With the development of marine resources and its emerging markets, the marine equipments such as offshore drilling platform, subsea oil and gas transportation pipeline, and storage equipment of oil and gas are developing actively. It is urgent to develop a new type of steel with low cost and excellent toughness to satisfy the demand of marine equipment. In this work, the morphology of austenite and phase transition process in medium manganese steel during quenching and tempering (QT) and quenching, lamellarizing and tempering (QLT) heat treatments were investigated by using EBSD, HRTEM and thermal dilatometer. The results show that the discrepancy of stability in austenite caused by its shape, size and location leads to the more excellent toughness during QLT heat treatment compared to QT. It has been found from thermodynamical and dynamical viewpoint that the formation rates of reverted austenite during QLT heat treatment are significantly larger relative to QT due to the partition process of C and Mn elements in the lamellarizing stage. Moreover, two growth models of film-type reverted austenite are distinct during two heat treatments: the unidimensional two-sided growth mode during QT and unidimensional one-sided growth mode during QLT.

Microstructure and welding residual stresses in ferritic heat-resistant steels such as P92 have been considered as one of the most important factors in the structural integrity and life assessment of power plant weldments. Applying computational tools to predict microstructure and residual stress distribution in practical welded structures is a preferable way to create safer, more reliable and lower cost structures. In this work, the effects of volume change, yield strength variation and transformation induced plasticity (TRIP) on the generation of residual stresses in P92 steel welded joints were investigated experimentally and numerically. Optical microscope and Vickers hardness tester were used to characterize the microstructure and hardness of the weldments. The hole-drilling strain-gage method was employed to determine the residual stress distribution across the weldments. Based on SYSWELD software, a thermal-metallurgical-mechanical finite element method (FEM) was developed to simulate welding temperature field and residual stress distribution in P92 steel joints. Firstly, numerical simulations of Satoh test were carried out to clarify the influence of solid-state phase transformation on the formation of residual stresses. The simulation results show that the volume change and the yield stress variation have a great effect on the magnitude and distribution profiles of residual stresses in the fusion zone (FZ) and heat affected zone (HAZ), and even alter the sign of the stresses, while TRIP have a relaxation effect on the tendency of stress variation during phase transformation. Secondly, a FEM was established to calculate the welding residual stress distribution in a single-pass bead-on P92 steel joint. In the FEM, three main constituent phases (austenite, untempered martensite and tempered martensite) in P92 steel were taken into account. Finally, the simulation results of welding residual stress were compared with the experiments obtained by hole-drilling method. The numerical simulation results are generally in a good agreement with the measured data.

9Ni steel has been widely used in liquid natural gas tanks and pipelines because of its excellent toughness at low temperature after quenching, larmellarizing and tempering heat treatment. Recently, in the cryogenic field it is used in some forgings, which have a strict demanding on the strength of this material. In order to clarify the relationship between the strength and the reversed austenite in the 9Ni steel after different temperature tempering, a systematic investigation on the amount of reversed austenite, deformation induced phase transformation (DIPT) of reversed austenite and its influence on the mechanical properties of 9Ni steel has been carried out by dilatometer, in situ synchrotron high-energy X-ray diffraction, XRD and TEM. The experimental results indicated that the amount of reversed austenite showed a parabolic trend with increase of tempering temperature and obtained the highest value after 600 ℃ tempering. And the DIPT of reversed austenite occurred after yielding during uniaxial tension test. This phenomenon induced that the yield strength of the experimental steel decreased to a minimum value after 600 ℃ tempering, and then, the value increased with further the increase of tempering temperature. However, the tensile strength of experimental steel increased with the increase of tempering temperature and reached the maximum after 640 ℃ tempering, because almost all of the reversed austenite transforms to martensite before necking.

Recently, increasing attention has been focused on the high strength low alloy (HSLA) steels mircoalloyed with multiple miroalloying elements, such as Nb-Ti, Nb-V and Ti-Mo, which can form synthetic carbide in steel, such as (Nb, Ti)C, (Nb, V)C and (Ti, Mo)C. Compared with the simplex carbide, such as NbC, TiC, those synthetic carbides with nanometer size exhibiting a superior thermal stability to exert their powerful influence mainly through their precipitation hardening in ferrite. It is reported that the precipitation hardening of approximate 300 MPa which can be obtained in Ti-Mo-bearing steel was developed by JFE steel, attributing to the synthetic (Ti, Mo)C particle precipitated in ferrite. However, as common microalloying elements, Nb and Mo are added synchronously in steel. The strengthening mechanism of Nb-Mo mircoalloyed as-rolled steel and the role of the carbide precipitated in Nb-Mo mircoalloyed as-rolled steel are rarely reported. Therefore, in the present study, the strengthening mechanism, microstructure and the precipitate characteristics of Nb and Nb-Mo microalloyed steels produced by thermo mechanical control process (TMCP) were comparatively investigated by means of SEM, EBSD, HRTEM and physical and chemical phase analysis, in order to systematically study the synergistic effect of Nb-Mo addition on the strength of as-rolled steel. The results shows that the microstructure is finer and the density of low-angle grain boundaries is higher in Nb-Mo microalloyed steel compared with that of in the Nb microalloyed steel. What's more, the Mo addition could increase the precipitation ratio of Nb, and the amount of the MC-type carbide with nanometer size in Nb-Mo microalloyed steel is evidently larger than that of in Nb microalloyed steel. Those MC-type carbide were identified as synthetic carbide (Nb, Mo)C, exhibiting low coarsening rate than that of NbC precipitated in Nb microalloyed steel, which thus contributed to a higher precipitation hardening. This is main reason of the difference in strength between Nb and Nb-Mo microalloyed steel.

Solute partition coefficient plays an important role in determining the microstructure and mechanical properties of Ni-based superalloys. Melt superheating treatment can greatly affect the melt structure and redistribution of solute atom in the melt. However, up to date, there are few investigations of the influence of melt superheating treatment on the solute partition coefficient, especially for the new-generation Ni-based single crystal superalloy with additions of Re and Ru. Therefore, in this work, the influence of melt superheating treatment temperature on the solute partition coefficient of a new Ni-based single crystal superalloy with Re and Ru elements under planar solid/liquid (S/L) interface under directional solidification conditions was systematically investigated by using EPMA. It was found that the distribution of solute elements, such as Al, Ta, Ru, Re, W, Co, showed remarkable change in both sides of the S/L interface with increasing melt superheating treatment temperature, but there was little change for the solute elements of Mo and Cr. With increasing the melt superheating treatment temperature, the concentration of solute elements for Al and Ta increased firstly and then decreased, but it showed an opposite trend for Re, W, Ru and Co. Additionally, the change of the solute partition coefficients for Ru, Co, Mo and Cr were small with increasing the melt superheating treatment temperature. The main reasons related to the above changes can be ascribed to that the variation of melt superheating treatment temperature affects the size of the atom clusters in melt, which gives rise to the variation of atomic distribution, and thus leads to the change of solute partition coefficients.

Freckles are a detrimental grain defect formed during directional and single crystal solidification of superalloy components leading to a high rejection rate. Based on the experimental and theoretical studies over the past forty years, the occurrence of freckles is generally believed to be mainly dependent on the alloy chemistry and process parameters, while the geometrical factor of castings was hardly taken into account. In the present work, a series of superalloy castings with complex geometry were directionally solidified in a production-scale Bridgman furnace. Some new features of freckle appearance have been observed, indicating new aspects of freckle formation. The freckles are preferably formed on the edges instead of on the plane surfaces of the castings. Correspondingly, freckles were found exclusively on the casting surface having positive curvature, whereas no freckles formed on the surface with negative one. The casting portions having inward sloping surfaces are very freckle prone, while those with outward sloping surfaces are absolutely freckle free. Therefore, as an independent factor the geometrical feature of the castings can more effectively affect the freckle formation than the local thermal conditions. It was also observed that freckles could occur not only on the external surfaces, but also inside the castings where a core was inserted, because both the shell and the core wall can provide very high permeability for freckling convection in the mushy zone. Based on this wall effect, all the important phenomena observed in the present work, such as the edge effect, the step effect, the sloping effect and the curvature effect on freckle formation in complex castings of superalloys, can be reasonably explained.

U720Li, a kind of precipitation type nickel-based superalloy, shows excellent mechanical properties at elevated temperature, which is also known as the difficult-to-deform alloy because of the high-alloying. To solve its deformation problem, new methods would be developed to enlarge the temperature deforming window and improve its plasticity. The hot compression deformation behaviors of directionally solidified and equiaxed grain U720Li alloys were studied by the MMS-300 testing system, as well as the dynamic recrystallization nucleation and growth mechanisms during the hot deformation were discussed. The microstructural characteristics of the alloy under different deformation conditions were examined using OM, SEM and EBSD. The results show that the deforming resistances of both directionally solidified and equiaxed grain U720Li alloys decrease with the increasing of deforming temperature. When the angle θ between the compression deforming direction and dendrite growth direction is 90°, the deforming resistance of directionally solidified U720Li alloy would be lower. With this direction, the coordination deformation between the dendrites becomes better and no crack can be found after deformation, which indicates that the deforming ability is best along θ=90° and it can be considered as the optimal deforming direction for directionally solidified U720Li alloy. Compared with equiaxed grain alloy, directionally solidified U720Li alloy performs higher deformation ability and more homogenous microstructures. During the deformation of directionally solidified U720Li alloy, bulging nucleation of grain boundary migration and dislocation pile-up induced nucleation are found as the main mechanism for the nucleation of dynamic recrystallization. In addition, the deformation activation energy of directionally solidified U720Li alloy is 766 kJ/mol, which is 482 kJ/mol lower than that of equiaxed grain alloy, indicating the directionally solidified U720Li alloy exhibits better hot-working plasticity.

It is widely acknowledged that topologically close packed (TCP) phases are detrimental to comprehensive properties of superalloys, as TCP phases deplete strengthening elements from matrix and easily become crack initiations. In this work, the precipitation kinetics and morphology of topologically close packed μ phase in FGH4097 powder metallurgy (PM) superalloy with (0~0.89%)Hf and the effect of μ phase on the mechanical properties of FGH4097 PM superalloy billet with 0.30%Hf has been investigated. The results showed that μ phase precipitated obviously in the alloys with 0.30%Hf and 0.89%Hf after long-term ageing at 750~900 ℃, the amount and size of μ phase increased as the ageing temperature, ageing time and Hf content increasing. μ phase mainly precipitated in grains with strip and flake shapes. After long-term ageing at 550~650 ℃, no μ phase precipitated in FGH4097 PM superalloy billet with 0.30%Hf and the tensile properties and stress-rupture properties at high temperature were not decreased, which showed excellent microstructure stability. After long term ageing at 750 ℃, precipitated μ phase had little effect on tensile strength at high temperature, however, the tensile ductility increased and high temperature stress rupture life reduced, and the stress rupture ductility increased by about 20%. In this work, the precipitation behavior of μ phase, the redistribution of elements in γ solid solution and the FGH4097 PM superalloy fracture morphology characteristics have been discussed in detail. The mechanism of the brittle and ductile dual effect of μ phase on the mechanical properties has been explained. The methods of controlling and avoiding excessive μ phase precipitation which leaded to performance deterioration have been proposed.

Primary MC carbide is one of the most important phases in cast Ni-based superalloys. During long-term thermal exposure, the primary MC carbide is not stable and tends to degenerate, exhibiting various degeneration reactions, such as MC+γ →M₆C+γ′, MC+γ→M₆C + M₂₃C₆+ γ′ and MC+γ→M₆C + M₂₃C₆+η. It is widely known that the degeneration of primary MC carbide has obvious influence on the microstructural evolutions of superalloys, including coarsening of γ′ phase, coarsening of grain boundaries and precipitation of topologically close-packed (TCP) phase, and consequently the mechanical properties of alloys. Much research work has focused on the degeneration mechanism of primary MC carbide during long-term thermal exposure, however, it is not very clear so far. In this work, a cast Ni-based superalloy is fabricated and thermally exposed at 850 ℃ for 500~10000 h in order to study the degeneration mechanism of primary MC carbide. The degeneration of primary MC carbide is observed by OM, SEM and TEM. High-angle annular dark field (HAADF) mode of TEM is used to clearly observe the degeneration of primary MC carbide and the element distribution in the degeneration areas. The results show that the primary MC degeneration is an inter-diffusion process which occurs between the primary carbide and the γ matrix. During the degeneration, C is released from the primary carbide, Ni, Al and Cr are provided by the γ matrix, while Ti, W and Mo come from both primary MC and γ matrix. The precipitation of η phase is determined by the atomic fraction of Ti+Nb+Ta+Hf and atomic ratio of (Ti+Nb+Ta+Hf)/Al and its amount is affected by the degeneration degree of primary MC carbide. The higher the degeneration degree, the larger the tendency for the precipitation of the η phase.

Mg and its alloys are regarded as potential candidates to replace steel and other heavier materials in some applications due to low density and high specific strength. However, the application of Mg alloys is limited because of their low strength, poor formability and corrosion resistance. Grain refinement and Mg-Al composite have been applied successfully to improve the strength and formability of Mg alloys. The accumulative roll bonding (ARB) is one kind of severe plastic deformation process which can produce bulk ultra-fine grained metallic materials. In the present work, the ultra-fine grained alternative Mg/Al multilayered composite sheets were fabricated at room temperature by ARB process using commercial pure Mg and AA1050 Al sheets up to 3 cyc. Some of Mg/Al sheets after 3 cyc ARB were annealed at 200 ℃ for 15, 60 and 90 min, respectively. The microstructure of ARBed sheets were invesgated by OM and SEM. The global texture evolution of these ARBed sheets were measured by neutron diffraction. It is found that the grains in both Mg and Al layers are refined gradually with the increase of ARB cycles. Although the grains in the Mg layers didn't grow up obviously after annealing at 200 ℃ for different times, the homogeneity of the microstructure was improved. The Mg layers of ARBed sheets showed typical rolling texture which enhanced with the increase cycle of ARB process up to 2 cyc and decreased sligthly after 3 cyc. The Al layers exhibited a combination texture types of rolling and shear texture, including Copper, S, Brass and rotated cube (RC) texture components. After 200 ℃ annealing, the Mg layers remained typical rolling texture component and it's intensity enhanced significantly after 15 min annealing and kept stable during the following annealing processing. The Al layers maintained a combination of rolling and shear texture components, the intensity of rolling components became stronger after 15 min annealing, then decreased after 60 and 90 min annealing. The yield strength and tensile strength were improved while the ARB cycle increased.

It is quite different from those low to medium stacking fault energy face-centered cubic metals, Al and most its alloys are not applicable to twin-induced grain boundary engineering processing due to their high stacking fault energy. In order to optimize the grain boundary character distribution so as to remarkably better the properties of Al and its alloys, it is necessary at first to study the grain boundary plane distributions. In this work, two parallel high purity (99.99%) Al specimens, which were prepared by multi-directional forging followed by recrystallization annealing resulting in a homogeneous microstructure with averaged grain size around 20 μm, were separately processed by equal channel angular pressing (ECAP) and direct rolling (DR) with true strain ε≈2 followed by a recrystallization annealing at 360 ℃ for 8~90 min. Then, the grain boundary plane distributions were characterized by five-parameter analysis (FPA) based on stereology method and electron backscatter diffraction (EBSD). The results show that the grain boundary planes of the specimens as processed mainly orient on {111}, mostly corresponding to the <111> twist high angle boundaries. It is due to the energy minimum of {111}. The primary difference of grain boundary plane distributions between ECAP and DR specimens lies in the behaviors of grain boundary planes orienting onto {111}. For ECAP specimens, it is slow the grain boundary planes orienting onto {111}. However, for DR specimens, it is quite easy the grain boundary planes orienting onto {111}. Discussions pointed out, compared with ECAP deformation, DR deformation is more efficient for grain boundary plane orienting onto {111} in the subsequent recrystallization annealing and thus is more in favor of the optimization of grain boundary character distribution. It could be attributed to the development of <110>//ND textures during DR deformation which results in the fast grain growth in the subsequent recrystallization annealing.

CoCrW alloy is a kind of stellite alloy, which has high strength, good wear resistance, and is widely used in aviation industry, nuclear industry and other fields. For a long time, the CoCrW alloy is mainly used as coating to have an effect on wear and the corrosion resistance. With the development of the industry, the conventional cast CoCrW alloy has been widely studied. The mechanical properties of the cast CoCrW alloy can be changed by heat treatment, which is of high hardness and great brittleness. In this work, hardness and wear-resistant properties of the CoCrW alloys as-cast and after solution treatment were studied by combining XRD, SEM, EDS, hardness test and wear resistance test, and effects of the solution temperature on the microstructure and wear-resistant properties were also investigated. The results showed that the microstructures of the CoCrW alloys as-cast and after solution treatment were both composed of M₂₃C₆, M₆C and γ-Co matrix, but the size, morphology and distribution of carbides occurred in the alloy changed obviously by solution treatment. The dissolution of a large amount of carbides in the alloy after solution treatment was mainly responsible for the decrease in hardness and wear resistance of the alloy compared with that of the as-cast one. With the increase of the solution temperature, the Cr, W and other alloying elements in the carbides were dissolved into the γ-Co matrix so as to strengthen the matrix resulting in the improvement of the hardness and the associated wear resistance of the alloy. The mechanism that controlling the wear-resistant property of CoCrW alloys as-cast and after solution treatment was the interaction of abrasive wear, adhesive wear and oxidation wear.

Nanotwinned materials have attracted widespread attention due to their superior mechanical properties, such as high strength, good ductility and work hardening. Experimental and molecular dynamics (MD) simulation results had indicated that there are three distinctly different dislocation-mediated deformation mechanisms in nanotwinned metals, namely dislocation pile-up against and slip transfer across twin boundaries (TBs), Shockley partials gliding on twin boundaries leading to twin boundary migration, and threading dislocations slip confined by neighboring twin boundaries. However, most of the previous studies are focused on the homogenous plastic deformation under tension and compression tests, the non-homogenous deformation and its deformation mechanism, especially under low strain and complex stress condition/confined condition, of nanotwinned metals are still not explored so far. In this study, the electrodeposited bulk Cu samples with preferentially oriented nanotwins were cold rolled with the normal of the rolling plane parallel to the growth direction (ND//GD) to strain of 15% at room temperature. The microstructure features of as-rolled Cu were investigated by SEM and TEM. Microstructure evolution indicates that many detwinning bands appeared in the direction about 30°~45° with respect to the rolling direction, which is the direction with the largest shearing stress. The twin lamellae in the detwinning bands coarsened obviously. Based on calculation of the local shear strain and strain gradient of TBs in a selected detwinning band, it indicates that the maximum shear strain occurs in the middle of the deformation bands, and its detwinning mechanism is directly related the localized shear strains (γ). The twin lamellae in the detwinning bands were coarsened obviously. When 0.3<γ<0.8, the detwinning process via producing amount of Shockley dislocations on twin boundaries dominates the deformation. After detwinning, Shockley partial dislocations stored at the area with the maximum strain gradient and formed incoherent twin boundaries (ITBs). The present investigation indicates detwinning process dominates the plastic deformation and sustains the local shearing strain in nanotwinned Cu at small strains under cold rolling.

Monotectic alloys are characterized by a miscibility gap in the liquid state. Many of them have great potentials to be used in industry. For example, alloys based on Cu-Pb and Al-Pb are good candidates to be used as advanced bearing materials if the soft Pb phase is dispersed in the Al or Cu matrix. Cu-Cr alloy is a high-strength, high conductivity material and Cu-Co alloy is an excellent magneto-resistive material, etc.. However, when a homogeneous monotectic alloy melt is cooled into the miscibility gap, it will transform into two liquids. The liquid-liquid decomposition generally causes the formation of a phase segregated microstructure. In recent years, considerable efforts have been made to investigate the solidification behavior of monotectic alloys. A lot of experiments have been carried out under microgravity conditions in space as well as under the gravitational conditions on the earth. The solidification behaviors of monotectic alloys under the conventional and rapid solidification conditions as well as the effect of external fields, such as electric current, magnetic field etc., are investigated. Models describing the solidification process have been built and the microstructure formations under different conditions have been calculated. It has been demonstrated that the microstructure evolution during cooling an alloy in the miscibility gap is a result of the concurrent actions of the nucleation, growth, Ostwald ripening and motions of the dispersed phase droplets. The nucleation of the dispersed phase droplets has a dominant influence on the solidification microstructure of monotectic alloys. In this work, solidification experiments were carried out to investigate the effect of micro-alloying element Bi on the solidification of Al-Pb alloys. The experimental results demonstrate that micro-alloying element Bi can cause an obvious refinement of the Pb-rich particles. The refining effect increases with the increase of the Pb content of Al-Pb alloys. The affecting mechanism of micro-alloying element Bi on the solidification process of Al-Pb alloys was analyzed. The microstructure formation process was calculated. The numerical results indicate that the addition of micro-alloying element Bi causes a reduction in the interfacial energy between the two liquid phases and, thus, enhances the nucleation rate of the Pb-rich droplets and promotes the formation of Al-Pb alloys with a well-dispersed microstructure.

Safe and efficient hydrogen storage remains a grand challenge in the widespread implementation of hydrogen fuel cell technology. Recently, chemical hydrogen storage has emerged as a promising alternative for vehicular and portable applications. A number of hydrogen-rich materials have been experimentally demonstrated to deliver large amounts of hydrogen under mild conditions with controllable kinetics. Among these materials of interest, hydrous hydrazine (N₂H₄H₂O) is a promising but yet not fully explored candidate. The development of highly efficient catalyst and its reaction kinetics law are the key issues of N₂H₄H₂O-based hydrogen generation (HG) systems. Herein, a supported Ni-Pt/La₂O₃ catalyst was prepared by a combination of co-precipitation and galvanic replacement methods. Via optimizing preparing processes, the developed catalyst enabled a complete decomposition of N₂H₄H₂O to generate H₂ at a reaction rate of 340 h^-1 at 323 K, which outperforms most reported N₂H₄H₂O decomposition catalysts. Phase/structural analyses by XRD, TEM and XPS were carried out to gain insight into the catalytic performance of the Ni-Pt/La₂O₃ catalyst. In addition, the effects of temperature, concentration of N₂H₄H₂O and NaOH, and amount of catalyst on the N₂H₄H₂O decomposition were investigated over the Ni-Pt/La₂O₃ catalyst. The kinetic rate equation may be represented by the expression: r = -d[N₂H₄H₂O]/dt = 2435exp(-51.53/(RT))[N₂H₄H₂O]^0.3[NaOH]^0(0.12)[Ni]^1.03. The obtained results should lay the experimental and theoretical foundation for developing practical application of N₂H₄H₂O-based HG system.