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Silicon oxide-based materials for Li-ion battery anodes have attracted extensive attention due to their higher capacity than graphite materials and better cycling performance compared to Si-based materials. However, the cycle performance needs to be further enhanced if they are to be widely used in commercial applications. In this study, we propose a simple strategy to prepare porous SiOx materials. Zn and SiO were combined by a high-energy mechanical milling process. The Zn/SiO composite was then heated to 900 °C and the Zn-based materials were removed by evaporation as Zn is a metal with a relatively low melting point (419.5 °C) and boiling point (907 °C). This process resulted in the production of porous SiOx materials with a large number of mesopores. Characterizations of the materials by X-ray diffraction analysis, X-ray photoelectron spectroscopy, and electron microscopy confirmed the synthesis of porous SiOx materials. The cycling performance of these materials was found to be improved. Carbon incorporation was performed in an effort to further enhance their performance, and the cycling performance of these porous SiOx/C composite materials was considerably enhanced, thus indicating that the strategy involving both a porous structure and carbon incorporation is very effective for improvement of the cycling stability of Li-alloy-based active materials.

Non-destructive testing (NDT) is the process of evaluating and inspecting materials without damage. The purpose of present study is to evaluate the residual lifetime of materials by proposing a new concept of non-destructive testing. The residual lifetime of service-exposed 1.25Cr-0.5Mo (P11) steel cold-reheater pipes in a coal-fired power plant, which were used for approximately 23 years at 310 °C, was investigated with accelerated degradation tests of a fresh P11 steel. The hardness and the microstructure of the P11 steel aged at 550 °C for 1000 h were similar to those of the service-exposed cold-reheater pipe. The creep-rupture tests of the steel aged at 550 °C for 1000 h were performed under various temperature and stress conditions together with the fresh steel. The creep-rupture properties analyzed using a Larson–Miller parameter (LMP) approach suggested that the residual lifetime of the service-exposed pipes is approximately 53% at the current conditions. The proposed methodology is expected to serve as a useful tool to check the timing of materials replacement and to maintain safety in a coal-fired power plant.

Mechanically alloyed Ni-20Cr (wt%) nanocrystalline powder was cold spray deposited on SA 516 Grade 70 steel. The microstructure and high temperature oxidation characteristics of Ni-20Cr nanostructured alloy coated steel have been investigated. The cyclic oxidation was conducted in air at 900 °C for periods up to 50 h. The coated steel was examined prior to and after cyclic oxidation using SEM, EDX, FIB, TEM and SAED. The TEM analyses showed that the coating exhibits equiaxed nanoscale microstructure. Interestingly, the high temperature cyclic exposure significantly influences the grain growth despite the segregation and preferential formation of nano-islands of Cr2O3 within the coating matrix. Furthermore, the enhanced grain boundary diffusion of Cr in the nanostructured coating promotes the formation of denser Cr-rich protective surface oxide layer, which possesses a superior oxidation resistance, i.e. prevents the corrosion of substrate steel. The microstructural changes, crystallographic and chemical aspects of the Ni-20Cr nanostructured coating upon cyclic exposure are discussed in the context of its high temperature corrosion protection property.

Long-term creep life (5000–100,000 h) predictabilities, based on the short-term creep life data (~ 5000 h), for Grade 92 steel were investigated among major creep life prediction models, Larson–Miller parameter (LMP), normalized power law (NPL) and Wilshire models. The NPL and Wilshire models showed superior short-term creep data based long-term creep life predictabilities to the LMP model. In particular, the Wilshire model showed relatively accurate predictions (within an error range of 7%–10%), which seemed to be due to reasonable coupling of the normalized stress with the temperature-dependent rupture life term in a form of the cumulative distribution function. Both NPL and Wilshire models, calibrated by the short-term creep life data, showed a transition in creep mechanism at a similar normalized stress. Thermodynamics-based kinetic simulation (MatCalcTM) results for major precipitates (M23C6, MX and Z phases) of Grade 92 steel suggested that the creep transition is associated with coarsening of M23C6 precipitates at high temperatures (above 600 °C), which led to the degradation of the creep property.

Interatomic potentials for the Ni-Ti and Ni-Al-Ti systems have been developed based on the second nearest-neighbor modified embedded-atom method (2NN-MEAM) formalism. The Ni-Ti binary potential reproduces fundamental materials properties (structural, elastic, thermodynamic, and thermal stability) of alloy systems in reasonable agreement with experiments, first-principles calculations and thermodynamic calculations. Atomistic simulations using the Ni-Al-Ti ternary potential validate that the potential can be applied successfully to atomic-scale investigations to clarify the effects of titanium on important materials phenomena (site preference in γ′, γ-γ′ phase transition, and segregation on grain boundaries) in Ni-Al-Ti ternary superalloys.

The mechanism for the formation of Z-phase was investigated for samples of an 11CrMoVNbN steel aged at 593 °C for up to 50,000 h. X-ray diffraction indicates that Z-phase appears after 5000 h of aging, and its amount gradually increases up to 50,000 h aging at the expense of mainly Cr 2 N. Transmission electron microscopy shows that Z-phase nucleates in the V-enriched rim region of Cr 2 N precipitates, which forms due to the diffusion of V into the precipitates from the matrix. Nucleated Z-phase tends to grow relatively rapidly compared with the preexisting precipitates such as Nb(C,N) and M 23 C 6 . Z-phase seems to consume Cr 2 N by the nucleation and growth mechanism rather than the transformation mechanism. The main difference in the aged samples is that the Nb content of Z-phase is lower and that Z-phase does not replace Cr 2 N completely, compared to the results of the crept samples.

Double-pass Friction Stir Processing (FSP) was applied to fabricate an AZ31/CNT nano-composite for surface hardening of lightweight structural components. The effects of double-pass FSP as well as groove depth (i.e., volume fraction of CNT) on the CNT distribution, dynamically recrystallized grain size, and resulting microhardness were studied. Double-pass FSP was performed for the CNT-filled plate-type specimen with different groove depths of 2, 3, and 4 mm. By applying double-pass FSP, the average size of CNT clusters decreased, implying a more homogeneous distribution. Compared with the FSPed specimen without CNT, grain size was refined from 19 μm to 3 μm and microhardness increased from 52 Hv to 83 Hv (i.e., 71% increase).

In this study, we use the quasi-harmonic Debye model to predict the coefficient of thermal expansion of Ni- X binary alloys. The method bridges between the macroscopic elastic behavior and thermodynamic properties of materials without an expensive calculation of the volume dependence of the phonon density of states. Furthermore, the Gruneisen parameter is derived from the volume dependence of the Debye temperature, which is calculated from the first-principles elastic stiffness constants. The experimental coefficient of thermal expansion (CTE) of pure nickel is well reproduced, especially in the low temperature region. Among the few alloying elements tested, Al is predicted to slightly decrease the CTE whereas Mo and W are more effective in reducing the CTE. For the cases of Ni-X binary alloy systems, where the variation in the CTE is relatively small, the method used here appears to perform better than certain other formulations that rely entirely on the energy vs. volume relationship.

Effects of surface deformation and grain size on the humid air oxidation of INCONEL 740 have been investigated. Scanning and transmission electron microscopy as well as X-ray spectroscopy are utilized to characterize and quantify the oxides. Results indicate that severe deformation refines surface micro-grains to nano-grains, leading to an increase in the mass gain and oxide thickness. Surface deformation prior to oxidation enhances the metal-oxide interface adherence. This enhancement is associated with the mechanical interlocking at the interface which ultimately improves the resistance to exfoliation. The grain size effect on the oxidation of non-deformed samples was found insignificant due to Cr-oxide exfoliation and evaporation.

Alloy design is of prime importance for automotive steels to achieve desired properties, such as strength, hardenability and wear resistance. In the present study, CALPHAD-based computational techniques have been successfully utilized to develop advanced steels for automotive applications. The first part of this series describes an integrated computational approach for the compositional modification of bearing steels. A conventional 100CrMn6 bearing steel has been precisely redesigned to achieve strength enhancement with optimized cementite size distribution. The strength of the modified bearing steel was further improved by the addition of 0.2 wt% V using fine vanadium carbide precipitates. Experimental verification of the calculated results confirmed the reliability of the computational method employed in this study.

The CALPHAD-based computational techniques established in the first part of this series [1] have been applied for the development of advanced carburizing steels used for gears in vehicle transmissions. To improve the strength and hardenability of a conventional carburizing steel, the alloying composition has been modified based on the calculation of phase equilibria and the prediction of mechanical properties such as the yield and tensile strengths, hardness, and volume fractions of martensite and bainite. The size and density of carbides precipitated with V, Nb or Ti microalloying elements as well as their austenite grain size have been predicted by thermo-kinetic simulation to optimize the microstructure. The reliability of the computational results has been experimentally confirmed by comparing the austenite grain size and the hardness of the newly developed carburizing steels with those of the conventional carburizing steels.

An interatomic potential for the Ti–Al binary system has been developed based on the second nearest-neighbor modified embedded-atom method (2NN MEAM) formalism. This potential describes fundamental materials properties (structural, thermodynamic, elastic, defect, deformation and thermal properties) of Ti–Al alloys in good agreements with experimental or first-principles data. The transferability and applicability of the present potential to atomic-scale investigations for Ni-based superalloys or Ti–Al based alloys are demonstrated.

The acoustic emission (AE) energy was analyzed to monitor the pitting process on a vertically positioned 304-stainless steel. The gravity-dependent morphology of the elongated corrosion pits was observed. A scatter plot of the duration and energy indicated two AE clusters with different energy levels. There was a time delay after the detection of the low-energy hydrogen-bubble signals. Subsequently, high-energy signals were observed, whose AE source was attributed to large-scale cracks formed during the rupture of the elongated pit cover. An in-situ analysis of the AE energy evolution provided detailed insights into the corrosion process in relation to the specimen position.

An attempt has been made to improve the oxidation resistance of the INCONEL 740 nickel-base superalloy by altering its composition. The effects of aluminum and titanium were studied in particular. Electron microscopy was utilized to characterize and quantify the oxides. The addition of aluminum up to 2.4 wt.% at low titanium contents provides the lowest mass gain. Aluminum internal and grain boundary oxidation, especially at nickel high-angle boundaries, was observed. The surface oxide consists of predominantly Cr2O3. The oxide thickness and defect density increase with the increasing titanium content.

We investigated the effect of preexisting plastic deformation with the strain of 30% on the creep behavior of TP347 austenitic steel at 650 °C with different applied stresses. The results showed that the creep rupture life of the prestrained sample increased more than that of the unstrained sample tested at 300, 250 and 200 MPa due to the strengthening of the mechanical twins induced by preexisting plastic deformation. Conversely, the prestrained samples were ruptured earlier than that of the unstrained sample tested when the lower creep stresses (150 and 125 MPa) were applied. The shorter creep rupture life of the prestrained sample was due to the reduced amount of time to a minimum creep rate and the shorter tertiary-stage creep time which can be related to the accelerated precipitation of sigma phase at the grain boundaries in the strained sample.

s First-principles calculations are performed to derive promising alloying elements which inhibit inter-diffusion between vanadium and palladium-coating layers in vanadium-based hydrogen separation membranes. Paying attention to the inhibition of inter-diffusion by the grain boundary segregation of impurities, several physical features of various solute elements, grain boundary segregation, grain boundary embrittlement and solute–solute binding tendencies, are evaluated to construct a large database. Several alloying elements and their combinations similar to or even better than a previously reported yttrium are suggested for future experimental works by comprehensively analyzing these physical features.

The effect of long-term aging at 550 °C on the hardness variation and precipitation evolution of a 1.25Cr-0.5Mo steel with ferrite/pearlite structure was investigated. Also, creep-rupture test of the steel was conducted at 550 °C. The hardness value generally decreased with increasing aging time due to the spheroidization of cementite (M3C). Interestingly, the hardness value was almost maintained at the aging time ranging from 1,000 to 5,000 h most likely due to the formation of fine needle-like M2C precipitates in ferrite regions. After 5,000 h of aging, the hardness value again decreased with the decrease in the amount of M3C. X-ray diffraction and transmission electron microscopy indicate that the amounts of M2C and M7C3 increases instead of M3C and some of M7C3 precipitates nucleated at M3C particles, growing at the expense of M3C around ferrite/pearlite interfaces. On the whole, the simulated precipitation kinetics of the steel using the MatCalc software well described the precipitation sequence. The creep-rupture strength of the steel drastically decreased especially after 1,600 h of rupture time, although it generally decreased with increasing rupture time. The accelerated dissolution of M3C under creep stress/strain was responsible for the drastic decrease in creep-rupture strength.

Precipitate evolutions of 2.25Cr-1Mo heat-resistant steel during creep, under stress ranges of 30–300 MPa at 500–650 °C, were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) equipped with energy dispersive spectroscopy (EDS). As-tempered microstructure was composed of ferrite and pearlite. Major precipitates in the pearlite region were identified as M3C, M7C3 and M23C6 (M = Fe, Cr, Mo) carbides. It was found that the precipitation sequence was changed according to the creep exposure temperatures and times. The composition of the carbide was shifted to the Cr-rich side as the creep exposure time at 550 °C or lower increases without changing the ferrite and pearlite structures. On the other hand, M2C was formed in an early stage of creep at 600 °C. The amount of M2C was decreased and M6C was newly precipitated after a long time creep exposure at 600 °C or higher. The amount of M3C decreased with increase in exposure time and fully disappeared even after a very short time creep exposure at temperatures higher than 600 °C. The overall precipitation sequence in the 2.25Cr-1Mo steel during creep was found to be M3C, M7C3, M23C6 → M3C, M7C3, M23C6, M2C → M7C3, M23C6, M6C. It was considered that the slope change in Larson-Miller Parameter (LMP) plot was attributed to the formation of Mo-rich M2C and dissolution of M7C3 precipitates during creep in the 2.25Cr-1Mo steel.

During aging at 900 °C for 100 h of IN740H Ni-based superalloy, unexpected needle-like MC carbide formed at or near incoherent twin boundary having a twin relationship with surrounding matrix. Similar to the known needle-like M23C6 precipitation, the nucleation and growth of MC carbide in needle-like morphology resulted from emission of Shockley partial dislocations and inherent directional stress field originated from incoherent twin boundary, respectively. Also, this precipitation process essentially gave rise to the twin relationship with its surrounding matrix.

The creep behavior of high-Cr martensitic heat-resistant steels was investigated to discuss the dominant factors determining the creep-rupture life in the temperature range from 839 to 894 K. Variation in the content of Nb, a carbide former, induced a difference in creep-degradation at the long-term creep condition, which was attributed to the formation of a Z-phase at the expense of M 2 N precipitates. Due to the continuous evolution of the microstructure during creep service, a simple form of the Monkman–Grant equation could not properly describe the creep-rupture life of the alloys; however, the modified Monkman–Grant equation, which incorporates the creep rate at the tertiary creep region, resulted in a reasonable estimation of creep-rupture life.

A low carbon ferritic steel was alloyed with Ti, Mo and Cu with the intention of achieving greater increment in strength by multiple precipitate strengthening. The steel is hot rolled and subjected to interrupted cooling to enable precipitation of Ti–Mo carbides and copper. Thermodynamic calculations were carried out to determine equilibrium phase fractions at different temperatures. Microstructure characterization using transmission electron microscopy and composition analysis revealed that the steel contains ~5 nm size precipitates of (Ti,Mo)C. Precipitation kinetics calculations using MatCalc software showed that mainly body centered cubic copper precipitates of size

The effect of Nb addition on the creep strength of high-Cr martensitic heat-resistant steel was investigated at temperatures ranging from 839 to 894 K. The short-term creep properties were improved by Nb addition due to the enhanced MX precipitation. However, Nb addition deteriorated the steel's long-term stability resulting in an even shorter creep-rupture life than the original alloy. Onset of the accelerated degradation in Nb-containing steel was attributed to the gradual formation of Z-phase at the expense of M 2 N precipitates during creep exposure.

Acoustic Emission (AE) technique was employed for evaluating charge/discharge damage in a lithium-ion battery. A coin-type battery of lithium cobalt oxide/carbon electrodes was used for acoustic monitoring during accelerated charge/discharge cycle test. A number of AE signals were successfully detected during charge/discharge. Microstructural observation of the electrodes after the cycle test revealed mechanical damage such as micro-cracking of the cathode and chemical damage such as solid electrolyte interphase (SEI) layer formed on the anode. The detected AE signals were classified into two distinct types (i.e., type 1 and type 2) based on the AE waveform parameters (i.e., duration and amplitude). The main frequency component of the type 1 signal with short duration and high amplitude was in the range of 121­160 kHz, whereas the frequency of type 2 signals with long duration and low amplitude was between 81 and 120 kHz. Active AE source of type 1 and type 2 signal was attributed to micro-cracking in cathode and gas bubble accompanied by SEI layer formation on anode, respectively. These results demonstrate the feasibility of the AE technique for the evaluation of charge/discharge degradation of secondary battery. [doi:10.2320/matertrans.M2014396]

In this work, a new experimental setup for acoustic emission (AE) monitoring was designed to study the process of pitting corrosion of 304 stainless steel. Two AE sensors were employed in the present setup to investigate the AE behaviors of the pitting process in the working electrode and the hydrogen bubbles on the counter electrode separately and simultaneously. The AE signals from hydrogen bubbles on the counter electrode started to be detected with a delay time (defined as ¦t1) after the pitting potential was reached. Then the AE signals from the pitting process were detected after another delay time (defined as ¦t2). The parameters of ¦t1 and ¦t2 generally decreased with the increase in sodium chloride concentration. Another parameter, ¦t3, was defined as the time difference between the first detections of the AE signals from hydrogen bubbles on the counter electrode and the pitting process on the working electrode. The AE signal-derived parameter, ¦t3, decreased linearly with the increasing concentration of sodium chloride. Furthermore, the value of ¦t3 was directly proportional to open circuit potential (EOC) and pitting potential (EP) of 304 stainless steel, respectively. These correlations imply that the more easily pitting corrosion occurs, the lower the value of ¦t3 would be and vice versa. The AE-derived time-delay parameter reflecting the pitting corrosion process can offer an alternative idea for evaluation of the corrosion critical variables and the pitting corrosion resistance. [doi:10.2320/matertrans.M2014389]

An acoustical investigation into the hydrogen bubbles on the counter electrode during pitting corrosion of 304 stainless steel has been carried out with the potentiodynamic method. After reaching the pitting potential, there was a short time delay before an Acoustic Emission (AE) signal was detected. Given this, the obtained cumulative AE counts were divided into three distinct stages showing their own unique behavior in terms of rise time and duration. The bubble evolution characteristic in each different stage was supposed to be responsible for the feature of AE signal. In the AE Stage III, the cumulative counts of the AE signals provided empirical correlations with both the total number and the total volume of corrosion pits, thus demonstrating the potential of utilizing the AE signal of hydrogen bubble on the counter electrode as a nonintrusive monitoring tool of pitting corrosion. [doi:10.2320/matertrans.M2014373]

The high-temperature mechanical behavior of cross-weld specimens prepared from a dissimilar weld joint between T92 martensitic and Super304H austenitic heat-resistant steels incorporating Ni-based weld metal was evaluated at temperatures up to 650 degrees C. For both high temperature tensile and creep tests, failure took place in T92 due to its faster degradation with temperature increase. The heat-affected zone of T92 played a critical role during creep deformation, resulting in type IV failure under the long-term creep condition. For the creep specimens, the location of failure shifted from the base metal region to the fine-grained heat-affected zone as the creep duration time increased from the short-term to the long-term condition. The massive precipitation of Laves phase on the grain boundaries of the fine-grained heat-affected zone during creep deformation was observed and found to be responsible for the accelerated void formation in the area leading to the premature failure.

In this research, the phenomenon of strain-induced precipitation of M 23 (M = Cr, Fe, Mo) C 6 precipitates in 347 austenite stainless steel was systematically studied. During annealing at 650 °C, the experimental results exhibited significantly earlier formation of M 23 C 6 precipitates in the plastically strained specimen, than in the no-strain specimen. The various microstructural characterizations showed that the accelerated formation of M 23 C 6 precipitates occurred around deformation bands that were generated by deformation twinning. Extensive investigation suggested that the strain-induced precipitation of M 23 C 6 is attributed to the formation of strain-induced α′ martensite during plastic deformation near deformation bands.

The creep properties of an 18Cr9Ni3CuNbVN austenitic stainless steel have been investigated at temperatures ranging from 923 to 1073 K and stresses from 120 to 250 MPa. The rupture lives ranged from 10 to 20,105 h. The stress dependence of the minimum creep rate obeyed a power law, with stress exponents ranging from 6 to 8.6. The activation energy was determined to be 460 to 485 kJ/mol/K. The microstructural evolution during the creep test was investigated. The V-rich Z-phase and metallic Cu precipitates began to precipitate in the middle of the creep deformation, resulting in an increase of the creep strength, while only Nb-based MX precipitates were present from the beginning of the high-temperature creep test.

Amorphous SiO materials are considered to be alternative high-capacity anode materials for Li-ion batteries. However, the conventional preparation method is a high-temperature and energy-consuming vacuum process. In this study, a simple strategy to produce a SiOx material is proposed. Low-cost micron-sized Si and amorphous SiO2 powders are mechanochemically mixed using a high-energy mechanical milling process. As a result, SiOx materials are produced that have the potential to replace the current commercial SiO powders. The X-ray photoelectron spectra exhibit that the chemical bonding state of Si in the synthesized SiOx materials is similar to commercial SiO. The electron microscopic analyses demonstrate that the secondary particle sizes are reduced and Si crystallites with sizes of a few nanometers to a few tens of nanometers are embedded in the silicon oxide matrix. Due to the morphology and microstructure, the initial coulombic efficiency and cycling stability of the SiOx materials are greatly improved compared to the commercial SiO sample. To further enhance the electrochemical properties, carbon incorporations are carried out, which result in an excellent cycle performance being achieved for the Li-ion battery anodes. The ball-milled SiOx material has ample potential as an alternative to conventional SiO materials.

The effect of vanadium addition on the creep property of niobium-containing 18Cr–9Ni austenitic heat-resistant steel was studied. After solution treatment, the MX precipitates of vanadium-free steel contained Nb only. On the contrary, high Nb and low V contents were detected from the MX precipitates in the steel with 0.3 wt% of vanadium. Vanadium-rich MX carbo-nitride was not observed in the matrix and at the grain boundaries after solution treatment. The vanadium precipitated as a form of Z-phase during early-stage creep deformation was attributed to the improvement in creep strength at 700 °C with applied stress higher than 150 MPa. And metallic Cu precipitates were confirmed in the nanometer scale using TEM and EDS technique. The Cu precipitates are believed to contribute to the strengthening of austenitic heat resistant steel independently. The precipitation behavior is discussed using transmission electron microscopy and thermo-kinetics simulation technique.

The effect of Nb and Cu addition on the creep properties of a high Mn–N austenitic stainless steel was investigated at 600 and 650 °C. In the original high Mn–N steel, which was initially precipitate-free, the precipitation of M23C6 (M = Cr, Fe) and Cr2N took place mostly on grain boudaries during creep deformation. On the other hand, the minor addition of Nb resulted in high number density of Z-phases (CrNbN) and MX (M = Nb; X = C, N) carbonitrides inside grains by combining with a high content of N, while suppressing the formation of Cr2N. The addition of Cu gave rise to the independent precipitation of nanometer-sized metallic Cu particles. The combination of the different precipitate-forming mechanisms associated with Z-phase, MX and Cu-rich precipitates turned out to improve the creep-resistance significantly. The thermodynamics and kinetics of the precipitation were discussed using thermo-kinetic simulations.

The low cycle fatigue (LCF) properties of 24Cr ferritic stainless steel for solid oxide fuel cell interconnects were investigated. The fatigue strength of 24Cr stainless steel decreased with increasing temperature, but the fatigue life increased at 600 °C and 700 °C. The fatigue behavior at room temperature (RT) was characterized as cyclic hardening followed by saturation and cyclic softening while marginal cyclic hardening was observed at 600 °C and 700 °C. The superior oxidation resistance of 24Cr stainless steel allows preventing the fatal impact of oxidation on the high-temperature fatigue life. Microstructural analysis showed that persistent slip bands (PSBs) developed prevalently at RT but not at 600 °C and 700 °C. Such temperature-dependent microstructural difference retarded the crack initiation and prolonged fatigue life at high temperatures.

The effect of Mo- and W-addition on the precipitation hardening of Ti- and Nb-microalloyed high-strength low-alloy steels was investigated. The variation in simulated hot-coiling temperature caused a significant difference in strength, which is attributed mainly to different precipitation-hardening behavior. Among the four alloys with different combination of the microalloying elements: Ti–Mo, Nb–Mo, Ti–W, and Nb–W, the steel with Nb–Mo turned out to be the most effective in precipitation hardening. The strengthening mechanism was discussed based on the microstructural evolution observed by electron microscopy and the theoretical considerations combining the different hardening models.

The interface energies and electronic structures of the interfaces between BCC Fe and transition metal carbides have been investigated using first-principles calculations based on density functional theory. The effects of the composition and configuration of the carbides on the interface properties have been determined. It was shown that the Fe/TiC interface has the highest interface energy and the formation of complex carbides leads to a significant decrease in the interface energy. The complex carbide of (Ti0.5Mo0.5)C, which has Mo present at the interface, has been found to be the most stable. From the analysis of the density of states, the stability of the Mo-segregated (Ti0.5Mo0.5)C carbides has been revealed to be due to the hybridization of the d-orbitals in the t2g local symmetry between Mo and its first nearest neighboring Fe atoms.

Creep behavior of a new precipitation-strengthened 15Cr–15Ni austenitic stainless steel with optimized content of Nb, C, N and Mo, subjected to a special multicycled aging-quenching heat treatment process, was investigated at a temperature of 750 °C and stress range of 78–200 MPa for up to 10,000 h. The steel exhibited excellent creep rupture strength which exceeds even that of the commercial NF709 alloy. The value of the creep exponent n at the applied testing temperature/stress conditions was found to be of around 5.6, demonstrating that dislocation creep is the dominant deformation mechanism during the performed creep tests. The crept microstructures showed the presence of a high number of copper precipitates, and of fine dispersion of densely distributed intragranular nano-sized (Nb,Cr)N nitride precipitates which transformed to more coarsening-resistant Z-phase precipitates with the prolonging creep exposure time. The nitrides, occurring in dominant quantities in the microstructures, showed superior coarsening-resistance during creep exposure while copper precipitates exhibited a relatively high coarsening rate. A comparison between creep rupture strength of the studied steel and of its commercial equivalent grade SUS 304 JI HTB revealed that the nano-sized (Nb,Cr)N nitrides and/or Z-phase precipitates essentially played the key role in the observed improved resistance to dislocation creep of the material. Considerations over the observed creep behavior and aspects of high-temperature microstructural stability such as coarsening of strengthening precipitates, issues related to phase transformation processes suggested that the steel in the present study exhibits capability for applications in USC fossil power plants with steam parameters of around 700 °C/35–40 MPa.

Evolution of precipitate phases in a precipitation-strengthened high nitrogen 15Cr-15Ni-4Mn-3Cu-1.5Mo-0.46Si-Nb-C-N-Al-B austenitic heat-resistant stainless steel during 10,000 hours of isothermal aging at 1083 K (810 °C) was investigated. The major precipitate phases found in microstructure after 330 hours of aging are coarser residual primary Nb(C,N) precipitates, Z-phase precipitates, (Nb,Cr)N precipitates with fcc crystal structure, Cu precipitates, and coarser Cr2N precipitates. No significant changes were noted in the chemical compositions, crystal structure of the precipitate phases, or the phase species after 10,000 hours at 1083 K (810 °C). The coarsening behavior of Z-phase, (Nb,Cr)N, and Cu precipitates was the only observed notable change in the microstructures during the performed aging. Z-phase and (Nb,Cr)N precipitates exhibited superior dimensional stability and morphology with extremely low coarsening rates, while Cu precipitates coarsened at the fastest rate. In addition, Z-phase particles showed a remarkable lower coarsening rate as compared to that of (Nb,Cr)N precipitates. The abnormal slow coarsening kinetics of Z-phase can be explained by the nucleation energy barrier (NEB) limited coarsening mechanism applied for faceted ceramic crystals.

The effect of creep deformation on the microstructural evolution of an 11CrMoVNb ferritic heat resistant steel during high temperature creep tests was investigated. The creep test was performed within a temperatures range from 593 to 621 °C under different stress levels. For the detailed understanding of precipitate coarsening behavior, transmission electron microscopy (TEM) was used to analyze thin foils and replica films obtained from the crept specimens. The major precipitates were found to be M 23 C 6 , MX and Cr 2 N phases and there was no change in precipitation sequence during creep. Coarsening behavior of the precipitates and growth behavior of martensite laths of the crept specimens were carefully examined in both regions of the grip and gage parts of the specimens in order to identify the effect of creep deformation. On the whole, precipitate size increased with increasing creep rupture time. On the other hand, precipitate coarsening and martensite lath widening were pronounced in the gage part compared to those in the grip part. This suggests that the creep deformation accelerates the rates of precipitate coarsening and martensite lath widening.

A high nitrogen 15Cr-15Ni niobium-stabilized austenitic alloy has been produced and subjected to a special heat treatment consisting of 5 hours of solution treatment at 1270 °C followed by hot rolling, quenching and subsequent aging at temperatures of 700 °C to 800 °C. It was found that fine dispersion of nano-sized thermally stable primary Nb(C,N) precipitates had already formed in the as-cast condition. The particles were presented at all examined stages of the TMT process (as-homogenized, as-solution treated and as-aged conditions). Secondary precipitates Nb(C,N) were densely formed during subsequent aging; these precipitates had sizes of 4 nm to 5 nm. Both the primary and secondary Nb(C,N) particles showed excellent thermal stability within the temperature range of 700 °C to 800 °C. The creep properties of the studied alloy at 750 °C were superior when compared to those of commercial type 347 stainless steel.

Conventional thermo-mechanical treatment (CTMT) and modified thermo-mechanical treatment (MTMT) process were applied for manufacturing a high nitrogen niobium-stabilized 15Cr–15Ni austenitic alloy. CTMT process consists of 5 h of solution treatment at 1270 °C followed by water quenching and subsequent aging at 820 °C for 50 h. MTMT process differs from CTMT process in hot plastic deformation performed immediately after the solution treatment at 1270 °C and longer aging time. Microstructure and creep properties of the steel obtained by both processing routes were investigated. Creep rupture tests at 750 °C showed double increase in rupture time brought about by MTMT process. Examination of crept microstructure by transmission electron microscopy revealed that the improved creep properties in MTMT process were mainly due to improved distribution uniformity of fine nano-sized carbonitride precipitates in the austenitic matrix and that MTMT process has no effects on the number density and distribution of copper precipitates present in the steel. However, the creep ductility in MTMT process drastically reduced comparing to CTMT process. The higher density of grain boundaries due to finer grain recrystallized microstructures and the formation of higher volume fraction of coarser M23C6 precipitates at the boundaries are believed to be the main reason for the lower creep ductility in MTMT process.

Austenitic stainless steels are expected to be a major material for boiler tubes and steam turbines in future ultra-supercritical (USC) fossil power plants. It is of great interest to maximize the creep strength of the materials without increasing the cost. Precipitation strengthening was found to be the best and cheapest way for increasing the creep strength of such steels. This study is concerned with improving creep properties of a high nitrogen Nb-stabilized 15Cr-15Ni austenitic alloy through introducing a high number of nanosized particles into the austenitic matrix. The addition of around 4 wt pct Mn and 0.236 wt pct N into the 15Cr-15Ni-0.46Si-0.7Nb-1.25Mo-3Cu-Al-B-C matrix in combination with a special multicycled aging-quenching heat treatment resulted in the fine dispersion of abundant quantities of thermally stable (Nb,Cr,Fe)(C,N) precipitates with sizes of 10 to 20 nm. Apart from the carbonitrides, it was found that a high number of coherent copper precipitates with size 40 to 60 nm exist in the microstructure. Results of creep tests at 973 K and 1023 K (700 °C and 750 °C) showed that the creep properties of the investigated steel are superior compared to that of the commercial NF709 alloy. The improved creep properties are attributed to the improved morphology and thermal stability of the carbonitrides as well as to the presence of the coherent copper precipitates inside the austenitic matrix.

The influence of the cooling condition after solution treatment on the high temperature fatigue resistance of 23Cr-26Ni heat resistant steel was investigated. Two different cooling conditions were applied to the steel after solution treatment at 1230oC for 3 hours. One specimen was water quenched immediately after the solution treatment. The other one was furnace cooled at a rate of 0.5oC/min down to 750oC after the solution treatment. Then, both specimens were aged at 750oC for 5 hours. The low cycle fatigue (LCF) test was conducted to investigate the influence of high temperature on the LCF behaviors of the heat-resistant 23Cr26Ni alloy. Under two different heat treatment conditions, the LCF test was performed at total strain amplitudes ranging from ±0.4~0.9% at room temperature (RT) and 600°C. During the test, initial cyclic hardening occurred at both experimental temperatures. This phenomenon was attributed to the increase in the dislocation density due to cyclic deformation, which resulted in the interaction between the newly created dislocations and precipitates. Cyclic softening was observed in the later stages of the LCF test at RT. The formation of precipitates and increase in the dislocation density were observed using TEM. Also, the XRD and EDS techniques were used to verify the type and composition of the precipitates.

Modified embedded-atom method (MEAM) interatomic potentials for Nb-C, Nb-N, Fe-Nb-C, and Fe-Nb-N systems have been developed based on the previously developed MEAM potentials for lower order systems. The potentials reproduce various fundamental physical properties (structural properties, elastic properties, thermal properties, and surface properties) of NbC and NbN, and interfacial energy between bcc Fe and NbC or NbN, in generally good agreement with higher-level calculations or experimental information. The applicability of the present potentials to atomic-level investigations to the precipitation behavior of complex-carbonitrides (Nb,Ti)(C,N) as well as NbC and NbN, and their effects on the mechanical properties of steels are also discussed.

Recently, binary transition metal carbides such as (Ti1-xMox)C have attracted much attention for use in increasing the strength of steels. This study aims to understand the ground state structure of various compositions of that carbide. Using a combined approach of first-principles calculation and the cluster expansion method, ground state structures of the compositions of TiC and MoC were determined. Only 29 symmetrically inequivalent structures were sufficient to determine the ground state structures of (Ti1-xMox)C for the whole range of the composition. The linear chains of solute atoms predicted by calculation were favored when the compositions of MoC were between 20 at.% and 70 at.%. When the composition of MoC was 50 at.%, an alternating layer of Ti and Mo formed along the direction. This layered structure is expected to show peculiar mechanical and electronic properties.

Modified embedded-atom method (MEAM) interatomic potentials for the Fe–Ti–C and Fe–Ti–N ternary systems have been developed based on the previously developed MEAM potentials for sub-unary and binary systems. An attempt was made to find a way to determine ternary potential parameters using the corresponding binary parameters. The calculated coherent interface properties, interfacial energy, work of separation and misfit strain energy between body-centered cubic Fe and NaCl-type TiC or TiN were reasonable when compared with relevant first-principles calculations under the same condition. The applicability of the present potentials for atomistic simulations to investigate nucleation kinetics of TiC or TiN precipitates and their effects on mechanical properties in steels is also demonstrated.

Molecular dynamics simulations of the interaction between an edge dislocation and nanosized Cr precipitates in bcc Fe have been performed to investigate the hardening effect of α′ phases in high Cr ferritic/martensitic steels. The critical resolved shear stress needed for an edge dislocation to overcome Cr precipitates of diameter between 3 and 6 nm is larger than for dislocation glide in the bcc Fe lattice containing 10% Cr solute atoms. This indicates that the precipitation of α′ phases leads to hardening in high Cr ferritic/martensitic steels. The MD simulations reveal that the interspacing of Cr precipitates plays a more crucial role in the hardening of Fe–Cr alloys than the precipitate size. An attractive interaction exists between an edge dislocation and nanosized Cr precipitates, which is evident as a decrease in total energy when an edge dislocation is placed within in a Cr precipitate.

The microstructure and stress rupture behavior of 11Cr ferritic/martensitic steels with 0.02 wt.%Ti (low Ti) and 0.14 wt.%Ti (high Ti) have been studied. The steels are prepared by vacuum induction melting followed by hot forging and rolling into plates. The results show that titanium is easy to combine with oxygen and other elements to form complex inclusions. Large MX particles with 1–3 μm are found in the high titanium steel. Most of the large MX particles have a TiO 2 cored structure. After normalizing at 1100 °C for 1 h, cooled in air and tempering at 750 °C for 1 h, nano-sized MX precipitates distribute densely near martenstic lath boundaries in the high titanium steel. The large MX particles cannot be dissolved even at austenitizing temperature up to 1300 °C. Creep cracks nucleate at the interface between matrixes and the large MX particles or titanium-containing oxide inclusions.

The strengthening effect of nanosized Cu precipitates in bcc Fe has been studied by performing molecular dynamics simulations of the interaction between a screw dislocation and a coherent bcc Cu precipitate of 1–4 nm diameter in bcc Fe. The dislocation detachment mechanism changes from shear at a precipitate diameter of 4 and 2.5 nm in the twinning and anti-twinning directions, respectively, due to the coherency loss caused by the screw dislocation assisted martensitic transformation of the precipitate. The screw dislocation detachment mechanism with the larger, transformed precipitates involves annihilation-and-renucleation, or Orowan looping in the twinning vs. anti-twinning direction, respectively. The critical resolved shear stress (CRSS) of the screw dislocation-precipitate interaction increases with increasing precipitate size, and is strongly dependent on the precipitate structure and detachment mechanism. The CRSS is much larger in the anti-twinning direction. [doi:10.2320/matertrans.M2009040]

The effect of nitrogen on creep properties of 22Cr-25Ni austenitic heat-resistant steels with different nitrogen content was studied. The tensile and creep strength increased with increasing nitrogen content. The tensile strength increased due to grain refinement by Nb-rich carbonitride. The creep strength increased with increasing nitrogen content because nitrogen retarded the formation and growth rate of Cr-rich carbide and the growth rate of Nb-rich carbonitride during creep.

The relationship between precipitation behavior of MX(M=V, Nb) nitrides and mechanical property during aging in 7Cr-1.5Mo ferritic heat resistant steels were investigated in this study. Artificial aging was carried out at 700°C for different time from 100 to 10000 hours to simulate the environment of fossil power plant. The Ostwald ripening equation is related to the containing elements can explain the accelerated coarsening of MX in the model alloys. The Ostwald ripening equation was modified by introducing the effective diffusion coefficient as a sum of the lattice diffusion coefficient and dislocation diffusion coefficient, based on an assumption that V, Nb and Ti might diffuse not only by the lattice diffusion mechanism but also by the dislocation diffusion mechanism.

The precipitation behavior of MX nitrides during aging was investigated for Cr-Mo-N-X (X=V,Nb,Ti) ferritic steels. MX nitrides, which are finely dispersed in the matrix of Cr-Mo ferritic heat resistance steel, increase the creep strength of the steels by the precipitation strengthening mechanism. However, MX particles usually coarsen during aging, and this results in a decrease of creep strength. In order to clarify the coarsening process of MX particles, TEM samples were prepared from the aged for up to 3000hours specimens. During the aging, V, Nb and Ti containing nitrides (called MX nitrides) were precipitated out. From TEM observation, particle size distribution was confirmed and size distribution follows a typical log-normal distribution. The observed coarsening behavior well agreed with the calculated coarsening behavior of precipitates by Oswald ripening equation.