Zirconium alloys have been used as nuclear fuel claddings for decades, owing to their low thermal neutron absorption cross-section, good thermal conductivity, suitable mechanical properties, and excellent corrosion resistance. During in-reactor service, zirconium alloy cladding undergoes a corrosion reaction with the coolant and absorbs part of the hydrogen produced due to corrosion, resulting in the formation of brittle zirconium hydrides. Hydrides impose great risk to the mechanical integrity of the fuel claddings during reactor operation and even during storage and transportation of spent fuel rods. Hydride morphological features such as size, distribution, and growth direction are closely related to the cooling rate, which also affects the microstructural characteristics of hydrides, including nucleation sites, crystal structure, and precipitation strain. These factors further influence the mechanical properties and corrosion resistance of zirconium alloy cladding. Therefore, investigation of the influence of cooling rate on hydride precipitation is crucial to develop a theoretical study that can aid in the prevention of hydride embrittlement in nuclear fuel claddings. Herein, multiscale characterization techniques including OM, BSE-SEM, and EBSD were used to systematically investigate the morphology and microstructure of hydride precipitation under various cooling conditions in a Zr-4 plate material. The fcc-structured δ phase, well aligned in the plane of rolling and transverse directions, is the predominant hydride formed in zirconium alloys was found. With rapid cooling rates, the thickness and spacing of the hydrides decreased, forming finely dispersed plate-like distribution morphology. Intragranular hydrides and metastable fcc-structured γ-hydrides increased in number density with rapid cooling rate. The two types of hydrides exhibited the same crystallographic orientation while sharing one α-parent grain, both holding an orientation relationship of {0001}//{111} and <112¯0>//<110> with the α-Zr matrix, independent of the cooling rate. Prior to complete transformation into the δ phase, the γ-hydride is proposed as a transitional phase during the initial stage of hydride precipitation, given that the γ-phase requires a lower hydrogen concentration for the phase transformation and exhibits lower precipitation strain than the δ phase. {111}<112¯> twinning structures were identified within the hydrides, which are expected to favor alleviating hydride precipitation strains. High angular resolution EBSD revealed that strong tensile strains induced by the volume expansion of hydride precipitation are present in the vicinity of the hydride tip, which might act as the preferential nucleation site for new hydride precipitation, promoting the formation of hydride plate morphology. Furthermore, nanohydrides were identified precipitating at the boundary of Zr(Fe, Cr)2 second-phase particles, which is expected to play a role in the morphologic development of plate hydrides.
Increasing the fraction of {111}/{111} near-singular boundaries ({111}/{111}-NSBs) has been reported as a primary solution to intergranular corrosion failure in Al-Zn-Mg-Cu alloys. The authors' previous work demonstrates that continuous static recrystallization resulting from a specific prestrain and annealing is conducive to the formation of {111}/{111}-NSBs in Al-Zn-Mg-Cu alloys. Therefore, the development of such boundaries in the alloys during dynamic recrystallization (DRX), particularly during discontinuous DRX (DDRX) and continuous DRX (CDRX) at elevated temperatures, should be elucidated. In the present work, an Al-Zn-Mg-Cu alloy containing 7.79%Zn, 1.53%Mg, and 1.68%Cu (mass fraction) was selected as the experimental material. A hot-rolled plate of the alloy was subjected to a two-stage solution treatment at 470oC for 12 h and 520oC for 6 h followed by cold rolling and recrystallization annealing. Three parallel samples cut from the recrystallized plate were compressed at 450, 480, and 520oC at a strain rate of 0.001 s-1 to a true strain of 1.20. The samples were water quenched immediately after the compression. Electron backscatter diffraction and grain boundary inter-connection measurement based on five-parameter analysis were performed to examine the microstructures and grain boundary character distributions of the compressed samples. The results indicate that the microstructures of the samples were uneven, exhibiting fine- and coarse-grained regions. Low-angle grain boundaries are dominant in the fine-grained regions, whereas high-angle grain boundaries are dominant in the coarse-grained regions. The fraction of {111}/{111}-NSBs increases with the compression temperature in fine- and coarse-grained regions. In the sample compressed at 520oC, the {111}/{111}-NSBs from the low-angle grain boundaries constitute 8.77% of all grain boundaries, while those from the high-angle grain boundaries constitute 4.53%. The stress-strain curves and the microstructures of the sample compressed at 450oC to a true strain of 0.36 show that primary DRX occurs at strains from 0.05 to 0.70. Furthermore, the coarse-grained microstructures and high-angle grain boundaries develop during the stage involving steady-state flow. When the strain increases from 0.70 to 1.20, secondary DRX (including DDRX and CDRX) occurs in some regions, leading to dramatic grain refinement and a sharp increase in flow stress. In this stage, CDRX intensifies with increasing compression temperature, and {111}/{111}-NSBs in the low-angle grain boundaries increase rapidly.
The LOX (liquid oxygen)/kerosene rocket engine is widely used in heavy launch vehicles owing to its low cost, high performance, and high reliability. However, the next-generation LOX/kerosene rocket engine requires a new superalloy that can resist oxygen-rich combustion. The K4061 alloy is a second-generation superalloy that has better resistance to oxygen-rich combustion compared to the first-generation superalloy due to the addition of the Cu element in its composition. However, there is limited research on the role of the Cu element in the superalloy. This study investigates the effect of Cu content (mass fraction) ranging from 1% to 10% on the microstructure and Cu distribution of K4061 alloy using thermodynamic calculations along with DSC, SEM, and TEM experiments. The results show that during the equilibrium solidification of K4061 alloy, the γ matrix precipitates first, followed by precipitating MC carbides at the end of solidification. The addition of Cu does not affect the equilibrium solidification path of the alloy; however, it lowers the solidus and liquidus temperatures of the alloy and the precipitation temperature of MC. During the nonequilibrium solidification, the δ phase is also precipitated at the late solidification stage. The types of precipitated phases of K4061 alloy with different Cu contents remain unchanged, consistent with thermodynamic calculations. However, Cu-rich phases are not found in the sample, and Cu does not dissolve in MC in large quantities or segregate into grain boundaries. TEM results show that Cu is enriched in the strengthening phases, and the size of the strengthening phases slightly increases with the addition of Cu during aging heat treatment. Additionally, the addition of Cu reduced the room temperature and 750°C tensile strength of the alloy.
The as-cast structure of high-alloyed wrought superalloys exhibits disadvantages such as high microscopic segregation and poor microstructure uniformity, severely affecting their subsequent hot working and deformation properties. To optimize the as-cast structure of the wrought superalloy, GH4068 alloy was smelted via electron beam smelting (EBS), and its ingots with low segregation were prepared by setting different EBS powers for 10 min. The results show that the bottom of the ingots after EBS appeared to be fine grain regions, with the presence of only cellular segregation structure and cellular dendritic crystal; the large area in the middle became vertically growing columnar crystal regions, the direction of secondary dendrite crystal growth was parallel to that of the columnar crystal growth; a small amount of equiaxed crystal was observed at the top, and the growth direction of dendritic crystals was disordered. Analyzing the compositions of ingots revealed that Cr volatilization in this alloy was the most obvious; the Cr content decreased by 1.97% when the EBS power was 17 kW. The ingot structure prepared via EBS was more highly distributed than that obtained using the traditional vacuum induction melting + electroslag remelting duplex process. When the EBS power was 12 kW, the secondary dendrite spacing λ2 was 44.6 μm, which was 32.2% less than that yielded using the duplex process, the degree of the microscopic segregation of the ingot dendrite region decreased considerably, and the degree of the microscopic segregation of the typical easily segregated elements Ti and W reduced by 20.4% and 18.6%, respectively. Furthermore, the massive precipitation of large interdendritic γ′ phases were observed, while the γ′ phases in the dendritic core were spherical and smaller in size than those in the interdendritic. Meanwhile, the ingot prepared with an EBS power of 12 kW achieved the smallest size for γ′ phases and least irregular γ′ phases in the interdendritic. In the EBS process, the actual melt temperature was considerably higher than the alloy melting temperature. After the overheating of the melt, the cluster structure effectively decomposed, elements were uniformly distributed, the degree of subcooling increased in the solidification process, and the uniformity of the melt was inherited to the solidification structure to refine the as-cast structure and reduce the degree of microscopic segregation. Meanwhile, during the EBS process, local high temperature generated due to the electron beam bombardment on the surface of the molten pool effectively reduced the N content in the alloy.
Due to the worsening of environmental pollution and energy shortage, nuclear energy has become an increasingly important energy source, offering clean, high-energy, safe, and stable power to meet the needs of modern life and production. P91 steel is an excellent heat-resistant steel, widely used in nuclear power plants for its good mechanical properties, high thermal conductivity, and low irradiation swelling rate. However, with the emergence of lead and lead-bismuth eutectic (LBE) cooled fast reactors (LFRs), P91 steel's inferior compatibility with LBE has limited its use in LFR construction. To address this issue, 9Cr ferritic/martensitic steel with high Si content (H-Si steel) was designed in this study to develop a compatible structural material for LFR heat exchange tubes. In addition, the effects of Si on the microstructure and mechanical properties of as-cast, homogenized, tempered, and aged H-Si steel were investigated using various techniques, including OM, FESEM, TEM, EBSD, tensile, and impact tests. The results show that the increase in Si content leads to 3.9% δ ferrite (volume fraction) in as-cast H-Si steel, which can be eliminated completely by homogenization heat treatment. Tempering produces a Si-enriched layer around M23C6 due to the high Si content in the H-Si steel, which can suppress the rapid growth of M23C6 and maintain a smaller size of M23C6 than that of M23C6 in P91 steel. Meanwhile, the Si-enriched layer can slow down the growth of M23C6 and promote the nucleation and growth of the Laves phase during aging at 550°C, which induces smaller M23C6 and larger Laves phase in aged H-Si steel than that in aged P91. Due to the higher Si content and smaller M23C6, the tempered H-Si steel exhibits significantly higher strength than P91. After aging, the strength of H-Si steel further increases because of the precipitation of the Laves phase. Both tempered alloy steels have similar impact energy (about 210 J) and the fracture mode is ductile fracture. However, the precipitation of the Laves phase after aging decreases the impact energy of both alloy steels. The aged H-Si steel had a larger and more Laves phase, resulting in a partial cleavage area in the impact fracture surface and inducing a lower impact energy compared to H-Si steel.
Dissimilar materials can achieve multifunction and multiperformance coupling and have broad development prospects in areas, such as aerospace, energy, automotive, and biomedicine. The properties of dissimilar materials can be improved by enhancing the compatibility of the heterogeneous interface. Herein, a laser melting deposition experiment of CoCrNiCu medium-entropy alloy (MEA) on the surface of 316L stainless steel was carried out. The microstructure morphology and interface characteristics of the dissimilar materials were characterized using SEM, STEM, EBSD, and transmission Kikuchi diffraction (TKD). The interfacial mechanical properties of the dissimilar materials were tested. The methods for promoting the bonding strength of dissimilar materials were then proposed by systematically exploring the interfacial compatibility of the microstructure and crystallography. The results show that a total solution transition zone of CoCrNiCuFe, a high-entropy alloy, was formed at the interface between CoCrNiCu MEA and 316L stainless steel. The shear strength of the dissimilar material can reach 324 MPa. Through the synergistic effect of austenite stability reduction caused by C interfacial partitioning and the plastic deformation induced by residual stress, some austenite grains of 316L stainless steel near the interface of the dissimilar materials undergo strain-induced martensitic transformation. This can promote the transformation-induced plasticity (TRIP) effect to improve the strength and ductility of the dissimilar materials while reducing interface matching. Therefore, as for the dissimilar materials with small physical discrepancies, single-phase matching with the same crystal structure should be maintained to increase the interfacial bonding strength by improving the interfacial crystallography compatibility. The TRIP effect can be used to design a duplex structure to improve the process of coordinated deformation for dissimilar materials with large physical discrepancies.
Saw blades are always running under a high resonance, large lateral pressure, large tensile stress, and corrosive environment. Nonmetallic inclusions in steel break the continuity of the matrix and easily cause stress concentration and crack formation. Furthermore, the inclusions, especially MnS/CaS, cause the initiation of pitting corrosion. Rare-earth elements in steel can play the role in liquid steel purification, inclusion modification, and solid solution alloying. Rare-earth inclusions can act as the nucleation sites for the formation of the δ-Fe/γ-Fe phase during the solidification of molten steel, thus refining the solidification structure, and have relatively lower pitting sensitivity. In this study, the effect of Ce treatment on the cleanliness, microstructure, and corrosion resistance of 75Cr1 steel was investigated via inclusion characterization and in situ observation of the microstructure evolution as well as electrochemical polarization experiments. Results showed that Ce can effectively remove O, S, and other impurity elements in steel. With the increase in Ce content, the typical inclusions in 75Cr1 steel changed from the initial Ca-Mg-Al-O + MnS + CaS + TiN inclusions to Ce2O2S and Ce2O2S-CeAlO3 inclusions and then to rare-earth sulfide inclusions. After the oxygen and sulfur contents were reduced to a certain extent, Ce started to combine with residual elements such as P and As to form rare-earth phosphide and arsenide inclusions. The size and number of inclusions firstly decreased and then increased. Meanwhile, the morphology of the inclusions firstly changed from irregular to spherical and then changed to irregular again when excessive Ce was added. The addition of appropriate Ce can refine the austenite grains and inhibit their growth; moreover, the corrosion potential and pitting corrosion resistance of steel are improved and the self-corrosion currents decrease. The 0.0195%Ce-containing 75Cr1 steel showed the highest cleanliness, a refined microstructure, and enhanced pitting corrosion resistance.
Cu-Co alloys demonstrate immense potential for industrial applications due to their excellent properties, including high electrical conductivity and giant magnetoresistance effect. As typical immiscible alloys, Cu-Co alloys are prone to liquid-phase separation during their preparation; as a result, their components undergo severe segregation, limiting their applicability. Thus, investigating and elucidating the evolution mechanism of solidification structures of the Cu-Co alloys is imperative. However, liquid-phase separation in these alloys occurs on miniscule time and space scales, and complex physical processes such as diffusion, convection, and heat transfer are also involved. Hence, investigating the kinetic characteristics of alloy solidification and the mechanisms of microstructure formation solely by experimental methods using the existing technology is challenging. Nevertheless, with the continuous advancement of theoretical foundations and computational capabilities of materials, numerical simulations have emerged as an effective tool for investigating the microstructure evolution of immiscible alloys. This study investigates the mechanisms involved in the formation of core-shell structures during the solidification of Cu-Co alloys using a combination of experimental and numerical simulation techniques. Based on the phase-field method, three parallel simulations, incorporating fluid flow and Marangoni motion, were conducted. The microstructure evolution at various stages and under different conditions was systematically analyzed. The simulation results indicated that the fluid flow resulting from liquid-phase separation could expedite the coarsening of the second-phase droplets. Furthermore, Marangoni motion driven by temperature gradients resulted in the coalescence of second-phase droplets at the center (high temperatures), accelerating the coarsening process. The Ostwald ripening phenomenon and coagulation process between the second-phase droplets were simulated, and the growth kinetic mechanisms of the second phase were revealed. In addition, three Cu-Co alloys were used for simulations to investigate the impact of the volume fraction of Co-rich phase on the microstructure evolution. The validity of the simulation results was confirmed by comparing the simulated solidification structures with those obtained experimentally.
The existence of elemental S in nickle-based superalloys negatively impacts their performance. The oxide film at the interface of the nickle-based superalloy peels off during the service process, leading to the failure of the alloy. However, the influence mechanism of the elemental S on the interface of the matrix and the coating layer is yet to be studied. Herein, the influence mechanism of the elemental S on the nickle-based superalloy and NiAl coating was studied using the first-principle calculation, especially focusing on the S segregation phenomenon. The interface adhesion work, segregation energy, and interface charge of the pure and S-doped interfaces of Ni3Al/NiAl and NiAl/Al2O3 were analyzed. The calculated results show that the interfacial adhesion work of the system decreases when the elemental S is present, resulting in reduced interface stability; in these systems, the elemental S tends to segregate toward the interface. By analyzing various aspects of the interface electronic structures (such as differential charge density, Bader charge, electron localization function, and densities of states), it was found that the bonding near the interface was weakened in the system with the elemental S, thereby reducing the tightness of the local connection. The influence mechanism of the elemental S on the interfacial stability of the system was finally revealed.
Ti-6Al-4V, a typical dual-phase titanium alloy, has mechanical properties largely determined by its microstructures. However, the absence of three-dimensional (3D) information regarding the relative orientation relationships of grain boundary α, α lamellae, and α side branches, hinders precise microstructure control. In this study, using thermodynamic data from Pandat and Thermo-Calc, along with kinetic data from DICTRA, the 3D morphology of α lamellae in Ti-6Al-4V alloy was simulated via the phase field method. This study simulated the influence of interfacial energy anisotropy on the growth of α lamellae at a heat treatment temperature of 820°C and analyzed the corresponding solute field. The findings reveal that interface energy anisotropy considerably affects the morphology of α lamellae. When the anisotropy of the interface energy increased from 0.4:0.1:1.0 to 0.8:0.1:1.0, the α lamellae transformed from a thick rod shape to a slender needle shape. Higher anisotropy levels lead to accelerated growth rates of α lamellae. Variation in interface anisotropy, primarily affect the density and growth rate of α lamellae, while their growth direction remains consistent. Additionally, the width of individual lamellae progressively widens under different interface anisotropies. The phase-field simulation results align closely with experimental findings. Notably, the 3D simulation results of α lamellae organization offer more detailed insights into the side branches of α lamellae than two-dimensional (2D) SEM images. In 3D simulation, it can be observed the growth morphology of side branches at different positions of grains. The results indicate that the angle between the main lamellae and the side branches includes experimental observations of 30° and random angles.
Fe-Cr alloys are essential materials for core reactor components. The long-term in-core service of these components under intense radiation, thermal, and stress coupling conditions may potentially expedite the degradation of their mechanical properties. Radiation defects and insoluble helium gas molecules are generally trapped in voids or grain boundaries, forming intra- or intergranular fission gas bubbles. These bubbles cause irreversible radiation volumetric swelling and brittleness. However, a comprehensive understanding of the bubble formation process, particularly the effects of Cr content and dislocation stress field on the formation, remains unclear. As a mesoscale simulation approach, the phase field model coupled with irradiation, temperature, and elastic stress has been employed to study bubble evolution influenced by alloy composition and dislocation configuration. This approach offers advantages when addressing bubble-formation-related issues on different spatial and temporal scales. In this work, the phase field method is employed to investigate bubble growth kinetics and the effects of Cr content and dislocation stress field on bubble formation and evolution in Fe-Cr alloy under radiation. The simulations reveal that in an oversaturated gas and vacancy system, gas atoms tend to cluster at heterogeneous nucleation sites, such as vacancy clusters and dislocations, and grow by absorbing vacancy and gas atoms. The bubbles maintain a constant gas concentration up to a certain size as they continue to grow by absorbing vacancies. However, when the vacancy saturation is high, a bubble will behave as a void if its outward pressure is lower than the equilibrium pressure of a bubble of the same size. Cr additives reduce the diffusion rate of gas atoms and vacancies, extending the nucleation period of bubbles and decelerating their growth and coarsening. Dislocations cause vacancies and gaseous atoms to aggregate in the tension stress regions of the edge dislocation, enhancing the bubble's preferential heterogeneous nucleation in that area. This work discusses key kinetic elements affecting bubble evolution, including intrinsic microstructures and diffusivity. Further, it provides inspiration for future material designs for improving irradiation resistance and long-term service stability.
The grain boundary (GB) energy is one of the fundamental structure-dependent properties of GB and plays a crucial role in the GB-related behaviors and properties of polycrystalline materials. An in-depth understanding of GB energy will help to explore the corresponding mechanisms and provide significant guidance for tailoring material properties based on GB engineering. The crystallographic orientation of GB strongly dominates the GB energy. However, a relatively comprehensive understanding of the orientation dependence of the GB energy is still lacking, especially for bcc materials. In this study, the energies of 1568 tilt GBs in bcc Fe, which covers the misorientation angle (θ) of 0°-180° and 40 distinct misorientation axes ( O ), were computed using the cutoff sphere bicrystal molecular dynamics model. The energy dataset was used to statistically analyze the correlation between GB energy (γ) and GB crystallographic orientation, thereby revealing the underlying mechanisms. The results show that the tendencies of γ-θ correlationcan be considerably different in the high-angle range for GBs with distinct O. Statistically, GB energies increase with θ and disorientation angle in the low-angle range and then level off for higher angles. The energies for noncoincident site lattice (non-CSL) GBs are not necessarily higher than those of CSL GBs and follow the same trend in the low-θ range as CSL GBs. The energies of the tilt GBs decrease with the variation of O from the central regions to the edge and then the corners of the stereographic triangle due to the increasing tendency of the symmetry of the boundary structure. Therefore, the lowest energies are observed for GBs with O close to <111>. Relatively low energies are not observed for GBs terminated by low-index or dense planes. The GB energy shows an overall increasing trend with the surface energy of the boundary plane until a plateau in the GB energy is reached. No distinct correlation is observed between the GB energy and coincidence index (Σ) value. However, the cusp in the γ(θ) curve for GBs with a common O is found to be generally located at the GB with a much lower Σ than its neighboring GBs. Additionally, the potential correlations and laws concerning GB energy and its crystallographic orientation for bcc metals are observed to be partially similar or consistent with those of fcc metals.
