For future carbon neutral society, a novel thermal power generation system with no CO 2 emission and with extremely high thermal efficiency (~ 70 %) composed of the oxygen/hydrogen combustion gas turbine combined with steam turbine with the steam temperature of 700°C is needed. The key to realize the thermal power plant is in the developments of new wrought alloys applicable to both gas turbine and steam turbine components under higher temperature operation conditions. In the national project of JST-Mirai program, we have constructed an innovative Integrated Materials Design System , consisting of a series of mechanical property prediction modules (MPM) and microstructure design modules (MDM). Based on the design system, novel austenitic steels strengthened by Laves phase with an allowable stress higher than 100 MPa for 10 5 h at 700°C was developed for the stream turbine components. In addition, for gas turbine components, novel solid-solution type Ni-Cr-W superalloys were designed and found to exhibit superior creep life longer than 10 5 h under 10 MPa at 1000°C. The superior long-term creep strengths of these alloys are attributed to the “grain-boundary precipitation strengthening (GBPS)” effect due to C14 Fe 2 Nb Laves phase and bcc α 2 -W phase precipitated at the grain boundaries, respectively.

This study investigates the role of grain-boundary precipitates in enhancing creep rupture strength of Ni-based alloys through analysis of Ni-15Cr-15Mo and Ni-15Cr-17Mo (at.%) model alloys. The investigation focused on the “Grain-boundary Precipitation Strengthening (GBPS)” effect from the thermally stable TCP phase, a phenomenon previously observed in Fe-Cr-Ni-Nb austenitic heat-resistant steels. Through multi-step heat treatments, specimens were prepared with varying grain boundary coverage ratios (ρ) of TCP P phase (oP56) and consistent grain-interior hardness from GCP Ni2(Cr, Mo) phase (oP6). In the 15 at.% Mo alloy, specimens with a higher coverage ratio (~80%) demonstrated significantly improved creep performance, achieving nearly four times longer rupture time (3793 h vs. 1090 h) at 300 MPa and 973 K compared to specimens with lower coverage (~35%). However, the 17 at.% Mo alloy showed unexpectedly lower performance despite high coverage ratios, attributed to preferential cavity formation at bare grain boundaries. These findings confirm that GBPS via thermally stable TCP phase effectively enhances creep properties in Ni-based alloys, with grain boundary coverage ratio being more crucial than intragranular precipitation density.

The formation of periodically arrayed rows of very fine Fe 2 Hf Laves phase particles was recently found in 9 wt. % chromium ferritic matrix through interphase precipitation along a reaction path of δ-ferrite → γ-austenite + Fe 2 Hf with a subsequent phase transformation of the γ phase into the α-ferrite phase. One of the problems on the formation of the fine Laves phase dispersion is a poor heat treatability; the interphase precipitation (δ-Fe→γ-Fe+Fe 2 Hf) is competitive with the precipitation of Laves phase from the δ phase in the eutectoid-type reaction pathway (δ→δ+Fe 2 Hf). In the present work, the effect of supersaturation on the precipitation of Laves phase from δ phase (δ→δ+Fe 2 Hf) and the δ→γ transformation in the reaction pathway was investigated by changing the Hf and Cr contents. The results obtained suggest that it is effective to have a high supersaturation for the precipitation of Laves phase and an adequately high supersaturation for the δ→γ transformation at the same time in order to widen the window of the interphase precipitation

Effects of alloying additions of Ti or Mo to a simplified chemical composition of the γ′′-Ni 3 Nb strengthened type Ni-based alloy 718 on the precipitation mode of δ-Ni 3 Nb phase were investigated to aim at designing grain boundaries using the δ phase for raising temperature capability of the γ′′ strengthened Ni-based wrought alloys. In the base alloy of Ni-22Cr-16Fe-3.5Nb, the δ phase precipitated at the grain boundaries of the matrix phase in a platelet form by continuous precipitation mode at temperatures above 1273K (1000°C) but in a lamellar morphology by discontinuous precipitation mode below that temperature. The boundary temperature where the continuous/discontinuous precipitation mode changes was raised by addition of 1 % Ti and lowered by addition of 5% Mo. The increase in the boundary temperature by Ti addition can be considered to have occurred by an increase in the solvus temperature of γ′′ phase. The decrease in the boundary temperature by Mo addition can be interpreted by the reduction of the strain energy caused by the coherent γ′′ precipitates and/or the volume change by the formation of δ phase from the γ/γ′′ phases, which may promote the continuous precipitation with respect to the discontinuous precipitation.

Electron beam melting (EBM) is one of the candidate manufacturing processes for TiAl alloys which have been considered as next generation high-temperature structural materials. The microstructure and mechanical properties of Ti-48Al-2Cr-2Nb (48-2-2) alloy bars fabricated using EBM were investigated, with a particular focus on the effect of processing parameters such as input energy density and building direction. We observed that the microstructure of the alloy bars fabricated using EBM depends strongly on the processing parameters used during the fabrication process of alloy. In particular, the alloy bars fabricated under appropriate processing parameters have a unique layered microstructure composed of duplex regions and equiaxed γ-grain regions (γ bands). Because of their fine microstructure and deformable soft γ bands, the alloy bars with the unique layered microstructure exhibit higher strength and higher ductility at room temperature (RT) than that of cast alloys. In addition, the alloy bars fabricated at an angle between the building direction and the loading axis of 45° show good fatigue properties at RT even without hot isostatic pressing treatment.

A novel salt-bath nitrocarburizing process recently developed forms a lithium-iron compound-oxide layer on the surface of steel in concurrence with a nitride layer by adding lithium ions to the molten salt. This process has already been successfully applied to mass-produced products. However, the microstructure and its formation process of the surface layer in alloyed steels during the nitrocarburizing process have not yet been fully understood. In this study, we focus on the effect of Si and Cr, which are included in a common die steel, on the microstructure of an oxide layer of a nitrocarburized alloy. The alloys used in this study are Fe-0.4wt%C, Fe-0.4wt%C-2.0wt%Si, and Fe-0.4wt%C-2.0wt%Cr. These alloys were arc melted into button ingots under an Ar atmosphere. The ingots were annealed at 1123 K for 1.0 h, followed by air cooling and double tempering at 873 K, similar to the heat treatments employed to hot-die steels. Salt-bath nitrocarburizing was carried out at 823 K for 0.1-10 h. The microstructures of the cross-sectional surface layers of the samples were examined using an optical microscope and FE-SEM. Elemental mapping as well as phase identification of the surface layers were done by EDS, XRD, and GD-OES. In the Fe-C binary alloy, a thin continuous oxide layer of α-LiFeθ 2 formed first on the outermost surface, and a thick iron nitride layer developed underneath the oxide layer, with aligned oxide particles along the grain boundaries of the nitrogen compound layer. In the case of Si addition, the outermost oxide layer became thinner and an additional oxide layer consisting of α-LiFeθ 2 and (Li,Fe) 3 Siθ 4 formed between the outer oxide layer and nitrogen compound layer, and the formation of the oxide particles in the nitrogen compound layer was fully suppressed. In the case of Cr addition, internal oxide particles formed in the nitrogen compound layer, similar to those in the binary steel, although an continuous oxide layer of CrfN,O) formed in between those layers. On the basis of these results, the inner oxide layer formed with Si addition contributes to improving the frictional wear characteristics in die steels.

The competitive effect of Nb and V additions on the high-temperature oxidation behavior of Ti- 30Al alloys were studied at 800°C in air. Oxidation performance increased with increasing Nb content, however, V additions eliminated the beneficial effect of Nb on oxidation performance, causing higher oxidation mass gains. In-situ high-temperature XRD by means of synchrotron source suggested dissolution of Nb 5+ but lower valence of vanadium ions in the TiO 2 oxide scale during oxidation. Dissolution of Nb and V ions with different valence in TiO 2 during oxidation could cause the beneficial and detrimental effects observed on the performance of high-temperature oxidation of Ti-30Al.

700°C advanced ultra-supercritical system and supercritical CO 2 turbine system are developed for high efficiency turbine systems for next generation. This study covered the feasibility of creep life assessment of γ’-Ni 3 (Al,Ti) precipitation strengthened Ni-based superalloy rotor material, TOS1X-2, a modified alloy of UNS N06617 for these systems, based on hardness measurement method. It was found that the hardness of TOS1X-2 was governed by the change in precipitation strengthening and strain hardening during creep. The clear relationship between hardness increase in crept portion and macroscopic creep strain was observed, suggesting that it might be possible to estimate the creep strain or initiation of acceleration from hardness measurement. Microstructure inhomogeneity and microstructure evolutions during creep especially focused on dispersion of creep strain were characterized by EBSD quantitative analysis. It was found that creep strain was accumulated along the grain boundary, while it was relatively absent in coarse grains with low Schmid factor of {111} <110> slip system in fcc structure. The upper limit of hardness scatter band is thought to be important, since it represents the local and critical creep damage of the alloy.

Tensile deformation behavior of γ-TiAl based alloys consisting of α 2 -Ti 3 Al/γ lamellar colonies, β-Ti grains, and γ grains were investigated by in-situ scanning electron microscopy and digital image correlation technique, in order to identify the role of each microstructure constituents in deformation. The alloy with nearly lamellar microstructure, in which the volume fraction of β/γ duplex ( V DP ) is 10%, shows elongation of only 0.14%, whereas the alloy with nearly globular β/γ duplex microstructure with V DP of 94% shows elongation of 0.49%. In α 2 /γ lamellar microstructure, obvious strain localization occurs along lamellae and develops at specific regions with loading. In the case of β/γ duplex microstructure, strain localization is observed in γ grains and in β phase regions near the β/γ phase boundary, although no obvious deformation is observed in the β grains. β/γ phase boundaries enhances room temperature ductility of TiAl alloys by inducing multiple slip in γ phase and deformation of β phase.

The creep behavior of a γ-TiAl based alloy at 1073 K was investigated, examining three different microstructures: equiaxed γ (Eγ), γ/γ fully lamellar (FLγ), and equiaxed γ with α 2 phase on grain boundaries (Eγα 2 ). The aim was to understand the influence of lamellar interfaces and grain boundary α 2 phase on creep behavior. Initially, creep rates were consistent across all specimens upon loading. However, Eγ exhibited a gradual decrease in creep rate compared to Eγα 2 and FLγ. Notably, the minimum creep rate of Eγ was one order of magnitude lower than that of Eγα 2 and FLγ. Conversely, Eγα 2 and FLγ displayed a slight acceleration and the longest rupture strain, albeit with the shortest rupture time compared to Eγ. Upon microstructural analysis of of the creep-test specimens, it was observed that numerous dynamic recrystallized grains (DXGs) and sub-grains formed along grain boundaries and interiors in Eγ, whereas they were limited to the region along grain boundaries in FLγ. In contrast, very few DXGs were formed in Eγα 2 . These findings indicate that γ/γ interfaces inhibit the extension of DXGs into grain interiors, suggesting that the grain boundary α 2 phase effectively suppresses the formation of DXGs.

Interstitial carbon (C) in β-Ti, α-Ti, α 2 -Ti 3 Al and γ-TiAl phases present in the γ-TiAl alloys with and without substitutional elements (M: transition element) is quantitatively analyzed using soft X-ray emission spectroscopy (SXES), in order to reveal the effect of solute carbon on the phase equilibria. SXES for carbon analysis was used and the peak intensity of the second reflection of carbon Kα is analyzed using the fully homogenized sample having different C content under the optimum condition to make the accurate calibration curves. The obtained calibration curve is in an accuracy of ± 0.07 at. % C. In all heat treated alloys, no carbide is observed. In Ti-Al binary system, the α+γ phase region shifts toward higher Ti side, and the volume fraction of γ phase increases slightly with the carbon addition. In all system, carbon preferentially partitions into the α phase, followed by less partitioning in the γ and β phases in order. The carbon content in the β phase remains unchanged of almost 0.05 at. % regardless of carbon addition in Ti-Al-V system and the partition coefficient of carbon between the α and γ phases becomes larger in Ti-Al-V system than that in TiAl binary system.

The precipitation behavior of various phases in austenitic heat-resistant model steels, including the Fe 2 Nb Laves phase (C14 structure) on grain boundaries (GB) and grain interiors (GI), and the Ni 3 Nb metastable γ“ phase and stable δ phase on GI, was investigated through experimental study at different temperatures and thermokinetic calculation. The steel samples were prepared by arc melting followed by 65% cold rolling. Subsequently, the samples were solution treated within the γ single-phase region to control the grain size to approximately 150 μm. Aging of the solution-treated samples was carried out at temperatures ranging from 973 K to 1473 K for up to 3600 hours. Microstructural observations were conducted using FE-SEM, and the chemical compositions of the γ matrix and precipitates of Laves and δ phases were analyzed using EPMA. The precipitation modeling was performed using MatCalc software, utilizing a thermodynamic database constructed by our research group to calculate the chemical potential of each phase. Classical nucleation theory was applied for nucleation, while the SFFK model was employed for the growth and coarsening stages. Distinct phases were defined for grain boundary and grain interior Laves phase, with all precipitates assumed to have spherical morphology in the calculations. The precipitation start time was defined as the time when the precipitate fraction reached 1%. Experimental results indicated that above 973 K, Laves phase nucleation primarily occurred on grain boundaries before extending into the grain interior, with the nose temperature located around 1273 K. To replicate the experimentally determined Time-Temperature-Precipitation (TTP) diagram, interaction parameters among elements were adjusted. Additionally, by introducing lower interfacial energy between the γ matrix and Laves phase, the TTP diagram was successfully reproduced via calculation, suggesting relative stability at the interface.

In the present study, the precipitation kinetics of topologically close-packed (TCP) Fe 2 Nb Laves and geometrically close-packed (GCP) Ni 3 Nb phases is studied quantitatively in experimental alloys with different Ta / Nb+Ta ratio, to clarify the mec4hanism of the Ta effect. The microstructure of alloys is changed from Widmanstätten structure to lamellar structure due to discontinuous precipitation, with increasing Ta / Nb+Ta. It is confirmed that Ta partitions into both Fe 2 Nb Laves and Ni 3 Nb phases. However, two phases stability is changed by added Ta content. Ta accelerates the formation kinetics of the precipitates at grain boundaries, as well as γ“-GCP phase within grain interiors, due to increased supersaturation by Ta addition. Besides, Ta retards the transformation kinetics of metastable γ“-Ni 3 Nb to stable the δ-Ni 3 Nb phase. The results indicate that Ta decreases the driving force for the transformation of the δ-GCP phase.

In this study the effect of Widmanstätten-type morphology α 2 plates on creep has been investigated by preparing nearly equiaxed γ (N γ ) and nearly equiaxed γ having Widmanstätten-type α 2 plates within grain (Wα 2 ). Creep tests were conducted at 1073 K under constant stresses, high stress and low stress, in air. At the high stress, Wα 2 shows creep rate smaller than N γ in transient stage, both specimens show similar minimum creep rate and the creep strain at minimum creep rate is 3 % for Wα 2 and 10 % for N γ, since N γ shows prolonged primary region. In acceleration stage, both show similar behavior with rupture time of about 50 h and rupture elongation of 60 %. At the low stress, on the other hand, reverse behavior occurs, that is, W α 2 shows creep rate higher than Nγ in transient stage. The regions near grain boundaries progressively deformed for both specimens at high stress level, whereas deformed region is extended within grain interiors. From these results it is suggested that α 2 plate act as the obstacle for dislocation motion in the γ matrix at high stress and that interfacial dislocation promote the creep deformation at low stress.

Strengthening of Ni-based superalloys is in principle designed using GCP (Geometrically Close-packed phase) of Ni 3 Al-γ' (L1 2 ). However, game-changing microstructural design principle without relying on γ' phase will be needed for further development of the alloys. We are currently constructing a novel microstructure design principle, using thermodynamically stable TCP (Topologically Close-packed phase) for grain boundaries, together with GCP other than γ' phase for grain interiors, based on grain boundary precipitation strengthening (GBPS) mechanism. One of the promising systems is Ni-Cr-Mo ternary system, where TCP of NiMo (oP112) phases, μ (hR13) and P (oP56), together with GCP of Ni 3 Mo (oP8) and Ni 2 Cr (oP6) exists. In this study, thus, phase equilibria among A1 (fcc)/TCP/GCP phases in Ni-Cr-Mo and Ni-Cr-W systems have been examined at temperature range from 973 K to 1073 K, based on experiment and calculation. In Ni-Cr-Mo system, Ni 2 (Cr, Mo) with oP6 Pearson symbol, which is stable at about 873 K in Ni-Cr binary system, is formed to exist even at 1073 K. oP6 phase is coherently formed in A1 matrix with a crystallographic orientation of {110} A1 // (100) oP6 , <001>Α1 // [010]oP6, indicating GCP at composition range around Ni-15Cr-15Mo as island. In Mo-rich region there is Α1/NiMo/oP6 three-phase coexisting region, whereas another three-phase coexisting region of Α1/P/oP6 exists in Cr-rich region. Based on vertical section, it is possible to design microstructure with TCP at grain boundaries, together with oP6 phase within grain interiors by two-step heat treatment.

Effects of microstructure constituents of α 2 -Ti 3 Al/γ-TiAl lamellae, β-Ti grains and γ grains, with various volume fractions on room-temperature ductility of γ-TiAl based alloys have been studied. The ductility of the alloys containing β phase of about 20% in volume increases to more than 1% as the volume fraction of γ phase increases to 80%. However, γ single phase alloys show very limited ductility of less than 0.2%. Microstructure analysis have revealed that intragranular fracture along γ/γ grain boundary occurred in γ single phase alloy whereas it does not along β/γ interphase in alloys containing β phase. In addition, local strain accumulations along β/γ interphase have been confirmed. The present results, thus, confirmed the significant contribution of β phase, especially the existence of β/γ interphase to enhancement of the room-temperature ductility in multicomponent TiAl alloys.

Steam oxidation of a novel austenitic steel, of which composition is Fe-20Cr-30Ni-2Nb (at.%), has been conducted at 973 K to evaluate steam oxidation resistance based on detail analyses of scale morphology and scale growth. Two types of scale morphologies were observed in the solution treated sample, depending on the grain of the steel. Although thin duplex scale with the Cr-rich layer was formed in the early stage, most of the surface was covered with thick duplex scale which consists of magnetite as the outer scale and the mixture of Fe-Cr spinel and metallic Ni as the inner scale. On the other hand, surface morphology of the oxide scale was independent of grain of the steel and thick duplex scale as seen on the solution treated sample was formed on the pre-aged sample. Steam oxidation resistance of the steel is almost the same as that of commercial austenitic steels and it can be improved by the surface treatment such as shot peening. Based on the results, this steel has both enough creep rupture strength and good steam oxidation resistance for A-USC power plants.

In this study, we have examined the creep of a novel austenitic heat resistant steel of Fe-20Cr- 30Ni-2Nb (at.%) steel at 1073K in steam and air atmospheres. Our studied steels were Fe-20Cr- 30Ni-2Nb (base steel) and that with 0.03 at. %B (B-doped steel) . The addition of boron is to intentionally increase the area fraction of Laves phase on grain boundaries (ρ). The specimen with ρ = 43% (base steel pre-aged at 1073 K/240 h) exhibits the rupture life of 262 h, whereas the rupture life of the specimen with higher ρ of 80% (B-doped steel pre-aged at 1073 K/240 h) is 833h, which is about three times longer than that of the specimen with ρ = 43%. The specimen with ρ = 80% exhibits smaller creep rate than those with lower ρ than 43% in the entire creep stage. In addition, all specimens show the creep rupture strain of about 60%. The creep rupture life is almost same to that tested under air, whereas the creep rupture strain is slightly smaller (a few percent) than that under air. In the surface of the creep ruptured specimen in steam, the intergranular oxides associated with voids or cavities are often present and grow along grain boundaries to over 100 μm in depth. The intergranular oxidation occurs more extensively in steam rather than air. These results demonstrate that stable Fe 2 Nb Laves phase on grain boundary could increase the creep resistance of the present steel at 1073K without ductility loss in steam as well as air, resulting in the pronounced extension of rupture life. The intergranular oxidation accelerated by steam would not give a serious effect on the creep properties of the present steel below 103 hours in rupture life.