The development of Gen-IV nuclear systems and ultra-supercritical power plants proposes greater demands on structural materials used for key components. An Fe–18Ni–16Cr–4Al (316-base) alumina-forming austenitic steel was developed in our laboratory. Its microstructural evolution and mechanical properties during aging at 950°C were investigated subsequently. Micro-structural changes were characterized by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. Needle-shaped NiAl particles begin to precipitate in austenite after ageing for 10 h, whereas round NiAl particles in ferrite are coarsened during aging. Precipitates of NiAl with different shapes in different matrices result from differences in lattice misfits. The tensile plasticity increases by 32.4% after aging because of the improvement in the percentage of coincidence site lattice grain boundaries, whereas the tensile strength remains relatively high at approximately 790 MPa.

The effects of aging at 700 °C on the microstructure and tensile properties of an alumina-forming austenitic (AFA) stainless steel were investigated. With increasing aging time, B2-NiAl and Laves phase precipitated first on grain boundaries (GB) and then in the grain interior. The GB precipitate coverage reached 74% after aging for 1000 h. The GB precipitates not only suppressed grain coarsening during aging, but also influenced the tensile fracture mode at 700 °C by partitioning stress concentration on triple junctions. Moreover plate-like and spherical NbC particles precipitated during aging. Spherical NbC with size of around 5 nm were stable, while plate-like NbC grew to 89 nm after aging for 1000 h. These precipitates played an important role on the tensile strength. Age hardening contributed to the increasing tensile strength at RT with aging time, while the softening mechanism of dynamic recovery dominated the tensile tests at 700 °C.

The oxidation morphologies of modified 310 steel exposed in 900 and 1100 °C air were investigated. A double layer morphology consisting of a (Cr, Mn)-rich outer layer and a fine Cr-rich inner layer was formed at 900 °C. It was related to the breakaway oxidation induced by the Cr-depletion and the Mn-segregation in inner layer. Some Cr-rich oxides with amorphous state were formed along grain boundaries. And some new finer oxide grains, voids and Cr-rich precipitates were observed in spallation areas at 1100 °C. Correspondingly, the oxidation kinetic curve dropped with the spallation of scale and increased with the formation of some new oxide grains. It was caused by segregation of Cr and the transformation of oxides from Cr2 O3 to the volatile oxides at elevated temperature. XRD analysis showed that the precipitates were mainly composed of CrO3. Segregation and depletion for solutions were also discussed by oxidation diffusion mechanisms.

•The Al-containing 9Cr ODS ferritic alloy was firstly fabricated by MA and HP.•The precipitates were identified as YAM, YAH, YAP, and YAG.•The volume fractions of precipitates were calculated by integrated intensity.•The YS and UTS at 700 °C are 245 MPa and 276 MPa, respectively.In this study, a 9Cr oxide-dispersion strengthened (ODS) alloy with additional corrosion resistant element Al was fabricated by mechanical alloying (MA) and hot pressing (HP) to explore the impact of Al on the microstructure and mechanical property of a 9Cr ODS alloy. It is found that the Al completely dissolved into the Fe–Cr matrix after milling for 30 h. The minor phases in the Al-containing 9Cr ODS ferritic alloy were investigated by a high-energy X-ray, and were identified to be orthorhombic-YAlO3 (YAP), bcc-Y3Al5O12 (YAG), monoclinic-Al2Y4O9 (YAM), and hexagonal-YAlO3 (YAH). These phases were further confirmed by selected area diffraction pattern (SADP), energy dispersive spectroscopy (EDS), and high resolution transmission electron microscopy (HRTEM). In addition, their volume fractions were also calculated from the integrated intensities. According to the analysis of the particles and their formation sequences, the larger particles (greater than 100 nm) are identified as mainly YAG and Al2O3 particles, while the particles with small size (less than 30 nm) are likely primarily YAM, YAH, and YAP particles. The yielding strength (YS) and ultimate tensile strength (UTS) at RT are 563 MPa and 744 MPa, respectively, while the YS and UTS at 700 °C are 245 MPa and 276 MPa, respectively. Although the addition Al in ODS alloys decreases the strength at RT, the values at high temperature are similar to those obtained for 9Cr ODS alloys strengthened by fine Y–Ti–O particles.Synchrotron X-ray diffraction line profile of the 9CrAl ODS alloy; (Ferrite matrix phases, along with minor phases, orthorhombic YAlO3 (yttrium aluminum perovskite, YAP), bcc Y3Al5O12 (yttrium aluminum garnet, YAG), monoclinic Al2Y4O9 (yttrium aluminum monoclinic, YAM), and hexagonal YAlO3 (yttium aluminum hexagonal, YAH) were recognized.).

•Different ODS austenitic powders showed different phase transitions during MA.•Nano-structural ODS austenitc powders were obtained by MA. The average grain size of as-hipped samples was around several hundred nanometers.•ODS-304 and ODS-316 austenitic powders completely transformed into α after MA, while ODS-310 stayed γ.•The element of Ti favored the transformation of γ to α in ODS austenitic powders during annealing and consolidationDifferent austenitic steel powders with additions of Y2O3 and Ti were fabricated by mechanical alloying (MA). The structural evolutions during the process of ball milling and subsequent annealing were studied by XRD, SEM and TEM. Nano crystalline austenitic powders were obtained by MA. Different ODS austenitic powders presented different phase transition during the process of MA and annealing, which were resulted from different contents of Ni and Cr. Both ODS-316 and ODS-310 showed a weak diffraction peak of α after annealing and consolidated by hot isostatic pressing (HIP) due to the addition of Ti. According to the TEM results, the grain size of all three ODS austenitic steels was around several hundred nanometers.

•The dispersed Y–Ti–O particles are finer and more homogeneous than Y–Al–O particles.•The plasticity of 14Cr–Al–ODS steel is superior to that of 14Cr–Ti–ODS steel.•14Cr–Al–ODS steel shows higher impact energy than that of 14Cr–Ti–ODS steel.Two kinds of ODS ferritic steels with nominal compositions (wt.%): Fe–14Cr–4.5Al–1W–0.35Y2O3 and Fe–14Cr–0.5Ti–1W–0.35Y2O3 have been fabricated by mechanical alloying of a pre-alloyed gas atomized powder with nano yttria powders and then consolidated by hot isostatic pressing. The effects of minor alloying elements Al and Ti on the microstructure and mechanical properties of the fabricated ODS steels have been investigated. Microstructural characterization revealed that Y–Ti–O and Y–Al–O dispersed particles were formed in the 14Cr–Ti–ODS steel and 14Cr–Al–ODS steel respectively, and the particle size of Y–Ti–O is much smaller than that of Y–Al–O. The tensile property test demonstrated that the 14Cr–Ti–ODS steel shows high strength but poor plasticity, while the 14Cr–Al–ODS steel has low strength but rather good plasticity. 14Cr–Al–ODS steel has much higher impact energy than that of 14Cr–Ti–ODS steel at the same testing temperatures.

•The tensile deformation of 9Cr ODS steel was studied by synchrotron irradiation.•The evolution of internal mean stress was calculated.•The evolution of dislocation character was determined by best-fit method.•Edge type dominates plasticity at RT and 300 °C, while screw type dominates at 600 °C.An application of high-energy wide angle synchrotron X-ray diffraction to investigate the tensile deformation of 9Cr ferritic/martensitic (F/M) ODS steel is presented. With tensile loading and in-situ X-ray exposure, the lattice strain development of matrix was determined. The lattice strain was found to decrease with increasing temperature, and the difference in Young's modulus of six different reflections at different temperatures reveals the temperature dependence of elastic anisotropy. The mean internal stress was calculated and compared with the applied stress, showing that the strengthening factor increased with increasing temperature, indicating that the oxide nanoparticles have a good strengthening impact at high temperature. The dislocation density and character were also measured during tensile deformation. The dislocation density decreased with increasing of temperature due to the greater mobility of dislocation at high temperature. The dislocation character was determined by best-fit methods for different dislocation average contrasts with various levels of uncertainty. The results shows edge type dislocations dominate the plastic strain at room temperature (RT) and 300 °C, while the screw type dislocations dominate at 600 °C. The dominance of edge character in 9Cr F/M ODS steels at RT and 300 °C is likely due to the pinning effect of nanoparticles for higher mobile edge dislocations when compared with screw dislocations, while the stronger screw type of dislocation structure at 600 °C may be explained by the activated cross slip of screw segments.

•The effects of mechanical alloying process on W–Y system powders were studied.•The microstructure evolutions of milled powders and consolidated compacts were investigated.•The super-saturated but homogeneous W–Y solid solution could be formed at the milling time of 15 h.•Excessive O impurity resulted in the decrease of density and degradation of bending strength.The W-3 Y composites were successfully prepared by spark plasma sintering of milled W-3 Y powders with different milling times (0 h, 5 h, 15 h, 30 h). X-ray diffraction (XRD), scanning electron microscope (SEM), and laser particle size analysis were used to study the microstructural evolution and morphological change during the milling process. The crystallite sizes exhibited a continuous refinement along with the increased milling time. The median particle sizes, measured by the laser diffraction method, showed a similar change tendency. Due to the existence of Y particles, the W–Y milled powders exhibited spherical-like morphology while pure tungsten milled powders exhibited lamellar morphology at the early milling stage (5–15 h). The microhardness of W-3 Y compacts showed a slight increase with the increase of milling time. The maximum bending strength of 795 MPa was obtained by sintering W-3 Y powders milled for 15 h. As the milling time was prolonged to 30 h, the increased oxygen impurity resulted in a slight decrease of density as well as the degradation of bending strength.XRD patterns of the strongest diffraction peak (110) of W showing change of W peak during milling. FE-SEM images inside the XRD patterns showing the mapping of W (green), Y (blue) and O (red) of consolidated W–Y alloys with different milling times.

•Y doping is beneficial to achieve fully dense tungsten alloys as compared with Y2O3 doping and La doping.•Y doping is more effective in inhibiting the grain growth of tungsten than Y2O3 doping and La doping.•The effect of different rare earth elements on the sinterability of tungsten alloys are investigated and compared.The effects of rare earth elements (Y2O3, Y and La) on the consolidation behavior, microstructure and mechanical properties of tungsten alloys were investigated in this work. The starting powders were mechanical alloyed (MA) and then consolidated by spark plasma sintering (SPS). It was found that Y doping was beneficial to obtain fully dense tungsten alloys with more refined grains as compared to any other rare earth elements. The maximum values of Vickers microhardness and bending strength obtained from W–0.5 wt.% Y alloy reached up to 614.4 HV0.2 and 701.0 MPa, respectively.

The effect of hot rolling on the microstructure and fracture behavior of a bulk W–Y2O3 alloy was studied. The as-sintered compact was comprised of polyhedral tungsten grains with an average grain size of 4 μm, spherical Y2O3 disperoids and trace amount of nano-sized/submicron pores. It showed typical inter-granular fracture. After hot rolling, it was observed that the tungsten grains were elongated along the rolling direction, showing much severe deformation than Y2O3 particles. The residual pores were reduced. The as-rolled alloy showed obvious anisotropic fracture behavior. For the longitudinal specimens, the fracture mode could be totally transformed to trans-granular cleavage fracture. Three point bending test showed that the longitudinal specimens of the bulk W–Y2O3 alloy performed appreciable ductility and superior fracture strength (2152.7 MPa) as compared to the as-sintered alloy (brittleness, 543.4 MPa) and transverse specimens (brittleness, 1105.0 MPa). The responsible strengthening and toughing mechanisms were analyzed and presented.

The hot deformation behavior of 9Cr oxide-dispersion-strengthened (ODS) steel fabricated through the process of mechanical alloying and hot isostatic pressing (HIP) as investigated through hot compression deformation tests on the Gleeble-1500D simulator in the temperature range of 1050–1200 °C and strain rate range of 0.001 s−1–1 s−1. The relationship between the rheological stress and the strain rate was also studied. The activation energy and the stress and material parameters of the hyperbolic-sine equation were resolved according to the data obtained. The processing map was also proposed. The results show that the flow stress decreases as the temperature increases, and that decreasing of the strain rate of the 9Cr ODS steel results in a positive strain rate sensitivity. It is clear that dynamic recrystallization is influenced by both temperature and strain rate. The results of this study may provide a good reference for the selection of hot working parameters for 9Cr ODS steel. The optimum processing domains are at 1200 °C with a strain rate of 1 s−1 and in the range of 1080–1100 °C with a strain rate between 0.018 s−1 and 0.05 s−1.

Tungsten based vanadium alloys have been fabricated by powder metallurgy and consolidated by spark plasma sintering (SPS) at temperature of 1600 °C for 3–5 min at 50 MPa. Four different concentrations of vanadium ranging from 1 to 10 wt.% were used to investigate the behavior of the developed alloys. X-ray diffraction analyses were performed for all four compositions of tungsten vanadium alloys. The morphology of cross sectional crack surfaces of sintered alloys was analyzed by scanning electron microscopy. The variations of vanadium concentration has not only shown an obvious impact on the microstructures, but also improved the densification and mechanical properties of the tungsten based materials. The maximum relative density of 98.5% was achieved for the highest concentration (10 wt.%) of vanadium alloy with micro hardness of 507 HV and good bending strength of 692.5 MPa.

The present investigation aimed at researching the mechanical properties of the oxide dispersion strengthened (ODS) ferritic steels with different Cr content, which were fabricated through a consolidation of mechanical alloyed (MA) powders of 0.35 wt.% nano Y2O3 dispersed Fe–12.0Cr–0.5Ti–1.0W (alloy A), Fe–16.0Cr–0.5Ti–1.0W (alloy B), and Fe–18.0Cr–0.5Ti–1.0W (alloy C) alloys (all in wt.%) by hot isostatic pressing (HIP) with 100 MPa pressure at 1150 °C for 3 h. The mechanical properties, including the tensile strength, hardness, and impact fracture toughness were tested by universal testers, while Young’s modulus was determined by ultrasonic wave non-destructive tester. It was found that the relationship between Cr content and the strength of ODS ferritic steels was not a proportional relationship. However, too high a Cr content will cause the precipitation of Cr-enriched segregation phase, which is detrimental to the ductility of ODS ferritic steels.

ODS-304 austenitic steel (pre-alloyed austenitic steel powders +0.35 wt% Y2O3+0.5 wt% Ti) was fabricated by the process of mechanical alloying and hot isostatic pressing. The milled powders were characterized by XRD and SEM. New peaks of YO1.401 appeared after milling of 5 h, which indicated that Y2O3 decomposed and dissolved into the matrix during MA. A bimodal distribution of grains was found in the as-HIPed specimen by TEM. It was found that fine oxide particles enriched in Y–Ti–O were dispersed in the small grains, while there were almost no dispersoids inside the large grains. Plastic deformations, including forging and hot rolling, were subjected to the as-HIPed specimens. Tensile properties were measured at 23 and 700 °C. At 23 °C, the UTS and elongation of the as-HIPed specimen reached 940 MPa and 24.5% respectively. Specimens showed much higher ductility after plastic deformations while there was just slight decrease of tensile strength at the same time.

The corrosion behavior of a 18Cr-ODS (oxide dispersion strengthened) steel exposed in supercritical water (600 °C/25 MPa) was investigated. The results indicate that the weight gain increases with the exposure time, following a parabolic law. A triple layer consisting of an outer layer, an inner layer and a diffusion layer forms on the sample. The phases of the outer layer are Fe2O3 and Fe3O4. The inner layer and the diffusion layer contain (Fe, Cr)3O4 and Cr2O3, respectively. Cracks, pores and exfoliations could be observed on the outer layer. Finally, the oxidation mechanism was discussed and the determining factors were given.Highlights► The corrosion behavior of a 18Cr-ODS steel was investigated. ► A triple oxide layer different from the conventional steels forms. ► The Fe-18Cr/O2 stability diagram is calculated, and oxide mechanism is discussed. ► Reaction rate is also proposed as a rate determining factor.

Pure Fe, Cr, Ni, W, Ti elemental powders and nano-Y2O3 powders (the compositions of the mixed powders are Fe–18Cr–8Ni–2W–1Ti–0.35Y2O3) were processed by high energy mechanical milling. The as milled powders were consolidated by hot isostatic pressing for 3 h at 1423 K under a pressure of 200 MPa. The microstructure of the fabricated ODS austenitic steels and chemical composition of the oxide particles were examined by field emission scanning electron microscopy with energy dispersive spectrometry. The dispersed fine oxides were determined to be Y–Ti–O oxides. The tensile properties were improved significantly by oxide dispersion strengthening but the ductility deteriorated.

12 wt.% Cr-ODS ferritic alloys were fabricated by mechanical alloying combined with hipping under different sintering pressure and then forging deformation work. The microstructures of the hipped and forged samples after annealing at different temperatures were investigated and compared. No obvious recrystallization occurred even at an annealing temperature of 1100 °C, both for as-hipped and as-forged materials. When hipping under high pressure (200 MPa), the ODS material is very dense, and almost no cavities could be found after annealing at high temperature. While when hipping under low pressure (70 MPa), obvious pores could be found for this material after forging, the pores assembled and formed big cavities up to ∼10 μm after annealing at temperature of 1300 °C.

A functionally graded material-based actively water-cooled tungsten-copper mockup with a dimension of 30 mm×30 mm×25 mm was designed and fabricated by infiltration-brazing method. The thicknesses of the pure W layer and W/Cu graded layer were 2 and 3 mm, respectively. High heat flux tests were performed on the mockup using an e-beam device. There is no damage occurring on the joint after heat loading at 5 MW/m2. The temperature on the pure W surface is less than 500°C after irradiation for 100 s at 5 MW/m2, while the temperature on the brazing seam/copper surface is around 200°C.

The consolidation behavior of tungsten powders with different particle size (ranging from submicron size to several microns) have been investigated by using a novel method combining resistance sintering with ultra-high pressures. The densification effects of the consolidation parameters, including pressure, input power and sintering time, have been investigated. High density is achieved for consolidation of micron sized tungsten powder and the microstructure retained its fine-grain size. The sintered samples achieve higher hardness and bending strength when the finer powder is used. The sintering mechanism of this method was quite different from other sintering techniques.

Resistance sintering under ultra-high pressure was developed to fabricate W–Cu composite containing 5–80 v/o copper. The consolidation was carried out under pressure of 6–8 GPa and input power of 18–23 kW for 50 s. The densification effect and microstructure of these W–Cu composites were investigated. The effect of W particle size on sintering density was also studied. The micro-hardness was measured to evaluate the sintering effects.

The possibility and development of fabrication of W coating by atmospheric plasma spraying (APS) is summarized and reported. SEM was used to investigate the surface and cross-section morphology of fabricated coatings, while EDS and XRD were used to investigate the compositions and phase of these coatings. Through controlling the condition and parameters of the APS, the oxidation of the W coating can be reduced significantly. The W coatings with different initial particle sizes resulted in different microstructures. The finer the particle size of the spraying powders, the lower the porosity will be. The effects of different interlayers on bonding strength were studied. The W coating deposited directly onto the copper substrate presented the highest bonding strength compared with coatings with different interlayers, while the coating with a graded W/Cu interlayer shows the best thermal shock resistance. The feasibility of fabrication of thick W coating by APS was also investigated. Pure W coating with a thickness of 2–3 mm and porosity less than 2% can be fabricated by APS.

Ultra-fine grained tungsten specimens with a grain size from the submicron range to several microns were fabricated by resistance sintering under ultra-high pressure. Transient heat loads were applied on the tungsten specimens at room temperature for a pulsed duration of 5 ms at different power density of 0.22, 0.33, 0.44 and 0.55 GW m−2, respectively, by using an electron beam facility. The crack formations and surface melting behaviors under transient heat load were investigated.

Amorphous ribbon-type filler-metals represent a promising selection for joining heterogeneous materials together. In this work, rapidly solidified ribbon-type Ti based amorphous filler with a melting temperature of 850 °C and a thickness up to 20 μm is used to join silicon doped carbon to pure copper. SEM examinations demonstrate that a high quality brazed joints could be acquired. The brazed seam has a uniform structure and pore free along its entire length. TiC and ZrC are formed near the interface of carbon and filler-metal when the brazing holds enough time. Using very thin Mo and Cu foil (0.2 mm in thickness) as multiple interlayer are very effective to mitigate the thermal stress occurred in the interface between carbon and copper. The shear strength of this carbon–multiple interlayer–copper joint is more than 20 MPa, and the rupture is mainly occurred on the carbon side.

Both tungsten coatings with or without a W/Cu graded interlayer on an oxygen-free copper substrate were fabricated by atmospheric plasma spraying. High purity argon gas was used for cooling the substrate and preventing the coating from oxidation. The thickness of both coatings is ∼1 mm. XRD and EDS measurements of the coatings show that minimal oxidation occurred during the deposition process. Transient high heat load tests by electron beam with a pulse duration of 5 ms were performed on both coatings. The single pulse loading was applied on the virgin surfaces at several power densities (from 0.22 to 0.9 GW/m2). Although the weight loss of the W/Cu FGM (functionally graded materials) based coating was slightly lower than that of the pure W coating, their transient high heat loading performances were quite similar.

•The tensile property and Charpy impact were tested.•Both strength and plasticity in LT direction are better than that of TL direction.•The LSE was more than 65% of the USE from absorbed energy curve.•The initiation and propagation energy at different temperatures were calculated.•High LSE and dimples on the fracture surface indicated good toughness at −60 °C.A 9Cr-ODS ferritic/martensitic steel with a composition of 9Cr–1.8W–0.5Ti–0.35Y2O3 was fabricated by mechanical alloying and hot isostatic pressing, followed by hot rolling. Tensile properties were measured at room temperature (23 °C) and 700 °C in the rolling direction (LT) and the transverse direction (TL). The ultimate tensile strength (UTS) of the as-rolled samples in both directions reached 990 MPa at 23 °C, and still maintained at 260 MPa at 700 °C. The tensile strength and elongation of the rolling direction was greater than that of the transverse direction. The Charpy impact was tested from −100 to 100 °C in the LT direction. The lower shelf energy (LSE) was more than 65% of the upper shelf energy (USE). The total absorbed energy was separated into the energies for crack initiation and propagation. The propagation energy was always higher than the initiation energy in the range of temperatures tested. The ductile-to-brittle transition temperature (DBTT) of the rolled 9Cr ODS evaluated by an absorbed energy curve was about 0 °C. However, the high LSE and the fracture surface that still contained dimples at lower shelf indicated good toughness of the as-rolled 9Cr ODS steels at temperature of −60 °C.

Thermal insulation materials have attracted increasing attention in recent years for energy conservation in thermal power plant. A novel ceramic composite insulation material (hereafter refer to CCIM) composed of alumina fibers and hollow silica powders with excellent pliability and thermal insulation properties has been designed and fabricated. The effects of alumina fiber and hollow silica microspheres on the performance of composite were characterized through microstructure observation using scanning electron microscopy (SEM) and thermal conductivity evaluation. In the novel CCIM, ceramic fibers and particles of different sizes were uniformly mixed to form multi-scale sizes of pores which can decrease the heat conduction and convective heat transfer at high temperature significantly. The comparison between the traditional mineral wool and the fabricated CCIMs focused on microstructure and thermal insulation property was also performed in this work. The novel CCIM shows much lower thermal conductivity than the standard value of thermal conductivity of traditional inorganic insulation materials when the mean temperature is between 126 °C and 538 °C. The novel CCIMs were applied in a supercritical Power Plant. The surface temperature and surface heat flux measured during service in the supercritical Power Plant further demonstrated that the novel ceramic composite has better insulation properties than traditional inorganic insulation materials.