropillars of the NbCo 2 Laves phase with different compositions were cut in the diffusion layers by focused ion beam milling. The fracture toughness and the critical resolved shear stress (CRSS) were measured by in-situ microcantilever bending tests and micropillar compression tests, respectively. The hardness, Young's modulus and CRSS are nearly constant within the extended composition range of the cubic C15 Laves phase, but clearly decrease when the composition approaches the boundaries of the homogeneity range where the C15 structure transforms to the off stoichiometric, hexagonal C36 and C14 structure on the Co-rich and Nb-rich, respectively. In contrast, microcantilever fracture tests do not show this effect but indicate that the fracture toughness is independent of crystal structure and chemical composition of the NbCo 2 Laves phase.

Cr-based alloys have potential as heat-resistant materials due to the higher melting point and lower density of Cr. Although oxidation and nitridation at high temperatures are one of the drawbacks of Cr and Cr-based alloys, addition of Si has been reported to enhance the oxidation and nitridation resistance. This study focuses on the microstructure and mechanical properties in the Cr-Si binary alloys with the Cr ss + Cr 3 Si two-phase structure. The Cr-16at.%Si alloy showed an eutectic microstructure and hypoeutectic alloys with the lower Si composition exhibited a combination of the primary Cr ss and the Cr ss /Cr 3 Si eutectic microstructure. Compression tests at elevated temperatures were conducted for the hypoeutectic and the eutectic alloys in vacuum environment. Among the investigated alloys, the Cr-13at.%Si hypoeutectic alloy including the Cr 3 Si phase of about 40% was found to show the highest 0.2% proof stress of 526 MPa at 1000 °C. Its specific strength is 78.1 Nm/g which is roughly twice as high as that of Ni-based Mar-M247 alloy. It was also confirmed that the 0.2% proof stress at 1000 °C depends on not only the volume fraction of the Cr 3 Si phase, but also the morphology of the Cr ss + Cr 3 Si two-phase microstructure.

The behavior of strain-induced abnormal grain growth (AGG) in superalloy 718 has been investigated using compression testing and subsequent heat treatment below the d-phase solvus temperature of 980 °C. The nuclei of AGG grains were slightly newly recrystallized grains by a nucleation because small grains without dislocation was observed in the as- deformed microstructure. AGG was caused by the difference in intragranular misorientation (related to the stored strain energy in a grain) between dynamic recrystallized grains and deformed matrix. The initiation of AGG was retarded with decreasing plastic strain and produced microstructures consisted of larger grains having more complex morphology. It was observed that grain boundary migrated locally in the direction perpendicular to, or mainly in the direction parallel to the S3 {111} twin boundaries along with the formation of high-order twins. As a result of multiple twinning, AGG grains seemed to evolve with the growing directions changed.

Understanding of the thermomechanical processing that affects microstructures is important to develop new alloys, because the mechanical properties of Ti alloys depend on the microstructures. In our previous study, we found Sn deteriorated the oxidation resistance, while Nb improved the oxidation resistance. Then, we have focused on Ti-Al-Nb-Zr alloys which Nb was added instead of Sn. Zr was added for solid solution strengthening. In this study, the formation of microstructures by thermomechanical processing and the effect of microstructure on the mechanical properties were investigated using the Ti-13Al-2Nb-2Zr (at%) alloy. The samples heat-treated in the β+α phase followed by furnace cooling after processed in the β+α phase formed the equiaxed or the ellipsoid α phase surrounded by the β phase. On the other hand, the sample heat-treated in the β+α phase followed by furnace cooling after processed in the β phase formed the lamellar microstructure. The compression strengths of the equiaxed α structure processed at two temperatures in the β+α phase were almost the same. While creep life of the bi-modal structure was drastically changed by processing temperature.

The US Advanced Ultra-Supercritical (A-USC) Consortium conducted an extensive program to evaluate available superalloys for use in rotors for steam turbines operating at a nominal temperature of 760 °C (1400 °F). Alloys such as 282, Waspaloy, 740H, 720Li, and 105 were tested in the form of bar supplied from the alloy producers. Ultimately, alloy 282 was down-selected for the turbine rotor based on its combination of creep strength, phase stability, ductility, and fatigue resistance. The next step in development was to produce a full-size rotor forging for testing. A team was established consisting of GE Power (project management and testing), Wyman-Gordon (forging and testing) and Special Metals (melting and billetizing) to pursue the work. A research license to melt the alloy was obtained from Haynes International. The first step of the development was to devise a triple melt (VIM-ESR-VAR) practice to produce 610 mm (24 inch) diameter ingot. Two ingots were made, the first to define the VAR remelting parameters and the second to make the test ingot utilizing optimum conditions. Careful attention was paid to ingot structure to ensure that no solidification segregation occurred. A unique homogenization practice for the alloy was developed by the US Department of Energy (DOE) and National Energy Technology Laboratory (NETL). Billetization was performed on an open die press with three upset and draw stages. This procedure produced an average grain size of ASTM 3. A closed die forging practice was developed based on compressive flow stress data developed by Wyman Gordon Houston for the consortium project. Multiple 18 kg forgings were produced to define the forging parameters that yielded the desired microstructure. The project culminated with a 2.19 metric ton (4830 lb), 1.22 m (48 inch) diameter crack-free pancake forging produced on Wyman Gordon’s 50,000 ton press in Grafton, MA. The forging process produced a disk with an average grain size of ASTM 8 or finer. Forging cut-up, microstructural characterization, and mechanical property testing was performed by GE Power. Fatigue and fracture toughness values of the disk forging exceeded those previously reported for commercially available rolled bar.

The aspiration to deploy Nb-based alloys as viable upgrade for Ni-based superalloys is rooted in their potential for superior performance in high-temperature applications, such as rocket nozzles and next-generation turbines. However, realizing this goal requires overcoming formidable design hurdles, including achieving high specific strength, creep resistance, fatigue, and oxidation resistance at elevated temperatures, while preserving ductility at lower temperatures. Additionally, the requisite for alloy bond-coatings, to ensure compatibility with coating materials, further complicates the design process. QuesTek Innovations has its Integrated Computational Materials Engineering (ICME) technologies to design a superior performance high-temperature Nb-based superalloy based on solid solution and precipitation strengthening. Additionally, utilizing a statistical learning method from very limited available data, QuesTek engineers were able to establish physics-based material property models, enabling accurate predictions of equilibrium phase fraction, DBTT, and creep properties for multicomponent Nb alloys. With the proven Materials by Design methodology under the ICME framework, QuesTek successfully designed a novel Nb superalloy that met the stringent design requirements using its advanced ICMD materials modeling and design platform.

Advanced 700°C-class steam turbines demand austenitic alloys for superior creep strength and oxidation resistance beyond 650°C, exceeding the capabilities of conventional ferritic 12Cr steels. However, austenitic alloys come with a higher coefficient of thermal expansion (CTE) compared to 12Cr steels. To ensure reliability, operability, and performance, these advanced turbine alloys require low CTE properties. Additionally, for welded components, minimizing CTE mismatch between the new material and the welded 12Cr steel is crucial to manage residual stress. This research investigates the impact of alloying elements on CTE, high-temperature strength, phase stability, and manufacturability. As a result, a new material, “LTES700R,” was developed specifically for steam turbine rotors. LTES700R boasts a lower CTE than both 2.25Cr steel and conventional superalloys. Additionally, its room-temperature proof strength approaches that of advanced 12Cr steel rotor materials, while its creep rupture strength around 700°C significantly surpasses that of 12Cr steel due to the strengthening effect of gamma-prime phase precipitates. To assess the manufacturability and properties of LTES700R, a medium-sized forging was produced as a trial run for a turbine rotor. The vacuum arc remelting process was employed to minimize segregation risk, and a forging procedure was meticulously designed using finite element method simulations. This trial production resulted in a successfully manufactured rotor with satisfactory quality confirmed through destructive evaluation.

Ti alloys are used as compressor blades and disks in jet engines due to their high specific strength and good oxidation resistance at operation temperature. However, Ti alloys cannot be used above 600 °C because creep properties and oxidation resistance deteriorate. To overcome the above problems, the effect of alloying element on oxidation resistance was investigated and it was found that Sn deteriorated oxidation resistance and Nb improved oxidation resistance. Then, we have attempted to design new Ti alloys without Sn, but including Nb because Nb improved oxidation resistance. To expect solid-solution hardening, Zr was also added to the alloys. In this study, the creep behavior of Ti-10Al-2Nb-2Zr and Ti-10Al-2Nb-2Zr-0.5Si alloys was investigated. The creep test was performed at temperature range between 550 and 650 °C and stress range between 137 and 240 MPa. The stress exponent and the activation energy for creep were analyzed using an Arrhenius equation. The stress exponent was 5.9 and 3.4, and the activation energy was 290 and 272 kJ/mol for Ti-10Al-2Nb-2Zr and Ti-10Al-2Nb-2Zr-0.5Si, respectively. This indicates the creep deformation mechanism is dislocation (high-temperature power law) creep governed by lattice diffusion.

High-temperature shape memory alloys (HTSMAs) are expected to be utilized for actuators in high temperature environments such as thermal power plants and jet engines. NIMS has designed TiPd shape memory alloys because high martensitic phase transformation temperature of TiPd around 570 ° C is expected to be high-temperature shape memory alloys. However, the strength of the austenite phase of TiPd is low and the perfect recovery was not obtained. Then, strengthening of TiPd by addition of alloying elements has been attempted, but the complete recovery was not obtained. Therefore, high entropy alloys (HEA, multi-component equiatomic or near equiatomic alloys) were attempted for HTSMA. The severe lattice distortion and the sluggish diffusion in HEA are expected to contribute strong solid-solution hardening of HTSMA. In this study, multicomponent alloys composed of Ti-Pd-Pt-Ni-Zr were prepared and the phase transformation, shape memory properties, and mechanical properties were investigated.

In order to establish a induction bending technique for Ni-based alloy HR6W large pipe, induction bending test was conducted on HR6W, which is a piping candidate material of 700°C class Advanced Ultra-Super Critical. In this study, a tensile bending test in which tensile strain was applied and a compression bending test in which compression strain was applied to the extrados side of the pipe bending part. As the results of these two types of induction bending tests, it was confirmed that a predetermined design shape could be satisfied in both bending tests. In addition, the wall thickness of the pipe was equal to or greater than that of the straight pipe section in compression bending. Therefore, if compression bending is used, it is considered unnecessary to consider the thinning amount of the bent portion in the design. Next, penetrant test(PT) on the outer surface of the bending pipes were also confirmed to be acceptable. Subsequently, metallographic samples were taken from the outer surface of the extrados side, neutral side and intrados side of the pipe bending portion. Metallographic observation confirmed that the microstructures were normal at all the three selected positions. After induction bending, the pipe was subjected to solution treatment. Thereafter, tensile tests and creep rupture tests were carried out on samples that were cut from the extrados side, neutral side and intrados side of the pipe bending portion. Tensile strength satisfied the minimum tensile strength indicated in the regulatory study for advanced thermal power plants report of Japan. Each creep rupture strength was the almost same regardless of the solution treatment conditions. From the above, it was possible to establish a induction bending technique for HR 6W large piping.

NiAl precipitates with the B2 structure are known to be effective in increasing the strength of ferritic heat-resistant steels. The strengthening mechanism by the NiAl precipitates was examined using Fe-21Al-2Ni and Fe-23Al-6Ni (at%) single crystals. As a result, the difference in primary slip system between the bcc matrix and the NiAl precipitates is responsible for strong hardening. The B2-NiAl phase was precipitated in the bcc matrix satisfying the cube-on-cube orientation relationship with small misfit strain. The primary slip direction of the bcc matrix and the NiAl precipitates are <111> and <001>, respectively. However, in the ferritic alloys, the NiAl precipitates were cut by paired 1/2<111> dislocations in the bcc matrix, resulting in the hardening. The size and volume fraction of the NiAl precipitates strongly influenced the strength. The stress increase by the NiAl precipitates was also discussed quantitatively based on the precipitation hardening theory. Based on the experimental results obtained by the single crystal study, we developed Fe-Al-Ni-Cr-Mo ferritic heat-resistant alloy containing the NiAl precipitates. The alloy exhibited excellent creep properties at 923 K.

The creep-fatigue properties of modified 9Cr-1Mo (grade 91) steel have been investigated for the purpose of design in cyclic service. In this paper test results from creep-fatigue (CF) and low cycle fatigue (LCF) on grade 91 steel are reported. The tests performed on the high precision pneumatic loading system (HIPS) are in the temperature range of 550-600ºC, total strain range of 0.7-0.9% and with hold periods in both tension and compression. Curves of cyclic softening and stress relaxation are presented. The CF test results and results obtained from literature are also analysed using methods described in the assessment and design codes of RCC-MRx, R5 and ASME NH as well as by the recently developed Φ-model. It is shown that the number of cycles to failure for CF data can be accurately predicted by the simple Φ-model. The practicality in using the life fraction rule for presenting the combined damage is discussed and recommendations for alternative approaches are made.

Finite element (FE) modeling has been applied to a stress relaxation cracking (SRC) test in order to evaluate the effects of changing sample geometry and material type. This SRC test uses compressive pre-straining to create a tensile residual stress in modified compact-tension specimens and has been used to test 316H stainless steel. The FE model is first used to verify that sample integrity will not be compromised by modifying the geometry. The FE model is then applied to candidate Advanced Ultra Supercritical nickel-base alloys 617, 740H, and 800. It is determined that this stress relaxation test will be appropriate for these alloys.

At present there is no recognized standard test method that can be used for the measurement of the tensile properties of additively manufactured lattice structures. The aim of this work was to develop and validate a methodology that would enable this material property to be measured for these geometrically and microstructurally complex material structures. A novel test piece has been designed and trialed to enable lattice struts and substructures to be manufactured and tested in standard bench top universal testing machines and in small scale in-situ SEM loading jigs (not reported in this paper). In conjunction with the mechanical tests, a finite element (FEA) modelling approach has been used to help cross validate the methodology and results, and to enable larger lattice structures to be modelled with confidence. The specimen design and testing approach developed, is described and the results reviewed for AlSi10Mg.

Ni-base superalloys used for hot section hardware of gas turbine systems experience thermomechanical fatigue (TMF), creep, and environmental degradation. The blades and vanes of industrial gas turbines (IGTs) are made from superalloys that are either directionally-solidified (DS) or cast as single crystals (SX). Consequently, designing and evaluating these alloys is complex since life depends on the crystallographic orientation in addition to the complexities related to the thermomechanical cycling and the extent of hold times at elevated temperature. Comparisons between the more complex TMF tests and simpler isothermal low cycle fatigue (LCF) tests with hold times as cyclic test methods for qualifying alternative repair, rejuvenation, and heat-treatment procedures are discussed. Using the extensive set of DS and SX data gathered from the open literature, a probabilistic physics-guided neural network is developed and trained to estimate life considering the influence of crystallographic orientation, temperature, and several other cycling and loading parameters.

Extended storage of spent nuclear fuel (SNF) in intermediate dry cask storage systems (DCSS) due to lack of permanent repositories is one of the key issues for sustainability of the current domestic Light Water Reactor (LWR) fleet. The stainless steel canisters used for storage in DCSS are potentially susceptible to chloride-induced stress corrosion cracking (CISCC) due to a combination of tensile stresses, susceptible microstructure, and a corrosive chloride salt environment. This research assesses the viability of the cold-spray process as a solution to CISCC in DCSS when sprayed with miniature tooling within a characteristic confinement in two different capacities: cleaning and coating. In general, the cold-spray process uses pressurized and preheated inert gas to propel powders at supersonic velocities, while remaining solid-state. Cold-spray cleaning is an economical, non-deposition process that leverages the mechanical force of the propelled powders to remove corrosive buildup on the canister, whereas the cold spray coating process uses augmented parameters to deposit a coating for CISCC repair and mitigation purposes. Moreover, both processes have the potential to induce a surface compressive residual stress that is known to impede the initiation of CISCC. Surface morphology, deposition analysis, and microstructural developments in the near-surface region were examined. Additionally, cyclic corrosion testing (CCT) was conducted to elucidate the influence of cold-spray cleaning and coating on corrosion performance.

Inconel alloy 740 was initially developed to enable the design of coal-fired boilers capable of operating at 700°C steam temperature and high pressure. The alloy successfully met the European program's targets, including 100,000-hour rupture life at 750°C and 100 MPa stress, and less than 2 mm metal loss in 200,000 hours of superheater service. However, thick section fabrication revealed weldability challenges, specifically grain boundary microfissuring in the heat affected zone (HAZ) of the base metal. This paper describes the development of a modified variant with significantly improved resistance to HAZ microfissuring and enhanced thermal stability, while maintaining desirable properties. The formulation process is detailed, and properties of materials produced within the new composition range are presented. Additionally, the microstructural stability of the original and modified alloy compositions is compared, demonstrating the advancements achieved in this critical material for next-generation power plants.

This study aims to examine the effects of grain boundary oxidation and creep on crack initiation and fracture behaviors in cold worked surface layer, under static tensile stresses in air. To determine these effects in relation to percent cold work and hardness scale, cold-rolled plates with a reduction ratios between 10% and 50% were prepared. Uniaxial constant load (UCL) tests were conducted at elevated temperature in air using smooth round bar specimen. UCL tests with a load of 0.9σy (926MPa) at 550°C show that rupture time for all cold- rolled materials were shorter than that of as-received material. From cross-sectional observation after UCL testing, surface crack at grain boundary and voids were observed in as-received material, whereas creep cracks were also observed in cold-rolled materials. This implied that crack initiation was assisted by cold working. Comparing test results with a load reduced to 0.8σy (823MPa), difference of rupture time was expected as a factor of 5 for as-received material, and measured as 2-3 for cold-rolled materials. It was suggested that cold worked layer was more sensitive to creep than base metal.

Oxide scale, whose ionic conductivity is larger than its electronic one, generate an electro-motive force between a metal/scale and a scale/gas interfaces. When the scale is thin, an electrical potential gradient is large. The large electrical potential gradient may have a possibility to break scales. To confirm the possibility, high temperature oxidation of zirconium on initial stage was observed by an acoustic emission (AE) technique. AE signal was detected before the scale thickness less than 3 μm. And an electrical response on sputtered zirconium oxide thin film was observed. When the applied voltage over 2.00 V, the electrical current was scattered.

The drive for increased efficiency and carbon reduction in next-generation boilers is pushing conventional materials to their limits in terms of strength and oxidation resistance. While traditional isothermal testing of simple coupons provides some insight into material performance, it fails to accurately represent the heat transfer conditions present in operational boilers. This paper introduces a novel test method designed to evaluate the degradation of candidate materials under more realistic heat flux conditions. The method, applied to tubular specimens using both laboratory air and steam as cooling media, demonstrates a significant impact of thermal gradients on material performance. Initial comparisons between tubular heat flux specimens and flat isothermal specimens of 15Mo3 revealed increased oxidation kinetics and altered oxide morphology under heat flux conditions. The paper details the design of this heat flux test, presents results from initial work on 15Mo3 under air and steam conditions, and includes findings from further studies on oxides formed on 2-1/4Cr material under both heat flux and isothermal conditions. This research represents a crucial step toward more accurate prediction of material behavior in next-generation boiler designs.