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Creep rupture can happen in two ways, brittle and ductile creep rupture. Brittle creep rupture of austenitic stainless steels proceeds with the nucleation, growth and coalescence of grain boundary cavities. A creep cavity nucleation model has been developed previously, which considers cavity nucleation at particles and sub-boundary corners due to grain boundary sliding. A modified constrained cavity growth model has been used to describe the cavity growth behavior with combination of the cavity nucleation models. In this paper, the brittle creep rupture has been analyzed by combining the cavity nucleation and growth models. The physically based models where no adjustable parameters were involved were used to predict the brittle creep rupture strength. On the other hand, previously developed basic models for ductile creep rupture based on exhaustion of the creep ductility have been used. The creep rupture strength of common austenitic stainless steels has been predicted quantitatively by taking both ductile and brittle rupture into account. The predicted rupture times for ductile rupture are longer than those for brittle rupture at high stresses and low temperatures with a reversed situation at low stresses and high temperatures. This reproduces the characteristic change in slope in the creep rupture curves. [ABSTRACT FROM AUTHOR]

Diffusion controlled cavity growth models tend to exaggerate the growth rate. For this reason it is essential to take into account the restrictions caused by creep rate of the surrounding material, so called constrained growth. This has the consequence that the stress that the cavities are exposed to is reduced in comparison to the applied creep stress. Previous constrained growth models have been based on linear viscoplasticity. To avoid this limitation a new model for constrained growth has been formulated. Part of the work is based on a FEM study of expanding cavities in a creeping material. Compared with the previous constrained cavity growth models, the modified one gives lower reduced stresses and thereby lower cavity growth rates. By using recently developed cavity nucleation models, the modified creep cavity growth model can predict the cavity growth behaviour quantitatively for different types of austenitic stainless steels, such as 18Cr10Ni, 17Cr12NiNb and 17Cr12NiTi. [ABSTRACT FROM AUTHOR]

The possibility of creep cavity formation at subboundaries in austenitic stainless steels is analysed. It is demonstrated that such nucleation is thermodynamically feasible. A minimum stress must be exceeded in order to create cavities. The nucleation is assumed to take place where subboundaries on one side of a sliding grain boundary meet subgrain corners on the other side (double ledge models). Alternative cavitation positions can be found where particles meet subboundaries. The nucleation model can quantitatively predict the observed nucleation rate. The model gives a nucleation rate that is proportional to the creep rate in agreement with many experiments. [ABSTRACT FROM AUTHOR]

Two models are presented for grain boundary sliding (GBS) displacement during creep. GBS is considered as crucial for the formation of creep cavities. In the first model, the shear sliding model, GBS is accommodated by grains freely sliding along the boundaries in a power-law creeping material. The GBS rate is proportional to the grain size. In the second model, the shear crack model, the sliding boundaries are represented by shear cracks. The GBS rate is controlled by particles in the boundaries. In both models, the GBS displacement rate is proportional to the creep strain rate. Both models are consistent with existing experimental observations for GBS during creep of austenitic stainless steels. For cavity nucleation at particles, Harris' model (1965) for the relationship between GBS and a critical particle size has been analysed and found to be in agreement with observations. [ABSTRACT FROM AUTHOR]

Herein, polyvinyl alcohol based anion exchange membranes (AEMs) doped with various cobalt and chloride salts are synthesized to investigate the structure-performance relationship of ion-doped AEMs systemically. The performances of ion-doped AEMs are found to be related to the hydrolysis degree (DH) of the doped anions and cations. It is found that cations with varying DH transformed into hydroxides with different sizes and dispersions, which plays a key role in determining the structures and properties of cation-doped AEMs. On the other hand, weak-acid anions remained in the AEMs after alkali immersion, hindering OH− conduction and leading to the degradation of the anion-doped AEMs. High DH cations mildly react with the matrix and transform into more dispersive complexes, while low DH anions are replaced by OH−.The direct borohydride fuel cell using CuCl 2 -doped AEM exhibits a maximum power density of 202.4 mW cm−2 at 30 °C. [Display omitted] • The properties of ion-doped AEMs are effected by the DH of doped-ions. • Cations transform into hydroxides with various sizes in the cation-doped AEM. • Weak-acid anions stay in the anion-doped AEM and hinder the OH− conduction. • Mildbase strong-acid salts are concluded as the most promising ionic additives. [ABSTRACT FROM AUTHOR]

Exploring efficient non-precious metal cathode catalysts is attracting ever-increasing interest in the development of direct borohydride fuel cell (DBFC). Herein, a cobalt-based nitrogen-doped bamboo-like graphene tube (Co-NGT) is prepared as an efficient cathode catalyst. The Co-NGT consists of bamboo-like graphene tubes (40–130 nm in diameter and ∼10 μm in length), as well as cobalt nanoparticles (50–100 nm in diameter) confined in the graphene tubes. The Co-NGT shows an onset reduction potential of 0.964 V and an almost four-electron pathway towards oxygen reduction reaction, as well as a negative shift of 8 mV in half-wave potential after 10,000 CV cycles. The Tafel slope of the Co-NGT catalyst is 47 mV decade−1, which is by ca. 37.8 mV decade−1 smaller than for Pt/C. A DBFC using the Co-NGT cathode yields the biggest power density of 453 mW cm−2 at 60 °C and a life span of over 120 h. The excellent electrocatalytic properties and superior durability of Co-NGT catalyst arise from the highly graphitized NGT, providing strong interaction with Co nanoparticles and mitigating the agglomeration of Co nanoparticles. Our study guides for composition and structural design of high-efficient cathode catalysts of DBFC. [Display omitted] • The Tafel slope of Co-NGT towards ORR is 47 mV dec−1. • The half-wave potential is only 8 mV negative shift of Co-NGT after 10,000 CV cycles. • Maximum power density of 453 mWcm−2 is achieved by DBFC using Co-NGT cathode. [ABSTRACT FROM AUTHOR]

[Display omitted] • The coupling effect of the BCC-B2 transition and Mn occupancies has been studied. • The BCC-B2 transition intensified the magnetic interaction between Fe and Mn atoms. • The increased covalent bonding component and Peierls stress enhanced the brittleness. • VEC, spin polarization, and Mn content correlate with the contradictory relationship. The magnetism-ductility contradictory relationship presents a significant challenge in the development of magnetic alloys. The impact of the BCC-B2 transition, along with Mn site occupancy, on magnetism and ductility have been investigated by using first-principles calculations. The calculations involved the evaluation of magnetic moments, density of states (DOS), phase stability and ductility of FeCoMn alloys. The results of binary alloys confirm the enhancement of magnetism due to the BCC-B2 transition. Furthermore, the ordering phase transition can strengthen the magnetic interaction between Fe and Mn atoms, which is associated with minimal variations in the density of states of Fe and Mn in the B2 structure. Regarding the ductility of FeCoMn alloys, two factors contribute to increased brittleness. Firstly, the increased covalent component in bonding, as a result of the strong hybridization between different elements, leads to an increased brittleness. Secondly, the increased Peierls stress provides a larger resistance to dislocation motion, which also contributes to the increased brittleness. Finally, the Pearson correlation coefficients and data analysis indicate that VEC, spin polarizations and Mn content provide major contributions to the contradictory relationship between magnetism and ductility. [ABSTRACT FROM AUTHOR]

The contradictory relationship between magnetism and ductility restricts further applications of FeCoV alloys in high-performance electrical machines. The role of the BCC-B2 transition, accompanied by vanadium (V) site occupancies, in magnetic moments and ductility has been explored using first-principles calculations. The variations in magnetism and ductility of FeCoV alloys are attributed to the coupling of the BCC-B2 transition and V occupancies. When V replaces Fe atoms in the equiatomic B2-FeCo alloy, the superior magnetism observed in B2-Fe 50- c Co 50 V c alloys is a consequence of the enhanced local magnetic moment of Fe and the ferrimagnetic-ferromagnetic transition in the magnetic state. Moreover, due to the preferential V occupancy in the B2 phase, the B2-Fe 46 Co 50 V 4 alloy exhibits comparable ductility to the BCC-Fe 50 Co 46 V 4 alloy. The results indicate that the increased brittleness in the B2 phase arises from the raised Peierls stress and the enhanced covalent component in interatomic bonding, which is caused by the strong hybridization between Fe and Co atoms. Pearson correlation analysis illustrates that valence electron concentration (VEC) and V content are significant factors in the contradictory relationship between magnetization and ductility. The theoretical results demonstrate that tuning the V content and atomic occupancies is helpful to achieve a trade-off between magnetization and ductility in B2-FeCoV alloys. • The coupling effect of the BCC-B2 transition and V occupations on magnetic moment and ductility. • The underlying origins of magnetism-ductility contradictory relationship. • A trade-off between magnetic moment and ductility can be obtained in B2-Fe 46 Co 50 V 4 alloy. • the magnetization can be improved by adjusting VEC and V content. [ABSTRACT FROM AUTHOR]

Designing efficient and stable single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is essential to promote fuel cell commercialization. In this study, the Bayesian Regularization Neural Network (BRNN) has been used to determine the carbothermal shock (CTS) conditions for preparing high-loading Pt-Fe/C SACs. The BRNN model effectively identified a low reaction temperature of 283 °C for achieving high-loading (15 wt %) Pt-Fe/C SACs. Based on this model, we successfully synthesized Pt-Fe/C SACs with a metal mass content of 11.31 wt % using the CTS method. These ultrahigh content metal Pt-Fe/C SACs consisted of dispersed single atoms and foam-like atomic constructions, namely Pt-Fe/C atom foam catalysts (AFCs), rendered them promising candidates for electrocatalysis applications. The Pt-Fe/C AFCs exhibited good catalytic activity and durability toward ORR in alkaline conditions. The Pt-Fe/C AFCs were employed in a direct borohydride fuel cell (DBFC) as cathode catalyst, which resulted in a maximum power density of 214 mW cm−2 at 60 °C and demonstrated stable discharge for 40 h at room temperature. The electrocatalytic performance of the Pt-Fe/C AFCs surpassed that of low-loading Pt-Fe/C SACs and commercial Pt/C catalysts. The rapid ramp-up rate and the rapid cooling rate of the CTS process enabled the incorporation of high-loading Pt and Fe metal atoms into the graphitized carbon layer, resulting in an increased number of active sites of the Pt-Fe/C AFCs. This work provides an effective strategy for manufacturing efficient and stable SACs for practical applications in fuel cells. [Display omitted] • The Bayesian Regularization Neural Network is used to determine the CTS conditions. • Pt-Fe/C AFCs with metal content of 11.31 wt % is successfully synthesize by the CTS method. • The Pt-Fe/C AFCs exhibit good catalytic activity and durability towards ORR. • The DBFC using Pt-Fe/C AFCs cathode achieves a maximum power density of 214 mW cm−2. [ABSTRACT FROM AUTHOR]

Designing an efficient and stable non-precious metal cathode catalyst is essential to reduce fuel cell cost and promote fuel cell commercialization. Herein, ZIF-67 and carbon powder are rapidly heated using a transient Joule heat device to pyrolyze ZIF-67 to form a CoO/N/C nanocatalysts. The prepared CoO/N/C-90 is comprised of 3–4 nm CoO nanoparticles loaded on a carbon substrate. In alkaline medium the Tafel slope of CoO/N/C-90 is 50 mV dec−1, and a half-wave potential (E 1/2) of 0.82 V towards oxygen reduction reaction (ORR). The E 1/2 only experiences a negative shift of 4 mV after 40,000 cyclic voltammetry (CV) cycles, demonstrating excellent catalytic reaction kinetics and durability. The CoO/N/C-90 is assembled into direct borohydride fuel cells (DBFC) as a cathode, achieving a maximum power density (P max) of 526 mW cm−2 at 60 °C. This work indicates that the CoO/N/C-90 is an efficient and stable non-precious metal catalyst for DBFCs. [Display omitted] • CoO/N/C nanoparticle with a size of 3–4 nm is prepared by CTS method. • CoO/N/C exhibits superior stability and highly electrochemically active area. • A DBFC with maximum power density of 526 mW cm−2 is achieved at 60 °C. [ABSTRACT FROM AUTHOR]

Temperature-dependent properties are very useful in modeling the behavior of materials servicing at high temperature such as austenitic stainless steels. Besides, for predicting the mechanical properties of such alloys, experimental data at high temperature are very important, but scarce. In this work, we use first-principles calculations combined with a quasi-harmonic Debye model to evaluate the Helmholtz free energy of paramagnetic Fe25Cr20NiMnNb austenitic stainless steel as a function of temperature and volume. The contributions due to electronic excitations, magnetic fluctuations, and lattice vibrations are taken into account. Elastic and other thermodynamic properties of the material, such as the equilibrium lattice parameter, elastic moduli, entropy, are derived from the free energy in the temperature range from 800 to 1100 K and compared with available experimental data. The influence of Nb concentration, as well as the effects of electronic excitations and lattice vibrations, on the elastic and thermodynamic properties are discussed in detail. The elastic moduli are found to decrease with increasing temperature. This elastic softening phenomenon is mainly due to thermal expansion; the contribution of thermal electronic excitations is found to be relatively small. [ABSTRACT FROM AUTHOR]

• In situ XANES was used to reveal the valence evolution of Fe-based electrocatalysts toward ORR. • In situ XRD was used to reveal the phase transition of Fe-based electrocatalysts during ORR. • A Fe2+/3+ redox transition was both found in Fe-PPy-C and FeS-PPy-C. • The Fe, FeS and Fe 2 O 3 were all converted to FeOOH during ORR in Fe-based electrocatalysts. • The ORR activities of Fe-based electrocatalysts are governed by their static structure. [Display omitted] The commercialization of fuel cells relies largely upon the development of highly active non-precious metal electrocatalysts. Insufficient understanding of the reaction mechanism of Fe-based electrocatalysts towards oxygen reduction reaction (ORR) limits the further improving of its catalytic performance. Herein the valence state and phase transformation of three Fe-based electrocatalysts comprising different valences of iron during the ORR in alkaline medium are investigated by in situ synchrotron radiation X-ray absorption and diffraction studies in direct borohydride fuel cell. There is a dynamic structure associated with the 'Fe2+/3+' redox transition confirmed by in situ X-ray absorption near edge structure spectra in Fe-PPy-C and FeS-PPy-C. The metallic Fe, FeS, and Fe 2 O 3 of the three Fe-based electrocatalysts are all converted to FeOOH during ORR. The local structure of Fe atoms and valence transition barrier contribute to most of the ORR activities of the three Fe-based electrocatalysts. [ABSTRACT FROM AUTHOR]