Nanoindentation of Ni-based single crystal alloy which has a void defect is simulated by the molecular dynamics method. Three models with different voids which have a same radius but different depth (H=1.5 nm, 3.0 nm, 4.5 nm) are contrasted to the perfect model respectively. The influence of a void and misfit dislocation on nanoindentation process are analyzed using center symmetry parameter. Nucleation and growth of dislocation on various indentation depth are researched simultaneously. After relaxation, misfit dislocations occur in all models, which indicates that the void does not affect the generation of misfit dislocation in γ/γ' phase. The indentation load-depth curves show the shallow void (H=1.5 nm) has the greatest influence on nanoindentation. The results demonstrate that the void has two different ways to affect the nanoindentation process. Initially, the void softens the materials when the indentation depth is less than 0.375 nm. However, it will hinder the growth of dislocations because of a kind of surface force, which causes the increase of indentation load while the indentation depth is between 0.375 nm and 0.567 nm. The collapse of a void absorbs the strain energy, so the amount of stacking faults nucleation in γ phase in model with the shallow void is less than which in the perfect model. The indentation load-depth curves show that the indentation load in the H=1.5 nm model is larger than load in the perfect model at 1.263 nm indentation depth. But when the void collapses completely, dislocations tangle around the original location of the void and more stacking faults generate comparing to the perfect model at the same indentation depth h=1.743 nm. So the indentation load declines and becomes smaller than load in perfect model. If the void locates at the interface of γ/γ' phase (H=3.0 nm), it influence the nanoindentation process later than H=1.5 nm model. Dissociation of misfit dislocations is observed when the indentation depth arrives the maximum value 1.748 nm in H=3.0 nm model. Stairs form on the surface of γ phase because of the dissociation of misfit dislocations. There is almost no influence on the nanoindentation of Ni-based single crystal alloy when the void locates in the γ' phase (H=4.5 nm).

Ti and its alloys have potential application in micro-electromechanical systems (MEMS) for its excellent mechanical properties. The strength of micro- and nano-scale Ti and its alloys has been proven significantly increased as the sample size decreased, which is known as the "size effect", when dislocation and twinning are dominant plastic deformation modes. Martensitic transformation is an important plastic deformation mode in the Ti alloys. However, there is a limited research on the martensitic transformation in small-scale. Therefore, the study on mechanical behavior and deformation mechanism of [011]-oriented Ti-10V-2Fe-3Al (Ti1023) single crystal micropillars in a size range of 0.3~2.0 μm were investigated under uniaxial compression. The results show that Ti1023 micropillars exhibit smooth stress-strain curves in the regime of plastic deformation without a conventional strain burst phenomenon in the submicron pillars. It means continuous plastic strain hardening. The relationship between the yield stress (σ_0.2), the stress for stress-induced martensite phase (SIM) transformation (σ_cm) and the sample size can be expressed in the forms of σ_0.2∝d ^-0.18 and σ_cm∝d ^-0.28 , respectively. Strain hardening exponent (n) in creases with decreasing micropillar size. SEM examination together with crystallography analysis show that {112}<111> slip predominates plastic deformation mode in the Ti1023 micropillars. Transmission electron microscopy (TEM) observation of microstructures in the deformed and undeformed micropillars indicate that both nanoscale athermal ω particles and SIM phase α″ impede dislocation movement, and prohibit the formation of tangled dislocations in a collective, avalanche-like way resulting in strain bursts.

Cu-Ag material with excellent combination of high strength and high conductivity is an important conductor for both direct current resistive and pulsed high-field magnets. The strength and electrical conductivity of Cu-Ag microcomposite are closely related to the microstructure of proeutectic Cu because of its high volume fraction. The morphology of proeutectic Cu, Ag precipitation and concentration of Ag in Cu can be controlled by application of external field and the addition of the third elements. In this work, the microstructural evolution, concentration contributions, the resulting microhardness and electrical resistivity of Cu-6%Ag alloy, which was directionally solidified under a transverse magnetic field were studied. The effect of the magnetic field on the microstructure was analyzed by OM, SEM, TEM and EDS. The results demonstrate that in macro scales, the growth direction of columnar grains is gradually deflected along the axial and heating flow directions with increasing magnetic field intensity. In micro scales, the increasing magnetic field increases both the primary dendrite arm spacing and volume fraction of proeutectic Cu, and traps more supersaturated Ag in proeutectic Cu. No obvious effect on the secondary dendrite arm spacing of proeutectic Cu is observed. In nano scales, SAED pattern in TEM indicates a small quantity of fine nanostructured Ag precipitations in proeutectic Cu. A relationship among the primary dendrite arm spacing, external magnetic field intensity and the initial diffusion coefficient in liquid was established from the viewpoint of suppressed convection by the magnetic field. The increased supersaturated Ag in proeutectic Cu is thought to be caused by the influence of magnetic field on the solute redistribution coefficient. The changes of microstructure induced by magnetic field result in the increases of the microhardness and electrical resistivity in Cu-6%Ag alloy. A model was proposed to clarify the changes of electrical resistivity in terms of the resistivity of Cu matrix, the impurity-scattering resistivity from dissolved Ag in Cu and the scattering resistivity from vacancy, where the interface-scattering resistivity from precipitation of Ag is assumed to be ruled out. The result shows that the impurity-scattering resistivity from dissolved Ag in Cu, which is increased by the application of external magnetic field, plays an important role in determining the overall resistivity of the alloy.

The effects of microstructure on the fatigue crack growth behavior of hard-to-deformed GH4738 superalloy have been studied by a number of researchers. However, most of these studies are confined to a single factor, such as the effect of grain size on the fatigue crack growth rate, and show the effect of single factor which do not reflect the combined impacts of multi-microstructure factors. Therefore, there is a need to develop a quantitative approach to predict the effects of multi-microstructure on fatigue crack growth behavior in the design of GH4738 alloy with high damage-tolerant microstructure. A new multi-microstructure factors interaction equation is proposed for the prediction of the effects of grain size, γ′ size and carbide size on fatigue crack growth rate of GH4738 alloy in this work. Different microstructures of GH4738 alloy are produced by different heat treatments (HT) for this equation. The fatigue crack growth experiments are carried out under constant stress ranges on compact tension (CT) specimens at 650 ℃ in air. Subsequently, the effects of grain size, γ′ size and grain boundary carbides size on the fatigue crack growth rate of GH4738 alloy are analyzed by using the interaction equation of multi-microstructure factors. The results show that the equation can well predict the fatigue crack growth rate of GH4738 alloy under different microstructures. The growth rate of fatigue crack in GH4738 can be decreased with increasing grain size and reducing γ′ size and carbide size. The effect of grain size on fatigue crack growth rate is more notice able than that of γ′ and carbide sizes.

Co-based alloy has high strength, good corrosion and wear resistance, and is widely used in aviation industry, nuclear industry. The cast Co-based alloys has high hardness brittleness, but the toughness is low, which limits its wide use. The CoCrW alloy prepared by powder metallurgy process has high toughness, at the same time, the mechanical properties of the CoCrW alloy can be changed by heat treatment. In this work, the effect of solid solution treatment on the microstructure and mechanical properties of the hot pressed alloy was studied by SEM, XRD and TEM, hardness test, room-temperature tensile and wear experiment. The results show that the microstructure of the as-hot pressed and solid solution state CoCrW alloy are both composed of M₂₃C₆, M₆C, CrCo intermetallic compounds and γ-Co matrix, the contents of M₂₃C₆ and prior particle boundaries decrease remarkably after solution treatment, meanwhile, the toughness and wear resistance of the alloy are improved. With the increase of solid solution temperature and time, the hardness and tensile strength at room temperature of CoCrW alloys in crease first and then decrease.

The experimental alloy is designed and employed in high-performance industrial gas turbines as low-pressure turbine blades, working in temperature range of 750~900 ℃. The alloy contains high levels of refractory elements in order to increase the high-temperature mechanical properties. However, this can make the alloy prone to the formation of σ phase during service, which could deteriorate the properties further if the fraction of σ phase exceeds the safety allowances. In this study, the formation of σ phase during long-term thermal exposure, dissolution of the σ phase during rejuvenation process and their influence on stress-rupture properties of a hot-corrosion resistant nickel base superalloy have been investigated. During long-term thermal exposure at 800~900 ℃ for up to 1×10⁴ h, the σ phase formation is mainly in dendrite cores with a few at interdendritic regions. As the aging temperature increases, the precipitation rate of σ phase increases and the incubation time for nucleation of σ phase decreases. From the kinetic analysis, the σ phase form firstly in the vicinity or on the M₂₃C₆ in dendrite cores with the strong segregation of W, Cr and Co. The calculated activation energies of σ formation show that the early stage is related to Co and Cr diffusions and the steady stage is related to Mo diffusion. During solid solution process at 1000~1170 ℃, the σ phase precipitated during long-term thermal exposure dissolves to γ matrix. As the solid solution temperature is higher, the dissolution of σ phase becomes faster. Moreover, the σ phase does not embrittle the alloy. The reheat treatment of the alloy leads to the dissolution of precipitated σ phase and further prolongs the stress-rupture life efficiently.

Chaotic advection could be strengthened mix, mass transfer and heat transfer in the viscous fluid, the melt flow forming was affected by electromagnetic field in the metal solidification process, so were the macro and microstructure of materials. So it was necessary to study chaotic characteristics of semisolid Al alloy in the electromagnetic field. The characterization of chaotic convention and morphology of primary phase were mainly researched in semisolid A356 alloy under the electromagnetic field. The trajectories of the particle in semisolid A356 alloy melt was simulated by the computational fluid dynamics software Fluent, the Kolmogorov entropy and fractal dimension of the flow trajectories of semisolid A356 alloy melt were judged and analyzed. The results showed that chaotic advection may happen in semisolid alloy melt under electromagnetic field. Combined with experiment, the results of process parameters and different current frequencies were compared. The results showed that the primary phase with 59.86 μm in average equal-area circle diameter, 0.71 in average shape factor, 6829.5 nat/s of Kolmogorov entropy and 2.2439 of fractal dimension can be obtained in semisolid A356 alloy with pouring 650 ℃, stirring for 15 s at 30 Hz, and holding at 590 ℃ for 10 min. Meanwhile, the morphology of primary phase can be observed with the best parameters.

Magnesium alloys have attracted more attentions due to their low densities and excellent specific strengths. However, proper manufacturing methods are still needed to promote further applications of magnesium alloys due to the shortcomings of conventional processing methods. As one of the most promising additive manufacturing technologies, selective laser melting (SLM) was utilized to process the most commonly-used AZ91D magnesium alloy in this work. Element vaporization mechanism during the forming process and the influence of element vaporization on chemical composition, microstructure, and mechanical properties of the final products were investigated using OM, SEM, EDS, XRF and XRD. The results show that the relative content of Mg in the SLM-processed samples (86.61%~88.68%) was lower than that in the original AZ91D powders (90.63%) , whereas the relative content of Al in the former ones (10.40%~12.56%) was higher than its counterpart in the latter ones (8.97%). This variation matches well with the calculation by Langmuir model, demonstrating that element vaporization of AZ91D mainly targets at Mg. With the increase of laser energy density (E_V), weight ratio of Mg to Al (η) in the SLM-processed samples first increased, then decreased and finally tended to be constant. η of the sample prepared at 55.6 J/mm³ (sample No.8) presented a smallest difference with that of the original powders. A model illustrating analytic relationship between η and E_V was established by mathematical regression with the fitting index R² being 0.858. The sample processed at 166.7 J/mm³ (sample No.1) underwent one of the most remarkable compositional variation and exhibited a typical solidified microstructure similar to the die-cast AZ91D in which net-like β-Mg₁₇Al₁₂ precipitates were distributed around the α-Mg matrix. However, β-Mg₁₇Al₁₂ content as well as solid solubility of Al in α-Mg matrix was much higher in sample No.1. The enhanced tensile strength and micro-hardness as well as the deteriorated elongation of sample No.1 could be attributed to the composition variation during SLM process.

To reduce the weight of car body, Al-Mg-Si-Cu alloys have been widely used to produce outer body panels of automobiles due to their favorable high strength-to-weight ratio, corrosion resistance and good formability. However, their bake hardening responses still need to be further improved to enhance their dent resistance. In this work, an advanced Al-0.93Mg-0.78Si-0.20Cu-0.30Mn-0.40Fe-3.00Zn (mass fraction, %) alloy has been developed, its precipitation behavior has been investigated systematically through DSC, OM, SEM, TEM and tensile test. Five exothermic peaks were observed in DSC curve of the solution quenched alloy, these peaks were believed to be caused by the formation of GP zones and precipitate phases, the formation and dissolution of these precipitates were analyzed by Avrami-Johnson-Mehl method, a kinetic equation Y=1-exp[-2.03×10¹⁹exp(-23573/T)t²] has been established, which can be greatly used to predict the precipitation behavior. After artificial aging at 185 ℃for 90 min, the peak hardness of 133 HV can be obtained, corresponding to the predicted results. Additionally, the tensile properties in peak aging state, i.e. yield strength, ultimate tensile strength and elongation, are 346 MPa, 383 MPa and 13%, respectively, and ductile fracture is the main fracture feature as observed by SEM examination of fracture surface. Although 3.00%Zn is added in the alloy, yet, Mg-Si precipitates are still the main precipitates formed during artificial aging at 185 ℃. Both β′′ and pre-β′′ precipitates can be observed in the peak aging state, and the presence of the latter ones should be resulted from the formation of Zn-containing clusters. In addition, based on the microstructure evolution of the alloy, a schematic diagram of forming precipitates in the Al-Mg-Si-Cu-Zn alloy is put forwarded.

3D-IC integration realized by using through-silicon via (TSV) technology is the main trend in packaging industry. TSVs are usually fully filled by electroplated Cu, namely TSV-Cu, which can make products possess higher electrical performance, higher density and lighter weight. In a typical TSV forming process, the TSV-Cu is annealed to stabilize its microstructure. However, during annealing process, the Cu protrudes out of the TSV due to the large change in temperature and the mismatch of coefficient of thermal expansion between the Cu (16.7×10^-6 ℃^-1) and its surrounding Si (2.3×10^-6 ℃^-1) matrix. This protrusion is a potential threat to the TSV structural integrity, since it might lead to cracking or delamination. In this research, the effects of annealing process on microstructure evolution and protrusion of TSV-Cu are investigated. Four level sets of current density and additive concentration were used to fill Cu into the TSV by electroplating process to prepare test specimens. The TSV diameter was 30 μm, and depth was 100 μm. The pitch of two TSVs was 200 μm. The annealing process was conducted in a vacuum annealing furnace, the specimens were heated from 25 ℃ to 425 ℃, and then maintained for 30 min at 425 ℃. The microstructures of TSV-Cu before and after annealing were characterized by EBSD. The protrusion of specimens after annealing was measured by White Light Interferometer (WLI). The results show that, during the electroplating process, both current density and additive concentration have impact on the TSV-Cu grain size, higher current density and higher additive concentration help to gain a finer grained Cu, and the influence of the additive concentration is less significant than the current density. After being annealed, for all the specimens, the Cu grain size increases, the TSV-Cu protrudes with a crack along the Cu-Si interface within the Cu seed layer, and there is a positive correlation between the protrusion and the grain size of the TSV-Cu.

Advanced boiling water reactors (ABWRS) show optimistic application prospect in the future. However, in these reactors, influence of dissolved oxygen (DO) on the corrosion rate of zirconium fuel claddings should be seriously considered. In this work, the effect of the dissolved oxygen (DO) on the corrosion behaviors of Zr-4, N18 (Zr-1.0Sn-0.3Nb -0.3Fe-0.1Cr) and N36 (Zr-1.0Sn-1.0Nb-0.3Fe) alloys in 400 ℃ and 10.3 MPa steam was investigated. A recirculation loop was used to control the DO level at about 0.1×10^-6 and 1.0×10^-6, respectively. The results showed that, under the two DO level conditions, N18 had almost the same weight gain as Zr-4 after exposure for 90 d, and N36 had the highest weight gain. In the initial period of the corrosion test, the three alloys had lower weight gain under higher DO level condition. With the increase of exposure time, the weight gain under 1.0×10^-6 DO level exceeded gradually the weight gain under 0.1×10^-6 level for each alloy, and the time needed for exceeding was significantly shorter for the alloy with higher Nb content.

Non-metallic inclusions especially oxides are detrimental to the quality of ferritic stainless steel products. Accurate characterization on inclusions is conducive to further research on the inclusion control. There are some disadvantages in traditional 2D or 3D inclusion detection methods, tomography is thus employed to characterize inclusions in steel in the current work. Oxide inclusions in the slab of Ti bearing ferritic stainless steel were characterized 3 dimensionally using high resolution synchrotron micro computed tomography (Micro-CT), and the variations of quantity, volume and size of oxide inclusions along the thickness of continuous casting slab were analyzed quantitatively and compared with the 2D results detected by ASPEX, an automated scanning SEM. It was found that non-destructive detection could be well done by Micro-CT more accurately. The detected oxides by Micro-CT were mainly global, and the number of inclusions decreased with increasing size. In general, the number density and volume fraction of oxides were largest in the center of slab thickness, and decreased with the distance from center, reached the smallest value near the surface of slab. Contrarily, the average of equivalent diameter of oxide inclusions was largest near slab surface, and was smallest near quarter of thickness on the loose side.

High performance steels require not only high strength, but also the combination of high ductility, high toughness and good weldability. Retained austenite in multi-phase steels has been widely reported to be helpful for obtaining high strength, high ductility and high toughness. In this work, steel with multi-phase microstructure consisting of intercritical ferrite, tempered martensite/bainite and different volume fractions retained austenite was obtained by intercritical annealing and tempering at 600~680 ℃. The volume fractions of retained austenite were 2%, 5%, 10% for samples tempered at 600, 650 and 680 ℃, respectively. The effect of retained austenite on ductility and toughness was studied in detail. Results showed that there was no obvious change in strength by varying the volume fraction of retained austenite, yield strength of the steel was 540~590 MPa, tensile strength was 720~780 MPa. Retained austenite could largely improve both the ductility and toughness of the steel. With increasing the volume fraction of retained austenite from 2% to 10%, the uniform elongation and total elongation were enhanced from 10.3% and 23.8% to 20.4% and 33.8%, respectively. The underlying reason for the improvement of ductility was attributed to the transformation induced plasticity of retained austenite by providing sustainable high work hardening rate. The improvement of toughness by retained austenite became more obvious when testing temperature was lower. When impact test temperature was higher than -60 ℃, the Charpy impact energy of samples with 2%~10% retained austenite were larger than 120 J. When test temperature was -80 ℃, Charpy impact energy of sample with 2% retained austenite decreased to 14 J, while that of sample with 10% retained austenite remained as 60~80 J when test temperature was as low as -80 and -100 ℃. Results from instrument impact test indicated that retained austenite was helpful for enhancing plasticity before crack initiation at low temperature, leading to improvement of crack initiation energy, resulting in excellent low temperature toughness.

The cavitation erosion (CE) is a serious problem in engineering components in contact with a liquid in which the pressure fluctuates. The CE resistance of material is related to the microstructure, hardness, work hardening ability, superelasticity and superplasticity, or strain or stress induced phase transformation of material. The high nitrogen stainless steel (HNSS) is attractive for its low cost in application where a combination of good strength and toughness, high work hardening capacity, and corrosion resistance is required. These attractive properties cause the nitrogen alloyed stainless steels to be the good candidates with relatively high CE resistance. In this work, the CE behavior of HNSS in distilled water, 0.5 mol/L NaCl and 0.5 mol/L HCl solutions was investigated on the base of mass loss and polarization curve. The micrographs of damaged surface were observed by using SEM. The results showed that the cumulative mass loss of HNSS after subject to CE for 8 h was the highest in 0.5 mol/L HCl solution and lowest in distilled water. There existed an incubation period in mass loss rate curve and the incubation period shorted with the increase of the corrosive of tested solution. The plastic fracture was the dominant damage mode of HNSS subject to CE condition. The plastic deformation and dislocation motion of HNSS were facilitated by diffusion of hydrogen in HCl solution, therefore the initiation and propagation of crack were accelerated and removal of materials was accelerated by propagation and connection of cracks.

The stress state is important for properties and service life of mechanical parts, so finding an optimal method for evaluation of stress state is urgently needed to be solved. Because of convenience and fast detection speed, metal magnetic memory method has attracted attention of scholars, and some research findings also have been obtained. While current research mainly focuses on evaluation of stress state of single ferromagnetic material, the research on ferromagnetic composite material or ferromagnetic coating material is rare. Because of high energy density, laser cladding technology has been used widely in field of surface engineering. For this reason, the stress state of ferromagnetic laser cladding Fe314 alloy coating is evaluated with metal magnetic memory method. Distribution of stress state is usually affected by flaw including crack and gas hole in laser cladding Fe314 alloy coating, so the interaction influence of crack and load on evaluation of stress state of laser cladding Fe314 alloy coating is discussed in this work. Combing with equivalent method, different cracks, which were substituted with regular rectangular grooves, were machined in laser cladding Fe314 alloy coating. In order to obtain the relationship between burial depth and magnetic field intensity normal component H_p(y), the regular rectangular grooves that had the same width and different buried depths were machined. The microstructure of laser cladding coating was observed by SEM, and the influence of microstructure on magnetic field intensity normal component H_p(y) was also discussed. Based on magnetic-mechanical theory, interaction influence mechanism of crack and load on evaluation stress state of laser cladding coating with metal magnetic memory method was clarified, the relationship between burial depth of crack, load and gradient value K of magnetic field intensity normal component H_p(y) was also obtained. The results show that when zero crossing is seen as center, the magnetic field intensity normal component H_p(y) rotates clockwise as stress increases gradually, the slope and amplitude of H_p(y) curve increases, gradient value K of magnetic field intensity normal component H_p(y) corresponding to crack also increases as stress increases. Stress concentration in different zones is caused by anisotropic microstructure and layer interface of laser cladding Fe314 alloy coating, so the H_p(y) fluctuats obviously. When load is the same, gradient value K of magnetic field intensity normal component H_p(y) corresponding to crack decreases in the regular pattern of quadratic polynomial as burial depth increases. When burial depth is the same, gradient value K of magnetic field intensity normal component H_p(y) corresponding to crack increases as load increases. When burial depth is less, the influence of load on gradient value K is more obvious. When burial depth is bigger than 3.0 mm, advance the speed of gradient value K is relatively slow as load increases, and the difference in deformation capacity between laser cladding Fe314 alloy coating and 45 steel is seen as the main reason for above result.

{112}<111>-type twin is a common twinning structure in quenched carbon steel. As carbon content increases, the density of the twin becomes high in the quenched state. Researchers have suggested that understanding such twinning mechanism may help us to understand the martensitic transformation in steel. {112}<111>-type twin is also commonly observed in other body centered cubic (bcc) metals and alloys, especially deformed under the conditions of low temperatures and/or high strain rates. Yet, due to the intrinsic non-close-packed structure and the rapid speed of twinning process, the mechanisms of twinning nucleation, growth and termination have not been clearly understood although phenomenological mechanisms such as the classical shearing mechanism, dislocation mechanism, or shuffling mechanism, etc., were proposed. Recently, after reviewing numerous investigations on {112}<111>-type twinning process both experimentally and theoretically in bcc metals and alloys, it was found that the twinning boundaries are always embedded with ω phase, i.e., the displacement of the first layer of the twin is 1/12 <111> for ω instead of 1/6 <111> for twin, thus, an ω phase-related {112}<111>-type twinning mechanism (so-called ω lattice mechanism) in our previous study is proposed. In order to better understand the ω lattice mechanism, in this work, a detailed description of the whole process of nucleation, growth and termination of the {112}<111>-type twinning was offered by using the atomic lattice model. The model shows that the twin could nucleate during ω→bcc transition process, and then grow up by extending or merging of twin embryos, and finally terminate during encountering the different ω variants. Such two-dimensional atomic model can be extended to three-dimensional one, which can finally explain the formation mechanism of an internal twin in one bcc crystal. Moreover, the model suggests that the diffuse ω lattice (ω_diff) between the ideal ω lattice and bcc lattice (in the twin boundary) plays an important role in promoting the transition of ω↔bcc during twinning nucleation and growth processes. The results suggest that the {112}<111>-type twins are phase transition twin or phase transformation product.