The continuing development of X-ray light sources, optical devices and image analysis technologies enables us to nondestructively analyze internal structures of materials by using 3D imaging with a spatial resolution of micron, even dozens of nanometers. Innovations in materials science and technology will benefit from the new achievements of the X-ray tomography with higher resolution. This article devotes to review the origination and development of the X-ray tomography techniques for 3D imaging and introduce the principles and specifications of three imaging methods, including absorption imaging, phase contrast imaging and holographic imaging. The differences between synchrotron-based and laboratory-based X-ray tomography are also discussed. To explore new opportunities of the high resolution X-ray tomography in the development of material science and technology, particular emphasis is laid on the applications for traditional materials with a 3D view, such as the characterizations of holes, cracks, corrosion, composites, and in situ testing etc..

Stress corrosion cracking growth rates of domestic forged nuclear grade 316L stainless steel (SS) were successfully measured in high temperature and high pressure water at various temperatures and under various loading mode. A direct current potential drop (DCPD) technique was used to monitor the crack growth throughout the test. The crack growth rate decreases with the increasing dissolved hydrogen content and the decreasing dissolved oxygen content. The crack growth rate under trapezoidal loading mode is bigger than that under constant loading. The fracture surface has typical intergranular stress corrosion cracking (IGSCC) characteristics.

A low carbon steel containing Cu addition was treated by Q&P process using a CAS-200 continuous annealing simulator. The microstructure of the steel was characterized by means of SEM, EBSD, XRD and TEM and its mechanical properties were investigated by tensile testing at room temperature. Cu-rich precipitates formed during the Q&P process were observed as spherical particles in martensitic laths and are 9 nm to 20 nm in diameter. According to the Orowan mechanism, those fine particles may have a contribution to the yield strength of the steel about 134 MPa. Also observed are three different morphologies of the retained austenite phase in the test steel, i.e. thin film--like, fine granular and blocky, formed at different locations. The test steel has a good comprehensive mechanical properties, of which the product of tensile strength and elongation, the tensile strength and the total elongation are as high as 21.2 GPa·%, 1326 MPa and 16 %, respectively. The excellent combined properties can be attributed to the effect of transformation induced plasticity (TRIP) caused by the retained austenite.

Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glasses were remelted by pulsed laser at liquid N (Ar) cooling, room temperature and 473 K. The results show that the effect of specimen temperatures on the crystallization of specimens during laser remelting metallic glasses depends on the Ar blowing. If there is no Ar blowing, the crystallization of the specimen during laser remelting under liquid Ar cooling and room temperature is similar while the molten pool is deeper for liquid Ar cooling. And the specimen remelted at 473 K was most seriously crystallized with the deepest molten pool. But comparing the specimen at room temperature, the crystallization of the specimen at liquid N was suppressed during laser remelting with Ar blowing. The effects of specimen temperatures on the crystallization of metallic glasses during laser remelting are: First, the lower specimen temperature can increase the cooling rate of specimen and therefore reduce its crystallization. Secondly, the lower specimentemperature makes  the atmosphere around the specimen cooled down, reduces the shielding effect of plasma and increases the actual laser energy input into the specimen, resulting in more serious crystallization of the specimen. If there is Ar blowing, the atmosphere temperatures are similar to that at the liquid Ar cooling leading to the crystallization of the specimen to reduced. If there is no Ar blowing, the atmosphere temperature is similar to the temperature of specimen itself. In this case, the crystallization of the specimens is jointly determined by the temperature of the specimen itself and the atmosphere temperature around the specimens, which is not monotonous.

Cross rolling and multidirectional forging deformation modes are effective methods to improve the plastic forming ability of magnesium alloy. In these plastic forming process, multi-directional compression deformation is often involved. In this study, the deformation behavior during multi-directional compression of a AZ31 magnesium alloy was investigated using a micro-grid method in combination with electron backscatter diffraction (EBSD) tracking method. The sheets were compressed along transverse direction (TD) followed by compression along normal direction (ND). The experimental results show that {1012}<1011> tensile twinning dominates the deformation process during initial compression along TD, and that for most grains detwinning of these {1012}<1011> tensile twins dominates the deformation process during a following compression along ND. For some grains with a large deviation from the basal texture, a different deformation behavior is seen. No new twin variants are found in grains that underwent detwinning during compression along ND. This indicates that detwinning of {1012}<1011> tensile twins is favored over the activation of a different twin variants during deformation of the AZ31 magnesium alloy. The remaining twin area fraction can reduce to near-zero even when the local strain subjected during ND compression is less than that subjected during the initial TD
compression, demonstrating that a smaller strain is required for detwinning than for the initial twinning.

Fine-grained metals have attracted much interest due to the possibility to obtain both high strength and high ductility. Studies of the deformation mechanisms in fine-grained metals are therefore important, and can also help fill a gap in knowledge between nano-grained metals and conventional coarse-grained metals, which is an area of both scientific and industrial interest. For such an investigation it is very important to use a starting material with a simple microstructure. For this purpose spark plasma sintering (SPS) has been used to prepare samples of fully dense, fine-grained Al with average grain sizes ranging from 0.8 μm to 5.2 μm, in a fully recrystallized condition, with equiaxed grains and a random texture. The microstructures and mechanical properties of these sintered fine-grained Al samples have been studied using EBSD, TEM and compression testing. Based on these studies, relationships between the microstructure and mechanical properties have been established. The results show that the formation of deformation microstructure depends on grain size. During deformation, all grains in samples with an average grain size of 5.2 μm show grain subdivision by dislocation boundary formation, though few grains in a sample with an average grain size of 0.8 μm show dislocation boundary formation. The mechanical properties of fine-grained Al show a transition in behavior with decreasing average grain size. For samples with an average grain size of 1.3 μm, stress-strain curves show yield drop phenomenon, attributed to source-limited hardening. For a sample with an average grain size of 0.8 μm, the stress-strain curveshows only limited work-hardening after yielding, in agreement with the observation of the limited formation of dislocation boundaries inside grains during deformation. For a sample with average grain size of 5.2 μm the average dislocation boundary misorientation angle increases more quickly than in deformed conventional coarse-grained Al deformed to the same strain, due to the effect of increased volume fraction of grain boundaries and oxide particles. The strength of the SPS-prepared Al samples is higher than found for samples with identical grain sizes prepared by thermo-mechanical deformation, due to the presence of oxide particles, and to source-limited hardening for samples with average grain sizes smaller than 1.3 μm.

Aluminum alloys have been widely used in civil fields and war industry as structural materials due to their good corrosion resistance, electrical conductivity, thermal conductivity and high specific strength. But there is a serious problem of welding joints softening in welding process, which limits greatly the application of aluminum alloys. It is found that improving the composition of aluminum alloy welding wire can refine grains and enhance the mechanical properties of welding joints. Of all micro-alloying additions to aluminum, Sc offers the greatest potential for developing new light weight structural materials with excellent mechanical properties, good welding performance, desirable corrosion and creep resistance, due to the formation of extremely fine, coherent Al₃Sc particles, which can effectively refine grains and inhibit recrystallization. This aspect permits to increase the possible use of commercial aluminum alloys. The strengthening mechanism is due to the coherent thermodynamically stable particles with ordered L1₂ structure. Addition of sufficient amount of magnesium can enhance the nucleation rate of Al₃Sc. It was shown that clusters of Mg atoms were found at the center of Al₃Sc particles, suggesting that Mg promotes nucleation of these particles. Moreover, addition of zirconium substitutes for scandium(for up to half of the scandium atoms) to form Al₃(Sc, Zr) precipitates, which are more resistant to coarsening than binary Al₃Sc precipitates. Micro--alloying aluminum alloy welding wires which can be strengthened by heat treatment were designed by adding Mg, Sc and Zr to the traditional 4043 aluminum alloy welding wire, and the heat treatment process was optimized. Welding experiment was carried out using the traditional 4043 aluminum alloy welding wire and  micro-alloying aluminum alloy welding wires. After comparing the microstructures and mechanical properties of welding joints, it was found that the mechanical properties of welding joints could be improved when adding 0.25% Mg to the 4043 aluminum alloy welding wire. The yield strength increased by 12% and the tensile strength increased by 10%, while the elongation remained constant substantially. After 530℃, 2 h+170℃, 6 h heat treatment, the yield strength of the welding joints increased by 105% and the tensile strength increased by 54%. The additions of Sc and Zr to the welding wires resulted in the finer grains significantly, raising the number of strengthening phase and increasing the strength of the welding joints.

The study of liquid metal oxidation is of particular importance since some liquid metals are highly reactive with oxygen even under ultrahigh vacuum. The investigation of the oxidation behavior of liquid aluminum is strongly demanded due to its wide applications but relatively low melting point. However, it is still ambiguous about the products at the initial oxidation stage of liquid aluminum through the researches done in the past several decades. In order to investigate the initial oxidation products together with the oxidation mechanism of liquid aluminum, in-situ oxidation of nano aluminum powder was carried out at 720℃ in the environmental TEM (E-TEM). Prior to the oxidation, the nano aluminum powder was visualized and analyzed by E-TEM and DTA respectively.The results show that there is an amorphous Al₂O₃ film of 5—6 nm thick on the surface of the aluminum powder, which transforms toγ-Al₂O₃ at a temperature range of 350—550℃. By melting the nano powder to liquid followed by cooling under a total pressure of 10^-4 Pa in E-TEM, oxide-free solid aluminum is obtained. Re-melting of the solid aluminum at 720℃ yields oxide-free liquid aluminum. After oxidation of the oxide-free liquid aluminum under an oxygen partial pressure of 10^-2 Pa in E-TEM, α-Al₂O₃ nano wires appear on the liquid aluminum surface, demonstrating that the initialoxidation product of liquid aluminum isα-Al₂O₃. Moreover,the oxidation of liquid aluminum prepared from oxide-free bulk aluminum at 720℃ under an oxygen partial pressure of 10^-2 Pa was also studied. The XRD analysis of the products confirms that the oxide is constituted of singleα-Al₂O₃. Additionally, this particular oxidation result of liquid aluminum is attributed to its liquid structure and vapor oxidation process.

The laser welding test with Sn powder addition was carried out on DC56D+ZF galvanized steel with thickness 1.4 mm and 6016 aluminum alloy with thickness 1.2 mm. By using optical microscopy, scanning electron microscopy, X-ray diffraction and tensile test, the microstructure, fracture morphology, interface element distribution, main phase and mechanical properties of joints were studied. Elastic moduli and thermodynamic properties were calculated by using first-principles method based on density functional theory for FeAl and FeSn compounds. The results indicate that the morphology of welding surface can be improved and grain size is fine when the welding power is 2000 W, welding speed is 45 mm/s, the defocus distance is +2.0 mm, and Ar gas acts as the protection gas with flow rate 15 L/min. The average shear strength of the welding sample with Sn powder reaches 62.17 MPa. Compared to that without Sn powder addition, the average shear strength of weld joint increases by 1.46 times. Because of fine-grain strengthening and the improvement of the fluidity of the molten pool metal with Sn addition, it promotes the combination of the steel and aluminum interface and decreases the thickness of the intermetallic layer. FeSn intermetallic compounds has better ductility and is more stable than that of FeAl at high temperature that retard and reduce the generation of Fe-Al intermetallic compound, which can improve the mechanical properties of weld joint with Sn addition.

As a sort of quasi-brittle materials, fracture toughness of bulk metallic glasses (BMGs) is of paramount importance for their engineering application. Among the BMG families, Cu-based BMGs are of interest due to their low cost, high strength and less brittleness. As indicated in previous work, the Cu₄₉Hf₄₂Al₉ BMG exhibits a good combination of toughness and glass forming ability (GFA). Moreover, toughness of BMG significantly depends on alloy composition. In the Zr-Cu-Al system, it was suggested that increasing the Al content in the alloy does not favor to the plasticity of the glass. Then, it is expected that Al-free Cu-Hf-Ti BMGs may be tougher than the Cu₄₉Hf₄₂Al₉ BMG. In addition, notched cylindrical samples were used for the toughness assessment in previous investigation, which probably introduce an overestimation in toughness in comparison with archival data of engineering materials. To obtain the glassy plate samples for toughness measurements to meet the ASTM E399 requirement, alloys with robust GFA is necessary. In this work, the composition dependence of GFA for ternary Cu-Hf-Ti alloys was revisited. The alloys with the optimal GFA are located around the Cu₅₆Hf₂₇Ti₁₇ and Cu₅₇Hf₂₇Ti₁₆. The critical diameter to form the BMG rods was determined to be 5 mm. Then, the Cu₅₆Hf₂₇Ti₁₇ BMG plates of 2.5 mm in thickness can be fabricated as the specimens for toughness assessments. Using the single-edge notched specimen for three-point bending test, the notch toughness K _Q of Cu₅₆Hf₂₇Ti₁₇ BMG was determined to be(92±10) MPa·m^1/2. It is nearly doubled with respect to the Cu₄₉Hf₄₂Al₉ BMG (K_Q=(56±9) MPa·m^1/2).It means that the Cu₅₆Hf₂₇Ti₁₇ BMG is the toughest among currently-available Cu-based BMGs. Such high toughness of Cu₅₆Hf₂₇Ti₁₇ BMG also correlates with its moderate Poisson's ratio (ν=0.361) and low shear modulus (G=38.6 GPa). The enhanced toughness of Cu₅₆Hf₂₇Ti₁₇ BMG is associated with the extended plastic zone size at the notch tip with the proliferation of shear banding events. The fact that the Cu₅₆Hf₂₇Ti₁₇ superior to Cu₄₉Hf₄₂Al₉ BMG in toughness seemingly supports that Al element has an unfavorable effect on the toughness of Cu-Zr/Hf-based BMGs.

NiCoMnSn shape memory alloy (SMA) is expected to be a promising high temperature SMA. However, the brittleness has become a big obstacle for its practical application. It is known that, grain refining is effective in improving the ductility of a specific metallic alloy. The aim of this work is to investigate the effect of melt--spinning on grain refinement and martensitic transformation and provide a guideline for the development of NiCoMnSn SMA. Ni₄₃Co₇Mn₄₁Sn₉ high temperature SMA ribbon was prepared by a single-roll melt-spinning method. The microstructure and martensitic transformation were investigated by means of OM, SEM, TEM, XRD and DSC, respectively. The experimental results showed that, the ribbon had a chemical composition close to the master alloy and exhibited a thermoelastic martensitic transformation at about 160℃. The grains in the as-spun ribbon, ranging from 2 μm to 18 μm,were remarkably refined compared with the master alloy. In the as--prepared ribbon,most of the columnar grains grew along the direction vertical to the ribbon plane.At room temperature, non-modulated martensite (tetragonal structure) consisting of twin substructure is determined in the ribbon after relieving the internal stress. Transformation temperatures were lowered by 30℃ after heat treatment at 400℃ for 1 h and then kept nearly constant with the increase of heat treatment temperatures.

The kinetic scaling of the grain coarsening in the polycrystalline system containing the dispersive second-phase particles were studied by phase field method. The obtained results showed that the increase in the volume fraction of second--phase particles enhanced the growth resistance of grain, resulting in the remarkable deviation of the relationship between the average grain radius R_a and the time t from the non-linear relationship t=AR_a^m+B. The kinetic exponent m also increased with the increasing volume fraction of second-phase particles. No matter whether the second-phase particles existed or not in the system studied, the scaling rule had been satisfied at the late stage of grain coarsening. The increase in the volume fraction of the second-phase particles would cause the decrease in the peak value of structure factor profile. When the value of the wave vector k increased to a certain value, the structure factor curve of the studied system was essentially coincident. With the increase in the volume fraction of second-phase particles, The peak values of scaling function decreased and the peak width became wider. According to structure factor and scaling function, it was known that with the increase in the volume fraction of second-phase particles, the interaction among grains weakens and the grain size would become more uniform during the grain coarsening.

The research of silicon solar cells mainly focuses on reducing cost and improving conversion efficiency, and one of the effective methods of improving photoelectric conversion efficiency of solar cells is to decrease the reflection of incident sunlight onto the light-receiving surface. The porous Si layer can work for the antireflection of Si surfaces, which can be prepared by noble metal assisted chemical etching. In this work, the effects of HF, H₂O₂ and their volume ratio on morphology and growth of pores on single-crystalline Si surface by using Ag (with a small cluster) metal assisted etching were investigated in order to produce a highly efficient antireflecting structure. The metal particles were deposited onto Si wafer by electroless deposition from a metal salt solution including HF. The surface and cross-sectional morphologies of the porous Si surfaces were observed with field-emission scanning electron microscope. The reflectivity of the etched Si surface was measured with a UV-Vis spectrophotometer equipped with an integrating sphere accessory. The experimental results show that the growth rate and morphology of the pores formed on the Ag metallized Si surfaces are strongly dependent on the volume ratio of HF and H₂O₂. It is not beneficial for the pore growth when mole fraction ρ=[HF]/([HF]+[H₂O₂]) is too low or too high, and the etching goes well only whenρ is between 60%—80%; in this case, the pore growth rate is up to 1050—1260 nm/min, and the pores grow straightly and vertically with a relatively large pore density, pore size and connected pore net. The Si surface exhibits an average reflectivity as little as 5.9% in the wave range of 200—1000 nm, showing that a satisfactory antireflection is obtained. Additionally, the growth rate and morphology of the pores also depend on the size and morphology of catalyst Ag particles.

The poor oxidation resistance of titanium and its alloys limits their use at elevated temperature. To solve this problem, a large amount of surface engineering techniques to produce anti-oxidation coatings on titanium alloys were utilized. In the present research, a solid-state processing method of friction stir lap welding (FSLW) was used to fabricate Al cladding or coating on the surface of TC4 titanium alloy, with lower cost and simpler operation which are still desirable for the coating preparation on titanium alloys. The lap joint structure was smartly transformed into an interface structure of coating. In this work, the Al coating with a thickness of 500μm was fabricated via multi-pass FSLW process using a slight plunge depth of tool-pin. The mechanical milling was used as a post-treatment for a suitable coating thickness. The oxidation testing was conducted at 700℃ under air atmosphere. The microstructure, chemical composition analysis and phase determinations were performed using SEM, EDS and XRD methods. The evolutions of interlayer under the high-temperature oxidation procedure were detailed. It was found that the Ti- rich interlayer, with a thickness of 60μm, had a typical structure of mixed layers. The sufficient Al coating thickness played an important role in preventing the inter diffusion of oxygen, while the oxidation and melting phenomenon of Al coating occurred. The abundant Al content in the Al coating upper the interlayer, with a significant thickness, also benefited to the anti-oxidation performance and forming of the beneath Ti/Al interlayer at a rare oxygen environment due to the obstacle effect of the Al layer to oxygen diffusion, which exerted a main role in oxidation prevention for titanium alloy. As a result, the phases of outer surface were mainly Al₂O₃, Al₂Ti and Al₃Ti. The gradient distribution characteristic of Ti/Al interface structure occurred after the oxidation testing.

The matter of fracture in tension is also the issue of fracture elongation. The ability of superplasticity of materials is mainly characterized by excellent fracture elongations. Since first superplastic phenomenon was recorded, the investigations of superplasticity have not halted. Most of the existing literatures focused on physical or microstructural mechanisms while less attention was paid to mechanical theories on the superplastic deformation. However, superplastic phenomena on large elongation in superplastic tension are closely related to the mechanical stability and are finally dependent on the special fracture mechanism. Correspondingly, in this article, the studies of fracture mechanism of the superplastic deformation are reviewed, which involved nucleation, growth and coalescence of cavities. Then, the literatures related to the mechanical stability in superplastic tension are classified and reviewed, which involved the mechanical analysis and numerical simulation of the fracture elongation or the limit strain induced from neck’s initiation and development. The conclusions indicate that there has yet been no united and confirmed opinion on the superplastic fracture mechanism which has numerous versions from the microstructural or physical view, and the superplastic fracture mechanism would have maken no significant progress unless many long-term investigations will be carried out in the future. In order to interpret the essence of large fracture elongation, the current task should be thoroughly investigate the mechanical stability in superplastic tension based on the advanced technology of numeric analysis. In numeric analysis, the precise and quantitative constitutive equation should be adopted and the deformation conditons involving strain paths should be taken into account.

Ag-In-Cd alloy is widely used as the control rod material in the pressure water reactor (PWR),so it is very important to research the compressive creep behavior for understanding the mechanical property of control rod materials in pile. The compressive creep behavior of as-cast Ag-In-Cd alloy was investigated using a special apparatus at 300—400℃ and under compressive stresses in the range of 12—24 MPa in this work. The stress exponent n and apparent activation energy Q_a of the creep process have been calculated as well as the mechanisms of compressive creep behavior have been discussed. The results show that the compressive creep of the alloy increases with the increase of temperature and compressive stress. The relationship between steady creep rate and stress can be expressed in a power law form. The stress exponent n are 2.90, 4.09 and 5.77 at 300, 350 and 400℃ respectively. The apparent activation energy Q_a of the creep process are 68.1, 103.7 and 131.6 kJ/mol under compressive stresses of 12, 18 and 24 MPa respectively. Stacking fault is the primary rate controlling mechanism for the Ag-In-Cd alloy at 300—400℃ and the compressive stress range of 12—24 MPa, which was deduced from TEM observation.