Ni-based single crystal superalloy has been widely used in turbine blades due to its excellent high temperature mechanical behavior. In order to completely exhibit high temperature mechanical properties, the seed method has been used to produce Ni-based single crystal components for [001] orientation paralleling to main force direction. Stray crystals, which unexpectedly nucleate in the melt-back region, will competitively grow with seed during directional solidification. It is important to profoundly understand the mechanism of competitive growth to find ways of overgrowing stray crystal during producing Ni-based single crystal components. However, within the published research there are conflicting views on the mechanism of competitive growth at converging case. Bi-crystal converging competitive growth was investigated in Ni-based single crystal superalloy with different pulling rates using seed technology. A series of polishing and imaging quenching interface were done for the positional relationship of dendrites near grain boundary in 3D reference. It was found that solidification microstruc tures were different with different crystal orientations. Unfavorable oriented dendrite tilting to heat flux restrained favorable oriented dendrite aligning to heat flux mainly thought inserting into the favorable oriented dendrites channel, and this resulted in unfavorable oriented dendrite overgrowing favorable oriented dendrite at low pulling rate. However, at high pulling rate the unfavorable oriented dendrites mainly blocked by grain boundary favorable oriented dendrite and the grain boundary grew paralleling to favorable oriented dendrite core. Favorable oriented dendrite being depressed and vanished, owning to that unfavorable oriented dendrite inserting into favorable oriented dendrites channel result in adjusting primary dendrite spacing, is the main factor to favorable oriented grain overgrew by unfavorable oriented grain. According to above mechanism, effect of pulling rate on competitive growth at converging case was interpreted. This could broaden our understanding of competitive growth at converging case in 3D reference.

Titanium alloys are widely applied in aerospace, chemical and other related industries. The α+β alloys may obtain various microstructures and mechanical properties simply by varying their thermomechanical processing. Ti-6Al-4V alloy is the most common α+β titanium alloy. Its strength, ductility, fracture toughness and fatigue properties depend strongly on the microstructure especially texture. The understanding of the formation mechanisms of α micro-texture during processing is necessary for the optimization of the mechanical properties. In this work, the nucleation of α precipitates and micro-texture formation process under the influence of dislocations during the β→α transformation in Ti-6Al-4V alloy was simulated by phase field method. The stress field of an infinite straight dislocation was calculated by Willis-Steeds-Lothe method and used as input of the phase field model. It was shown that the normal stress component S₃₃ plays a dominant role in α variants nucleation in the presence of edge dislocation, while the shear stress component S₂₃ is the most important one for screw dislocation. The effect of edge dislocation on α variant selection is generally stronger than that of screw. V1 and V7 are the main variants selected by the edge dislocation while V7, V10 and V12 dominate around the screw dislocation, with V1/V7, V1/V4/V6 being the main variant cluster types around the edge dislocation, and V7/V10/V12 being the primary one for the screw dislocation. In a system with the presence of dislocations in the parent phase, the precipitate microstructure is determined by the combined effect of elastic interactions between the dislocation and different variants of a low symmetry precipitate phase, and elastic interactions among different variants. Variants with interfaces of relatively high energy may appear because of variants selection by dislocations.

TC4 titanium alloy is usually used to manufacture engine blades, blings or blisks and fatigue is the main failure of these components due to its high strength, good corrosion resistance and light weight. In engineering applications, three typical surface modification processes such as shot peening (SP), laser shock peening (LSP) and low plasticity burnishing (LPB) were employed to improve fatigue performance. In this work, SP, LSP and LSB were taken to enhance surface layer of TC4 titanium alloy. The surface integrity of specimens including surface roughness, microhardness, residual stresses and microstructure was investigated to obtain the effects of modification on surface layer by different methods. The rotating-bending fatigue performance was tested at room temperature and fatigue fracture surfaces were analyzed by SEM. Fatigue life was compared at the same stress 760 MPa with the reference machinced specimen. Fatigue strength was determined by stair method for 1×10⁷ cyc. The results show that both the rotating-bending fatigue life and fatigue strength of TC4 titanium alloy are increased by these surface modification processes. The fatigue life prolonging factor (FLPF) for SPed specimens is 20.4, and FLPF for LSPed specimens and LPBed specimens is 89.6 and 99, respectively. Meanwhile, fatigue strength improvement percentage (FSIP) for SPed, LSPed and LPBed specimens is 36.3%, 37.8% and 38.8%, respectively. Moreover, the fatigue cracks initiate beneath surface enhanced layer for surface-modified specimens, while they are located at surfaces for un-surface-enhanced ones. Based on dislocation theory, the subsurface cracks initiation resistance and fatigue strength for surface-enhanced specimens were analysied. Finally, surface modification mechanisms were discussed and some quantitative analysis methods on surface modification effects were proposed. For surface-enhanced smooth specimens, the FSIP limit is 40% based on proposed analysis model and it is verified in this work by different surface layer enhancement processes (36.3% for SPed specimens, 37.8% for LSPed and 38.8% for LPBed specimens are near to 40%). Fatigue total life including initiation and propagation is a complex problem, and therefore it is difficult to give accurate life prediction and analysis, especially for small crack growth, although some invesitigations on total fatigue life can be roughly estimated based on Basquin relation for stress fatigue life or Coffin and Marson eqution for strain fatigue life which have not any physical meaning or any mechanism.

Titanium alloys have been widely used as superior engineering materials because of their high specific strength, high temperature resistance and high corrosion resistance. In their engineering applications such as used in aircraft engines, titanium alloys may experience even 10¹⁰ fatigue cycles. Recently, faceted crack initiation was observed in high-cycle fatigue (HCF) and very-high-cycle fatigue (VHCF) regimes of titanium alloys, which resulted in a sharp decrease in fatigue strength. Therefore, the HCF and VHCF of titanium alloys have both scientific significance and engineering requirement. In this work, the effects of microstructure and stress ratio (R) on HCF and VHCF of a Ti-6Al-4V alloy have been investigated. Fatigue tests were conducted on a rotating-bending fatigue machine and an ultrasonic fatigue machine. All the fatigue fracture surfaces were observed by SEM. The results show that the HCF and VHCF behaviors of the fully-equiaxed and the bimodal Ti-6Al-4V alloy are similar. The observations of fracture surface indicate that two crack initiation mechanisms prevail, i.e. slip mechanism and cleavage mechanism. With the increase of stress ratio, the crack initiation mechanism switches from slip to cleavage. The S-N curves present the single-line type or the bilinear type. For the cases of rotating-bending and ultrasonic axial cycling with R= -1.0, -0.5 and 0.5, the S-N curves are single-line type corresponding to the slip mechanism or cleavage mechanism. For the cases of R= -0.1 and 0.1, the S-N curves are bilinear type corresponding to both slip and cleavage mechanisms. A model based on fatigue life and fatigue limit is proposed to describe the competition between the two mechanisms, which is in agreement with the experimental results.

High undercooling processing has long been studied, since crystal growth mode and microstructural evolution are dependent on the undercooling, ΔT. However, in traditional casting, the container wall acts as heterogeneous nucleation site and specimen undercooling is low, which makes it difficult to experimentally reveal the relationship between the crystal growth behavior and undercooling. In order to achieve different undercooling ranging from low to high, many methods have been proposed, such as drop-tube processing, flux processing and electromagnetic levitation (EML). The container wall effects on the purity of the specimen and on the heterogeneous nucleation of undercooled melt can be removed in these methods. Hence, melts can solid in homogeneous nucleation way and achieves high undercooling. Moreover, EML suspends melt droplet stably, and a freely suspended droplet gives the extra benefit to directly observe the solidification process by combining the levitation technique with proper diagnostic means. In this work, Al-70%Si alloy was undercooled by a laser heating EML. The solidification behavior of Al-70%Si alloy melts at different undercooling conditions was investigated during the solidification process by employing a high-speed camera (HSC). After the melts solidification, morphology on the surface of the samples was examined by SEM. The results show that undercooling has great effects on the growth of Si. The primary Si phases are coarse strip with special edges and faces, have obvious traces of twin, and show facet growth characteristic at low undercooling condition. However, the primary Si phases are dendrites and spherulites with smooth surface, and show non-faceted growth characteristic at high undercooling condition. Besides, the primary Si phases are coarse bulks with special edges and faces and dendrites with regular arrangement at moderate undercooling condition, which is the intermediary growth characteristic. As the undercooling increases, the primary Si is refined remarkably and the growth mode changes from facet growth to intermediary growth, and from intermediary growth to non-faceted growth. The critical undercoolings for the transition are 122 and 230 K, respectively. Furthermore, the critical undercoolings were also theoretically calculated using the physical and chemical parameter of Si, which are 108 and 209 K, respectively.

Mg alloys have been applied widely as structural materials over the past decades, with low density, high specific strength, stiffness, specific elastic modulus, and high recycling rate. However, their features of poor ductility and formability at room temperature have limited their application due to hexagonal close-packed crystal structure with less independent slip systems. Grain refinement and texture randomization are two means to activate other slip systems. In this work, the billets of Mg-9Al-3Zn-1Mn-6Ca-2Nd alloy produced by spraying deposition method (the Osprey process) were studied in order to analyze the effect of rolling deformation at 350 ℃ and pass reduction ε =20%, 25% and 30% on texture and microstructure evolution of an extruded size-asymmetry Mg alloy by SEM, TEM and XRD. The results show that under the condition of reduction of ε =20% at 350 ℃, a long-period stacking ordered phase with 24R structure was formed in (Ca, Nd)Al₂ phase (C15 Laves phase ). All of basal texture (0002), prismatic texture {100}<0001>, and pyramidal texture {102} were activated, with pole density level weakened while pass reduction increased (ε =20%, 25% and 30%), namely, texture randomization achieved in Mg alloy, with main causes of nanometer-sized dispersed C15 phase impeding dislocation movement and sub-cells inducing the process of recrystallization.

Advanced material processing techniques have been successfully used to produce metals or alloys with submicro- or nano-sized grain structures with some possibly required harsh working environment that limits their industrial application. Cryogenic deformation might promote extensively severe deformation or distortion of metals or alloys (such as Al or aluminium alloys, Cu or copper alloys, Ti, Zr, etc.) so as to accumulate higher deformation energy (e.g., higher defect density) for the depression of the (dynamic) recovery, which will contribute to the microstructure refinement. Presently, the macro-/micro-structural evolution, the martensitic transformation as well as its effect on the mechanical property during the cryogenic and room temperature rolling of 304 metastable austenitic stainless steel were studied. It shows that the cryogenic rolling can effectively accelerate the martensitic transformation, e.g., after 20% cryogenic rolling the volume fraction of the transformed martensitic is equal to that after 50% room temperature rolling, and finally the cryogenic rolling can promote the complete martensitic transformation. Also the through-thickness uniformity of the martensitic transformation after cryogenic rolling is significantly better than that of the room temperature rolled one, which can help to improve the through-thickness performance uniformity. It is found that the deformation mechanisms are different for cryogenic and room temperature rolling metastable austenitic stainless steel: the martensitic transformation and its deformation occur in the former while austenitic deformation in the latter. The cryogenic rolling can quickly induce higher hardness than that of the room temperature rolled one, and the hardness tends to be equal finally because of the minimized dislocation density difference between these two rolled steels. TEM results indicate that the orientation relationship between the transformed martensite and the old austenite in the cryogenic and room temperature rolled sheets can still keep the K-S (Kurduumov-Sachs) relationship.

The higher strength requirement of heavy generator retaining rings made of Mn18Cr18N austenitic stainless steel can be obtained by cold deformation strengthening. However, the yield ratio of Mn18Cr18N austenitic stainless steel is close to 1 gradually during the unidirectional tensile deformation, which will limit the unidirectional tensile deformation of cold deformation strengthening. In order to investigate the cold deformation strengthening by complex loading paths of Mn18Cr18N austenitic stainless steel, compression-tensile deformation behavior of Mn18Cr18N austenite stainless steel at room temperature was investigated by compression and tensile consecutive loading deformation experiments with the first compressive reduction range of 0%~40% and the second tensile range to fracture. Microstructure evolution, deformation dislocations, fracture behavior and mechanisms have been analyzed by OM, SEM and TEM. The results indicate that the subsequent tensile yield stress and the maximum tensile stress at the uniform plastic deformation stage, the reduction of cross sectional area and elongation increase at first and then decrease with the increase of compressive deformation. When the compressive deformation increases up to the critical reduction of 25%, the subsequent tensile yield stress and the maximum tensile stress reach up to the maximum values of 1039.97 and 1439.20 MPa respectively, and the reduction of cross sectional area and the elongation also reach up to the maximum values of 68.99% and 73.80% respectively. When the compressive deformation is less than the critical reduction, appearance of fractures shows the cup-cone shaped macroscopic fracture profiles, the dimpled microscopic fracture surfaces and the elongated grains. When the compressive deformation is greater than the critical reduction, fractures morphology is distinguished by the flat macroscopic fracture profiles, the crystalline microscopic fracture surfaces and the equiaxed grains with a lot twin structures. Several dislocation configurations with different density forms by dislocation slip when the compressive reduction is lower. Dislocation pile-up can be observed in the subsequent broken tensile specimen. Cross twins emerge in the specimen compressed up to 35% reduction. Twins with high density dislocation tangles arrange in parallel in the subsequent broken tensile specimen.

Stress corrosion cracking (SCC) in soil environments is one of the major failure and accident causes for oil and gas pipelines, which have induced hundreds of damages all over the world, resulting in serious economic losses and casualties. Previous study showed that acidic soil environments in Southeast of China are highly sensitive to SCC of pipeline steels. However, there is less research on the behavior and mechanism of growth behavior of SCC in this environment up to date. SCC behavior and mechanism of X80 pipeline steel in the simulated solution of Yingtan in China was investigated with electrochemical polarization curves, EIS, slow-rate-loading crack-growth test and SEM. Results showed that the applied polarization potential played an important role in SCC growth behavior and mechanism of X80 pipeline steel in the simulated solution of the acid soil environment. With the decreasing of the applied potential, the crack propagation rate increased constantly. In comparison to the crack propagation at the open circuit potential, the cracks extended faster in the initial stage of crack propagation when the applied potential was -850 mV; nevertheless, in the rapid propagation stage, the rate of the propagation was magnified with the application of -1200 mV potential. In addition, the crack propagation mode varied with applied potentials: it was mixed-controlled by both anodic dissolution (AD) and hydrogen embrittlement (HE) when the applied potential was more positive than -930 mV, and only in control of HE when the potential was less than -930 mV.

Nanotwinned metals have attracted widespread attentions recently, due to their unique overall properties, such as high strength, considerable ductility, enhanced work hardening and high electrical conductivity. The method of synthesized nanotwinned metals is an essential factor for influencing its application. To date, the direct-current electrodeposition technique is successfully used to fabricate bulk nanotwinned Cu samples. However, many parameters, such as the density of current, additive, the concentration of Cu²⁺, pH and temperature, influence the formation of nanoscale twins during electrodeposited process. To understand the effect of electrolyte additive on the formation of twins, in this work, gelatin with different concentrations was added into the electrolyte while other parameters are kept invariant. Bulk Cu with preferentially oriented nanoscale twins was synthesized in CuSO₄ electrolyte with different concentrations of gelatin. The nanotwinned Cu sample is composed of columnar grains with high density nanoscale coherent twin boundaries, most of them are parallel to the growth surface. It is found that the concentration of the electrolyte addition plays an important role in the twin lamellar spacing of the nanotwinned Cu samples, but has little effect on grain size. No twins or twins with micro-sized spacing are detected in electrodeposited Cu without the electrolyte addition. With the concentration of gelatin increasing from 0.5 mg/L to 5 mg/L, the average twin lamellar thickness of the bulk nanotwinned Cu samples decreased from 150 nm to 30 nm. Twin boundaries also grow longer in grains with the increase of gelatin. This is because that with the increase of the concentration of gelatin, the overpotential of cathode increases and nucleation of twins becomes easier, resulting in the reduction of twin spacing.

Nickel-based alloy Inconel 690 (hereinafter called alloy 690) is currently replacing alloy 600 as steam generator tubes in pressurized water nuclear reactors, owing to its excellent resistance to intergranular stress corrosion cracking (IGSCC) and good mechanical properties. The carbide precipitation is a major microstructural characteristic during heat treatment of stainless steels and nickel-based alloys. The carbide precipitation and chromium depletion in the grain boundary of alloy 690 were investigated. The grain size and carbide of alloy 690 with 0.001% and 0.03% (mass fraction) nitrogen contents were observed and analyzed. The extent of chromium depletion in the vicinity of grain boundaries was quantitatively determined as a function of thermal treatment time. The solution treatment of the samples was at 1080 ℃ for 10 min, and then the samples were thermally treated at 715 ℃ for 1~25 h. The results show that the nitrogen addition decreases the intergranular carbide density and the average carbide length but increases its distance. The level of chromium in the depleted regions in alloy 690 with 0.03%N is higher than that with 0.001%N. This is attributed to the beneficial role of nitrogen addition against grain growth and sensitization.

The aging hardening is the main strengthening mechanism of Al-Mg-Si alloy, and the hardening effect is determined by the microstructural features of precipitates including the morpholog, compositio, volume factio, nucleation density as well as the size distribution. In present wor, an integrated mathematical model coupling with the CALPHAD software is developed to simulate the precipitation kinetics and strengthening effects of needle/rod-shaped precipitates in ternary Al-Mg-Si aluminum alloys. This model takes into account the effects of morphology on the nucleatio, growth and coarsening of precipitates and on the strengthening effects. The yield strength model accounts for the whole precipitate size distributio, shape of precipitates and their specific spatial distribution based on the consideration of the competing shearing and bypassing strengthening mechanisms. Appli cation of the model to various aging treatments of Al-Mg-Si alloys is conducted and the predictions both for microstructural features and yield strength are validated with experimental results and the predictions by LSW model. Using this mode, the effects of aspect rati, interfacial energ, alloy composition and Mg/Si atom ratio in precipitates on precipitation kinetics and yield strength are investigated and analyzed. The results reveal that the different interfacial energy and aspect ratio will affect the predicted density and size of precipitat, and further have an influence on the prediction precision of yield strength. An increase of Mg content in the matrix of Al-Mg-Si alloy will accelerate the precipitation and improve the yield strengt, while increasing the Si content in the matrix will produce little influence on the yield strength.

Applications of shape memory alloys require them have the ability to undergo back and forth through the solid-to-solid martensitic phase transformations for many times without degradation of properties (termed “reversibility”). Low hysteresis and small migration of transformation temperature under cycling are the macroscopic manifestation of high reversibility. By the crystallographic theory of martensite, materials with certain crystalline symmetry and geometric compatibility tend to form no-stressed transformation interface and have exce-llent functional stability. In the theory, several conditions that corresponding to extremely low hysteresis are specified. Stronger compatibility conditions which lead to even better reversibility have been theoretically proposed, those conditions are called “cofactor conditions”. Recently, for the first time, experimental results find out the shape memory alloy Au₃₀Cu₂₅Zn₄₅ that closely satisfy the cofactor conditions. Enhanced reversibility with thermal hysteresis of 2.045 ℃, and the unusual riverine microstructure are found in Au₃₀Cu₂₅Zn₄₅. However, their studies are limited to crystallographic analysis, and haven't provided enough details of microstructural evolution in martensitic transformation. Furthermore, it is the evolution of microstructures that leads to an extremely low thermal hysteresis in this alloy. Thus, making clear of evolution of microstructures in martensitic transformation in this alloy is of great importance. So, in the present work, the phase field method was applied, in which the microstructure is described by Landau theory of martensitic transformation, Khachaturyan-Shatalov's phase field microelasticity theory, and thermodynamics gradient to study the microstructural evolution of martensitic transformation in Au₃₀Cu₂₅Zn₄₅, trying to figure out pathway of formation of the unusual microstructure with satisfying cofactor conditions. The simulation results show that during the martensitic transformation, quad-junctions composed of four different variants are formed. These junctions grow layer by layer, and the previously formed layer has larger size, thus leading to the formation of the experimentally reported “riverine” microstructure of martensite in Au₃₀Cu₂₅Zn₄₅. Further analysis based on the crystallographic theory of martensitic transformation shows that in Au₃₀Cu₂₅Zn₄₅ 6 groups of variants can form such kind of quad-junction, and each group of variants can form 4 kinds of type 1/type 2 twin pairs and two kinds of compound twin pairs. All of the quad-junctions in this transformation are composed of four of those 6 twin pairs in each variant group, and the twin walls of these four twin pairs are perpendicular to each other.

Owing to excellent mechanical properties, Al alloys are widely used in aerospace, automotive and civil industry. In order to optimize the properties and performance of the currently used Al alloys and/or even design novel Al alloys, the quantitative description of the microstructure during alloys preparation is the key. In recent years, the phase-field simulation coupling with the CALPHAD thermodynamic and atomic mobility databases has become an effective way to quantitatively simulate the microstructure evolution. So far, the accurate thermodynamic database for Al alloys has been established. However, it is not the case for atomic mobility database for Al alloys. The major obstacle lies in the lack of reliable diffusion coefficients in ternary and higher-order Al alloys, and thus there is an urgent need to remedy this situation. In this work, several semi-infinite and finite (thin film) single-phase solid-state diffusion couples in bcc Al-Fe-Mn and fcc Al-Cu-Ni alloys were first prepared. The concentration profiles for all the diffusion couples were then measured by means of EPMA. After that, the pragmatic numerical inverse method, which has been recently developed for high-throughput determination of the interdiffusion coeffi cients in ternary system and validated in several systems, was employed to compute the composition-dependent interdiffusivities in the corresponding systems at 1273 K. In order to eliminate the possibility that different interdiffusivities at the same composition would be obtained from different sets of diffusion couples, only one set of adjustable parameters was used for one system. All the obtained interdiffusivities satisfy the thermodynamic constrains. On the basis of the determined interdiffusivities as well as Fick's second law, all the experimental concentration profiles were reproduced nicely via numerical simulation, which verifies the reliability of the determined interdiffusivities. The further analysis indicates that the pragmatic numerical inverse method can not only realize the determination of reliable composition-dependent interdiffusion coefficients in ternary diffusion couples, but also cover the cases which cannot be dealt with by the traditional Matano-Kirkaldy method, such as the diffusion couples without intersection along their diffusion paths, and the finite (thin film) diffusion couples. In addition, the comparison between the interdiffusivities from semi-infinite diffusion couples and those from finite (thin film) diffusion couples was made, and the probable reason for their difference was also pointed out. All the presently obtained interdiffusivities in bcc Al-Fe-Mn and fcc Al-Cu-Ni alloys will be utilized to develop the accurate atomic mobility databases in ternary Al-Fe-Mn and Al-Cu-Ni systems in the next step.

Titanium/steel clad material with excellent mechanical properties and corrosion resistance has important application in storage and transportation equipment of oil and gas. Due to the metallurgical incompatibility of titanium and steel, the mechanical properties of weld joint would completely lose when the brittle intermetallic phase Ti_xFe_y and TiC appeared in the fusion welding process. Therefore, the gas tungsten arced welding (TIG), metal inert-gas welding (MIG) and metal active-gas welding (MAG) with V/Cu composite filler metals for butt joint in this study was carried out on TA1/X65 pipeline steel clad plates with thickness 16 mm ( titanium cladding with thickness 2 mm, X65 pipeline steel with thickness 14 mm). The microstructure, interface element distribution, main phase, microhardness distribution on cross section and mechanical properties of butt welds were investigated by using OM, XRD, EDS element mapping, microhardness and tensile test. The results indicate that the design of “U-type” circular groove advantageous to the MIG of Cu transition-metals, because of the “U-type” circular groove does not cause stress concentration and crack initiation. The deposited metal of Ti, V, Cu and Fe have obvious zoning, interdiffusion melting phenomenon is not severe, and by using solid solution phases to transit zonings of deposited metal. The microstructure of Ti and V transition interface was composed of Ti-based solid solution, the microstructure of V and Cu transition interface was composed of V-based solid solution, and the microstructure of Cu and Fe transition interface was composed of Cu-based solid solution. The high hardness region of butt weld cross section appeared in the Ti/V transition-interface and V/Cu transition-interface, the hardness value was respectively 326 HV₁₀ and 336 HV₁₀, and weakened the ductility of transition interfacial layer. A joint with a tensile strength of 546 MPa, mainly of that of the carbon steel was obtained.