The transition joint between austenitic stainless steel pipe and low alloy steel nozzle of the pressure vessel has attracted much attention due to the occurrence of failure during application. Usually, the low alloy steel vessel nozzle should be firstly buttered with several layers of austenitic stainless steel and then welded to the austenitic stainless steel pipe. Cracking phenomenon in the austenitic cladding layer sometimes occurs during fabrication of the transition joint, and the cracking mechanism is not very clear. It is worth noting that microstructure in the first buttering layer is largely dependent on the welding condition, because the variation of the buttering welding parameters would lead to different dilution ratios in the cladding layer. Therefore, it is essential to investigate the effect of dilution ratio of the cladding layer on the mechanical properties of the weld joint. In this work, microstructure of the 309L cladding layer under two kinds of buttering welding parameters was analyzed using OM, SEM, XRD, EPMA and EBSD, and its effects on the mechanical properties of the weld joints were further studied. The results show that duplex microstructure (austenite+martensite) are present in the 309L cladding layers under two kinds of buttering welding parameters, but the dilution ratio could determine the morphology and amount of martensite phase. Microstructure consisting of austenite and lath martensite is found in the 309L cladding layer with a lower dilution ratio. A higher dilution ratio could increase the amount of lath martensite. The formation of needle-like martensite occurs when the dilution ratio exceeds a critical value. The dilution ratio in the 309L cladding layers directly affects the mechanical properties of weld joint. For the weld joint with a lower dilution ratio, no cracking phenomonen is observed during three-point bending test, and the specimens fracture at the weld fusion zone after tensile test. For the weld joint with a higher dilution ratio, cracking phenomenon initiated at the 309L cladding layer is present during three-point bending test, and a significat reduction in the tensile strength and elongation is observed. During deformation, the strain incompatibility between needle-like martensite and austenite is produced, leading to the formation of microcracks at the interfaces. The preferential cracking at the 309L cladding layer with a higher dilution ratio leads to the degradation of mechanical properties of the weld joint.

The key index of grain-oriented silicon steel is the sharpness of secondary recrystallization Goss ({110}<001>) texture, which is determined by the matrix grain size distribution, texture environment and inhibitor level. In the widely used low-temperature slab heating process in virtue of high efficiency and low-cost manufacturing, the instability of inhibitor and the enlarged matrix grain size distribution seriously restrict the occurrence of secondary recrystallization and the sharpness of Goss texture. The investigation on orientation selection behavior during abnormal grain growth can explore the potential routines to solve the problem. In this work, the evolution process of secondary recrystallization texture in grain-oriented silicon steel has been studied by both experiment and calculation. It is found that single Goss texture is finally obtained by means of continuous orientation selection during secondary recrystallization. The kinetic model for secondary recrystallization, introduced with orientation-dependent relative grain boundary energy coefficient, can describe quantitatively the difference in growth rate between Goss grains with various deviation angles and non-Goss grains. The combined effects of grain size distribution, grain boundary characteristic between Goss and matrix grains, together with inhibition force level on orientation selection behavior are analyzed. Accordingly, a multi-parameter matching method for promoting the advantage of Goss grains in orientation selection is proposed.

Oxide dispersion strengthened (ODS) steel has excellent high-temperature performance and corrosion resistance. It has broad application prospect and development space in the key field of high temperature structural materials for nuclear power. 9Cr-ODS steel has become one of the most promising candidate materials in advanced nuclear reactors because of its excellent high temperature mechanical properties and radiation resistance. In this work, 9Cr-ODS steel was designed and prepared by powder metallurgy process. The as-hot isostatically pressed (HIPed) microstructure of the steel was studied and analyzed, including matrix grain distribution characteristics, micron-scale large size precipitated phase, and nanoscale oxide particles. In addition, the high temperature microstructure thermal stability of 9Cr-ODS steel aged at 650 ℃ for different time was researched by means of XRD, SEM, TEM and hardness test, and the microstructure change of matrix and hardness properties were analyzed. Based on the contrast analysis of the matrix microstructure and hardness properties, the hardness change of the austenitic ODS steel at high temperature was obtained. The results showed that the original as-HIPed microstructure of 9Cr-ODS steel is mainly composed of martensite lath and large amount of Y₂O₃. During ageing process, the lath martensite of 9Cr-ODS steel gradually coarsens and the number of dislocations decreases with ageing time increasing, and the Cr₂₃C₆ carbides begin to precipitate along the grain boundary and grow up. At the same time, the Laves phases with large size begin to precipitate in ageing and then grow with the increase of ageing time. Meanwhile, ageing treatment makes Y₂O₃ phase with larger size further grow, while Y₂O₃ phase with smaller size precipitate increase. This phenomenon can probably be associated with the dissolution of the fine particles induced from the particle coarsening, generally called the Ostwald-Ripening mechanism. The change of microhardness during ageing was related to the size of lath martensite and the number and density of the second phase precipitation, especially Cr₂₃C₆. The hardness test results show that the microhardness first decreases and then tends to be stable with the increase of ageing time.

During the dissimilar materials bonding of copper and 304 stainless steel, micro-voids and micro-cracks can propagate into the bond region because of Kirkendall effect, and have a strong impact on the mechanical and physical properties of conjunct. Copper and 304 stainless steel was bonded by utilizing vacuum solid-state diffusion method with an interlayer of CoCrFeMnNi high-entropy alloy, and the influence of temperature on diffusion reaction mechanism and properties was investigated by using SEM, EDS and microhardness test. The second Fick's law was adopted to calculate the diffusion coefficient of Cu/Fe in CoCrFeMnNi high-entropy alloy. The phase components of the diffusion interface were detected by XRD, and the famous phase-selection-criteria was also used to discuss the phase formation. The results showed that the diffusion interface was well bonded and all the elements diffused mutually at the temperature range of 800~900 ℃, the diffusion rate of Cu/Fe in CoCrFeMnNi high-entropy alloy was increased with the increasing temperature, and no intermetallic compounds were detected at the diffusion interface, and the microhardness increased continuously near the diffusion interface. It was investigated that CoCrFeMnNi high-entropy alloy can be used as an effective diffusion barriers for dissimilar materials bonding of Cu/304 stainless steel.

TiAl alloys are considered attractive structural materials because of their low density, excellent high-temperature strength, and oxidation resistance. However, their intrinsic characteristics, including low-temperature brittleness, poor workability, and narrow processing window, restrict their wide use in industrial applications. Various hot-work processes are conducted to enhance the inherent ductility of TiAl alloys, especially hot-pack rolling. In this work, the hot processing maps at different strains were developed based on isothermal compression tests and dynamic material model (DMM). The optimum hot-working parameters were selected and a crack free Ti-46Al-8Nb (atomic fraction, %) sheet was directly fabricated by hot pack rolling from ingot. Moreover, microstructure evolution and hot deformation behavior of the as-rolled alloys were investigated. The processing maps showed two typical dynamic recrystallization (DRX) domains which would facilitate the hot-work process, of which the temperature was at 1200 ℃, strain rates was 1 s^-1 with a peak efficiency of power dissipation of 0.38 and temperature of 1150~1200 ℃, strain rate of 0.01 s^-1 with a peak efficiency of power dissipation of 0.45. The instable-area temperature was 1100~1200 ℃ and strain rate was 0.06~1 s^-1 at low strain, which was expanded to low strain rate with the increasing strain. As the strain increased to 0.4, the region with the temperature of 1250 ℃ and strain rate of 0.006 s^-1 always became instable. The Ti-46Al-8Nb alloy sheet with thickness of 0.85 mm was produced within processing windows of 1150~1200 ℃, 0.01~0.03 s^-1 with engineering strain 18% per pass. The produced sheet showed uniform microstructure as a whole, though the local flow softening and deformation bands were inevitable. Furthermore, the main softening mechanism of Ti-46Al-8Nb alloy was DRX which began with the pile-up of dislocations, the formation of sub-boundaries and mechanical twins. Then the substructures would rearrange to inducing the formation of DRXed grains with the cumulative reduction increasing. The phase transitions of Lamellae (α/γ)→γ+α+B2/β and α→γ during hot-pack rolling combining with the growth of DRXed grains were simultaneously a main softening mechanism. The formations of plentiful mechanical twinning and twin lamellae also contributed to the uniformity of as-rolled microstructure.

Electron beam melting (EBM) is one of the additive manufacturing technologies which can be used to fabricate the complex structure and shape samples. Until now, there are few literatures published about the properties of Ti-Ni samples produced by EBM. In this work, the influence of two important manufacturing parameters of focus offset (FO) and speed function (SF) on the density, phase content and transformation behavior, microstructure and mechanical properties was investigated for the equiatomic Ti-Ni shape memory alloy fabricated by EBM used DSC, XRD, SEM, TEM and electronic universal testing machine. The results showed that all the Ti-Ni samples had a high relative density beyond than 97% for fabricated by different combinations of FO and SF in the selected range. The corresponding phase transformation temperatures for all the Ti-Ni samples fabricated by EBM were higher than the pre-alloyed Ti-Ni powder, due to the effect of evaporation of Ni element higher than that of the formation of Ni-rich Ti₂Ni phase during the quickly melting and solidification process. On the other hand, the EBM manufacturing parameters of FO and SF had limited influence on the phase contents, phase transformation temperatures and Vickers hardness. Due to the feature of the EBM fabricating method, the different types of defects would be introduced in the Ti-Ni solid samples. Though all the samples had similar high relative density, the performance of the compression behavior were shown great difference, and the crack defect had the larger effect than the gas and lack-of fusion porosities on the compression fracture stress and strain.

The microstructure evolution with grain boundary wetting phase transformation of α allotriomorph during the β→α phase transformation in Ti-6Al-4V alloy has been investigated by means of phase field modeling. A realistic microstructure was generated by coupling the Thermo-Calc thermodynamic parameters and phase field evolution equations. It is shown that the specially constructed thermal noise terms disturb the β/β interfaces and can produce heterogeneous nucleation of α phase at energetically favorable points such as triple junctions and β grain boundaries (GBs). A small amount of α_GB (grain boundary α) nuclei formed at the early stage of phase transition would lead to the formation of discontinuous α_GB; while a large number of α_GB nuclei can result in the formation of continuous α_GB. GBs can be "wetted" by a second solid phase through the reversible transition from incomplete to complete solid state wetting at a certain temperature without a new reaction. The volume fraction of α phase and the grain number increased gradually as the noise amplitude increased from 0.05 to 0.11, or noise duration from 50 s to 80 s. Both noise amplitude and time could control the formation kinetics of α_GB, which will influence the microstructure, and the fatigue properties of Ti alloys can be altered if these are controlled experimentally.

GH4169 has the advantage of excellent comprehensive mechanical properties, good oxidation and corrosion resistance, etc., which have been widely used in aero engine with the largest consumption. The GH4169 parts include high pressure compressor disk, turbine disk, shaft, gearbox and forged blade, et al. With the development of technology and the requirement of cost reducing, the size of GH4169 ingot and billet increases gradually at home and abroad. However, element segregation becomes more and more severe as the size of GH4169 ingot and billet increases, which will significantly degrade their mechanical properties. In this work, the large-scale GH4169 superalloy ingot (diameter 508 mm) was prepared by triple smelting, vacuum induction melting (VIM)+electro sag remelting (ESR)+vacuum arc remelting (VAR). Then, large-scale GH4169 billet (diameter 240 mm) was obtained from this prepared ingot via two-step high temperature homogenization heat treatment and cogging-forging. The element composition and microstructure at different positions of these large-scale ingot and billet were analyzed by SEM, TEM, EPMA and EDS. The results show that the segregation degree of element Al in GH4169 ingot is small, while those of elements Nb, Ti and Mo are large. Moreover, a lot of secondary phases were precipitated at the interdendritic regions of GH4169 ingot, including MC, Laves and δ phase. In the GH4169 billet prepared in our work, no "freckle" or "white spot" macro segregation was recognized, and the micro-element segregation was eliminated. Furthermore, combined with computational simulation, the chemical composition uniformity and main mechanical properties of GH4169 and Inconel 718 billets were compared. The statistical analysis using sample variance of macro chemical composition shows that the uniformity of chemical composition in GH4169 billet produced by different manufactures is different. The regional element segregation results in some vacillation on the mechanical properties of GH4169 billet. It is proposed that this regional element segregation can be further depressed by elaborately controlling the triple melting process and optimizing the homogenization heat treatment and forging process.

Nickel-based powder metallurgy superalloys have the characteristics of uniform structure, fine grains and no macrosegregation. Due to their excellent mechanical properties, such as excellent fatigue resistance, creep resistance, excellent high-temperature strength and crack propagation resistance, they have become the preferred materials for critical hot-end components such as aero engine turbine disks. Selective laser melting (SLM) has a high ability to form complex shape of parts, reducing post-machining procedures and completing efficient productions with low component volumes, so it has become a new technical route for the preparation of superalloys. In this work, the FGH4096M alloy was prepared by SLM technique with pre-powders prepared by vacuum induction argon atomization method. The microstructure and mechanical properties of the as build and heat-treated (HTed) alloys were investigated by OM, SEM, EBSD and so on. The as build alloy with a small number of γ' and carbide, mainly composed of austenite matrix γ phase, has the highest elongation. After heat treatment, a large amount of γ' phase precipitated in the alloy, which is one of the main factors affecting the mechanical properties of the alloy. The uniform and dense distribution of γ' precipitates in the alloy can significantly improve the strength. A higher lattice distortion between the γ' phase with cubic or petaloid shape and matrix can increase the strength of the alloy to some extent. Fine dendritic and equiaxed structures in SLM FGH4096M can improve the property of the alloy as fine grain strengthening. The higher solution temperature promotes the recovery and recrystallization of the SLM alloy, and eliminates the intra-crystal dendritics and equiaxed structures. The average elongation of the as build alloy is 24.97%. The yield strength and ultimate strength of the SLM FGH4096M alloy after direct ageing treatment are the highest, and the average values are 1459.46 and 1595.56 MPa, respectively.

The strain tensors are commonly defined by the local deformation of continuum. Unlike displacement, strain is not a physical quantity that can be measured directly, and it is calculated from a definition that relies on the gradient of the continuous displacement field. At the microscale, it is difficult to define the local deformation according to the position of each atom which is obtained from the adjacent discrete time interval, so there is no universally accepted definition of strain tensors of atomic scale so far, and none of the molecular dynamics software can be used to calculate the atomic strain until now. In order to define the atomic scale strain, a method for calculating the "deformation" both in the atomic scale and the continuum scale is proposed. In the definition, the discrete deformation gradient is proposed to describe the "deformation" in the atomic scale and the influence weight function of neighborhood atom is introduced. Then the weighted least squares error optimization model is established to seek the optimal coefficients of the weight function and the optimal local deformation gradient of each atom. After that, the advanced multilayer complex genetic algorithm can be used to calculate the atomic strain. Finally, take NiTi alloy as an example, the molecular dynamics evolution model of deformation and failure of NiTi alloy was established. Then the atomic scale strain nephogram at each time was calculated, and the microdefects such as twins were observed by strain nephogram. Compared with the micro-observation experiment of crack tip of NiTi alloy for three-point bending, the rationality of the atomic scale strain definition method established in this study and its application significance in identifying the evolution of microdefects are verified.

B₄Cp/Al composite has the advantages of light weight, good stability, high neutron absorption capacity and excellent mechanical properties, and is increasingly used in nuclear industry for storage and transportation of spent fuels. However, due to the obvious difference in the mechanical properties between the reinforcement and the aluminum matrix, the deformation of B₄Cp/Al composite is quite difficult. In this study, the hot compression behavior of 31%B₄Cp/6061Al (volume fraction) composite fabricated by powder metallurgy was investigated in the temperature range of 375~525 ℃ and strain rate range of 0.001~10 s^-1 with Gleeble-3800 thermal simulator system. Based on the modified dynamic material model (MDMM), the power dissipation efficiency and processing maps were established, the instability zones and stable area of hot deformation were determined, and the microstructure evolution during hot compression were analyzed. The results show that the temperature and strain rate have significant influences on the flow stress of 31%B₄Cp/6061Al composite, and the flow stress increases with decreasing temperature or with increasing strain rate. The optimum processing domains for 31%B₄Cp/6061Al composite are at temperatures of 480~525 ℃ with strain rates of 0.01~0.04 s^-1. However, the processing instability area is mainly concentrated in low temperature and high strain rate, and increases with the increase of strain. During the hot compressing, the microstructure evolution is influenced by hot processing parameters, such as the strain, temperature and strain rate. The higher the strain is, the more serious the grain deformation is. With increasing deformation temperature or decreasing strain rate, the size of the dynamic recrystallization grain in matrix increases obviously.

Compared with the mesh-based numerical calculation method, the meshless methods avoid the problems caused by geometric topology, nodal numbering and information transmission of discrete meshes or nodes, which shows significant advantages in solving the problems of complex computing domain boundaries, phase transformation, interface tracking, and crack propagation. Based on the moving least squares approximation and variational principles, a two-dimensional element-free Galerkin (EFG) model for heat transfer/solidification behavior of continuous casting round billets is derived and established in this work. Taking the measured heat flux as the boundary conditions, the non-uniform solidification behavior of the round billet is calculated and analyzed. The essential issues that affect the suitability and calculation accuracy of the meshless model are discussed, such as the nodal arrangement and the size of the supporting domain. The "concentric circle" nodal arrangement scheme in rectangular coordinate system is proposed, and the results show this scheme can conveniently deal with the problem of curve boundary and the solidified shell movement of round billet, showing great flexibility in node arrangement. When the supporting domain size is adopted to be 1.7 times of the average nodal spacing, the calculation accuracy is high under the regular and random nodal arrangement schemes. The results verify the feasibility and accuracy of EFG meshless model in the calculation of non-uniform heat transfer and solidification of billet. It shows a significant advantage in the phase transformation interface tracking, and provides a theoretical foundation for subsequent research on thermo-mechanical coupling and crack prediction analysis.

The fiber fracture is a very dangerous defect in the aeroengine integral bladed rings which is manufactured by continuous SiC fibers-reinforced titanium alloy matrix composite (Ti-MMC). Currently the lack of an effective method for inspecting the fiber fracture seriously influences the quality control of the aeroengine integral bladed rings, therefore solving the problem of non-destructive testing for deep buried radial micro-cracks is an insurmountable barrier for the application of Ti-MMC integral bladed rings in aviation. In this work, based on the theory of ultrasonic head-waves, the refraction longitudinal waves with the proper angle generated by the creeping wave probe are propagated on the interface, which means the creeping wave with high energy is generated. The finite element simulation model of the creeping wave is established for inspecting the defect of SiC fibers breakage in Ti-MMC aero-engine bling structure. Based on the results of the finite element simulation, significant defect signal waves are generated in the water layer above the defect. After that, not only the ultrasonic propagation characteristics at the interface of multilayer media but also the diffraction phenomenon of the creeping wave when they encounter the broken wire defect are analyzed. And the detection signal amplitude is not only affected by the incident angle of the creeping probe but also is related to the thickness of titanium alloy. In order to receive the diffracted waves with high sensitivity, the assembled probe, which is composed by a creeping wave exciter (5 MHz, 20°) and an immersion focusing sensor (5 MHz, diameter 6 mm), is specially designed and manufactured according to the results of the finite element simulation. In addition, the artificial sample with the broken fiber is prepared and then tested by the ultrasonic C-scan testing system with the assembled probe. The results show that the three-layer broken wire defects (the crack height is about 0.42 mm) with different depths in the artificial sample can be detected successfully. The C-scan results can match the defect positions of the artificial sample which means this detection method is valid. This method is simple, economical and fast, and is expected to solve the problem of nondestructive testing for broken wire in the deep embedded fiber ring in the Ti-MMC aero-engine bling structure.