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In order to investigate the diffusion bonding between Ni-based GH4099 alloy and Mo-based MHC alloy, the wettability of AgCuTi, TiZrCuTi and NiCrSiB filler metals on the surface of GH4099 alloy and MHC alloy was firstly performed. The results show that three kinds of filler metals have excellent wettability on the surface of two kinds of substrates, and are the suitable interlayers for joining GH4099 and MHC alloys. Secondly, diffusion bonding tests of GH4099 and MHC alloys were carried out by Gleeble thermally simulated experiments. The microstructures of three kinds of welding joints with different interlayers were compared and analyzed. It was found that Ti element had significant effects on improving the bonding strength of MHC side. A large number of Ni-Mo intermetallic compounds can easily be formed, which led to high brittleness of the joints. The experimental results showed that NiCrSiB is the best interlayer material for joining GH4099 and MHC alloys, and the shear strength of the joints can reach 116 MPa.

Oxide dispersion strengthened ferritic steel is considered an important structural material in fusion reactors due to its excellent resistance to radiation and oxidation. Fine and dispersed oxides can be introduced into the matrix via the powder metallurgy process. In the present study, large grain sizes and prior particle boundaries (PPBs) formed in the FeCrAlY alloy prepared via powder metallurgy. Thermo-mechanical treatment was conducted on the FeCrAlY alloy. Results showed that microstructure was optimized: the average grain diameter decreased, the PPBs disappeared, and the distribution of oxides dispersed. Both ultimate tensile strength and elongation improved, especially the average elongation increased from 0.5% to 23%.

In this work, laminated Al/Ti composites were fabricated by hot rolling at three different temperatures (400 °C, 450 °C and 500 °C). The deformation behavior and fracture mechanism of composites were studied by the in-situ tensile test. The results indicated that the ductile shear fracture (for 400 °C and 450 °C rolled specimens) and ductile delamination fracture (for 500 °C rolled specimen) were the main fracture failure modes of Al/Ti multilayers. Results shown that the tensile load and displacement of the 500 °C prepared Al/Ti were better than other samples due to the higher rolling temperature. The changes of slip lines and plastic bands were the main plastic deformation characteristics for the Al layer and Ti layer metals, respectively. The investigation found that the major crack nucleation type in Ti layer was fold belt cracking. The slip bands cracking, cross-slip cracking, particle interface cracking and grain boundary slide cracking were the primary crack nucleation patterns for the Al layer.

The 2Cr13/316L multilayered steel (MLS) composites were prepared by accumulative hot roll bonding and heat treatment. The element diffusion, interface bonding mechanism and interface bondin...

The 2Cr13/316L multilayered composite plates were fabricated by hot rolling with recycle heating step. The effect of rolling reductions on microstructure and properties was investigated. The 2Cr13 layer consists of martensite and lath ferrite, but the middle layer has less ferrite than both sides. The content and grains of ferrite increase with the increase of the reduction and number of reheating, which leads to a decrease in the hardness of the 2Cr13 layer. The hardness of the 2Cr13 layer is determined by the volume ratio of martensite and ferrite. Tensile strength of the specimens with the rolling reduction of 72% and 82% reached 815.8 MPa and 763.4 MPa, while elongations were 20% and 20.8%, respectively. With the increase of the rolling reduction, the fracture mode also changed from cleavage fracture to dimple fracture. There were no cracks and delamination when the 2Cr13/316L composite plate bent to 130° and 180°, which indicated better interfacial bonding.

The high tensile strength and irradiation resistance of oxide dispersion strengthened (ODS) ferritic steels is attributed to the ultrafine and dispersed oxides within the matrix. The high content of oxygen and yttrium is critical for the formation of dense Y-rich oxides. However, only few studies have reported the effect of oxygen content on the microstructure and mechanical properties of ODS steels. Herein, we employed gas atomization reactive synthesis to prepare pre-alloy powders and then hot isostatic pressing (HIP) to consolidate two 22Cr-5Al ODS steels with different oxygen content. Our results showed Y-rich precipitates at and near grain boundaries of the as-HIPed alloys. Moreover, with the oxygen content increasing from 0.04 to 0.16 wt%, more precipitates precipitated in the as-HIPed alloy, and the ultimate tensile strength of the alloy was improved. However, increasing the oxygen content to 0.16 wt% led to formation of stripe and chain precipitates at and near grain boundaries, which caused a partial intergranular fracture of the as-HIPed alloy.

Ceramic-based composites have been applied to turbine components of new-generation aerospace engines due to their excellent properties. Despite the superior high-temperature resistance of SiC/SiC composites, it is necessary to combine with film cooling technology to guarantee normal function of turbine components. However, SiC/SiC composites are typical difficult-to-process materials，and the depth of film cooling holes have increasingly high requirements, which have become one of the major problems in the manufacturing of aerospace engine turbine parts. In this paper, the spatial energy density distribution based on filament effect of femtosecond laser was theoretically studied, it is revealed that the filament effect of femtosecond laser with high single pulse energy will lead to the peak of energy density shift towards the negative defocus direction. Based on the theoretical research, the experimental study on the processing of deep micropore in SiC/SiC composites by femtosecond laser was carried out, main results show that in equal conditions, the depth of micropore in feed trepanning is deeper than that in trepanning, and that in trepanning is deeper than that in direct punching. This research provides both theoretical and technical support for processing of micropore with high aspect ratio by ultrafast laser on the difficult-to-process materials, which have high practical value in the aerospace field.

Nano B2 FeCu ordered clusters and multiphase microstructure consisting of intercritical ferrite, tempered martensite and nano-γ phase (reverted austenite) were obtained by two-step heat treatment involving intercritical annealing and intercritical tempering. The experimental steel with nano Cu precipitates and nano-γ phase exhibited high strength and high ductility combination. The yield strength and total elongation of the experimental steel increased from 758 MPa and 16.8% to 984 MPa and 29.5% after the second step intercritical tempering for 5 min. High resolution transmission electron microscopy (HRTEM) and three-dimensional atom probe (3DAP) studies provided evidence to support that high density of nano B2 FeCu ordered clusters contributed to high strength. First principle calculations suggested that the feasibility of B2 FeCu nano-ordered clusters, and the stability of B2 structure is related to the coherent stress field at the interface between clusters and the BCC-Fe matrix. The average size of B2 FeCu nano-ordered clusters was 4 nm with a lattice constant of 0.2893 nm and an orientation relationship of (1 1 0)B2//(1 1 0)α and [0 0 1]B2//[0 0 1]α. 9R Cu without twinned structure was discovered at different tempering times. The proportion of Cu in Cu precipitates varied from 24.4 at% to 61.2 at% with change in crystal structure and increase in the size of precipitates. The significant increase in yield strength is discussed in terms of ordered domain strengthening, modulus strengthening and lattice misfit strengthening mechanisms. Bright field TEM image and selected area electron diffraction (SAED) pattern discovered and proved that spindle nano-γ phase was present at phase boundary between tempered martensite and intercritical ferrite. The size of them were among 242–375 nm in length and 52–80 nm in width. The two needle tip ends of nano-γ phase were just at phase boundaries, indicating the preferred growth direction. (1 −1 1)γ plane of nano-γ phase was parallel to (1 −1 0)α plane of the adjacent tempered martensite. The effect of nano-γ phase on enhancing the plasticity was revealed by instantaneous work hardening index curve.

The microstructure and mechanical properties of 2Cr13/316L composite plates by hot-rolling at different rolling temperatures of 1130 °C, 1180 °C, 1250 °C, 1300 °C are investigated in details. With the increase of the rolling temperature, the tensile strength and bending strength of 2Cr13/316Lwere reduced first and then increased slightly due to the coarsening of grains and the increasing of ferrite content in 2Cr13 layers. The highest tensile elongation is obtained at the composite plate with rolling temperature of 1180 °C. 2Cr13/316L composite plates obtained by hot rolling processes were able to withstand larger bending deformation, and no cracks or interfacial delamination can be found in the bending process because of a strong combined interface.

This paper analyzes the micro-mechanism of decrease of impact toughness with increasing Silicon and increasing Copper in weld metal of a low-alloy high-strength X120 steel. By comparing the microstructures, measuring the extending length of the fibrous crack, and calculating the local cleavage fracture stress σf, and then using the basic principles of the micro-mechanism of cleavage fracture, the essential causes of change of toughness in the weld metal of high-strength steel welded joints are revealed in this work.

The behaviors and effects of Zn-coating on the welding-brazing processes of steel with aluminum and magnesium were investigated by Cold Metal Transfer (CMT) arc. The behaviors of Zn-coating at the very beginning of burning and during the spreading of the liquid aluminum and liquid magnesium droplet over the steel plate were observed respectively. The effects of Zn-coating on the wetting process of liquid droplets and on the fracture of welding-brazing joints were analyzed. The effects of Zn-coating on joining mechanisms and tensile load of Al-steel joints and Mg-steel joint were compared and analyzed. For Al-steel, Mg-steel, some common behaviors can be found, the main effect of Zn is its evaporation which decreases the temperature at the interface to prevent steel from melting and makes the steel to expose the fresh metallic surface which is much easier to be spread by a liquid droplet than an oxidized surface. Zn-coating improves the wettability of liquid Al or liquid Mg over the steel surface. However, some effects of zinc coating on wetting behavior, joining mechanisms and tensile load of Al-steel joints and Mg-steel joint are still different. For Al-steel arc, Zn-rich triangle zone is not formed by driving the liquid Zn from inside and accumulated at the toe, rather it is formed by the diffusion of the liquid Zn from outside Zn-enriched periphery into the droplet. In the case of Mg-steel arc, the Zn-rich zone forms partly due to the accumulation of Zn driven from the center during the Zn evaporation. For Al-steel joint, Zinc coating has not reacted to form the interface layer between Al weld metal and steel, however, zinc coating reacted to form the interface layer between Mg weld metal and steel. Zinc coating improved the strength of Al-steel joints, but, it is not beneficial to the strength of Mg-steel joint.

For the heat resistance steel weld metal, special post-weld heat treatments need to be performed to maintain the creep resistance, improve the impact toughness, and relieve the welding residual stress. In this paper, firstly, the three factors affecting variability and lower values of toughness were revealed. Secondly, two different post-weld heat treatment (PWHT) methods have been used to improve the impact toughness of a heat resistance steel weld metal. Two PWHT methods include post-weld direct tempering (PWDT) process at 690 °C/26 h/air cooling and post-weld normalized tempering (PWNT) process with re-austenitization at 960 °C/1.5 h/air cooling + tempering at 690 °C/26 h/air cooling. For three type specimens with as-welded (AW), PWDT, PWNT processes, the impact toughness and the hardness were measured, and the fracture surface, microstructures were observed and analyzed by scanning electron microscope (SEM). Results indicated that PWDT heat treatment can improve the impact toughness of the weld metals from the original 79 J - 169 J to 82 J–290 J because the microstructure is changed from granular bainite to block ferrite and granular bainite. PWNT heat treatment can remarkably improve the impact toughness of the weld metals from the original 79 J- 169 J to 148 J–325 J. It is attributed to that PWNT heat treatment process can remove the boundaries of the columnar grain zone and reheated zone, make the difference of the microstructure between the columnar grain zone and reheated zone decrease, and improve the homogeneity of the microstructure.

Oxidation and cracking behavior during thermal-shock experiments of a new Co-based alloy were analyzed in the present work. Comparing to the static conditions, the oxidation behavior during thermal shocks is stress-assisted and crack-accelerated. Cracks during the thermal shocks were initiated from the sample surfaces and their growth are driven by various stresses, including those induced by volume change, mismatch of coefficients of thermal expansion (CTEs), and temperature gradient between the matrix and the internal oxides. It is found that the crack propagation correlates to these stresses in the classic Paris’ law.

The demand for heat-resistant steel has increased owing to its utility in numerous devices that must withstand high steam pressures and high temperatures, such as turbine rotors and blades in ultra-supercritical power plants. It is inevitable to join heat-resistance steel part by welding method, so it is important to maintain the toughness of the weld metals. In this study, the microstructure, low-temperature impact toughness, and fracture surface of as-welded and post-weld heat treatment (PWHT) of 2.25Cr-1Mo-0.25V weld metal were investigated. The microstructures of the as-welded and PWHT specimens are granular bainite and ferrite, respectively. This work revealed the relationship between effective microstructure nearby crack initiation origin and low temperature impact toughness for both the as-welded and PWHT specimens. The evolution of the microstructure and prior austenite was then investigated using confocal laser scanning microscopy (CLSM) to observe the formation of coarse ferrite grain structures. A suggestion for enhancing the low-temperature toughness was provided based on the effect of adjusting Mn content and forming acicular ferrite.

The martensite/austenite MLS composite plates were fabricated by hot roll bonding of the alternately stacked 2Cr13 and 316L stainless steel plates. The effects of heat treatment on the microstructure transformation, interface behavior, tensile properties, fracture modes and failure mechanisms have been studied in depth. The microstructure of the composite plates is composed of austenite in 316L layers, martensite and a small amount of ferrite in 2Cr13 layers. After heat treatment, the microstructure of the composite plates changes from rolling texture to equiaxed grains. Most importantly, the recrystallization and thermal diffusion formed after heat treatment can effectively increase the overall plasticity of composite plates at the expense of small strength and hardness, improve the interface deformation compatibility and interfacial bonding strength, change the fracture mode from the quasi-cleavage fracture into the full dimple fracture, and fracture mechanism from the interface failure into the intralayer failure.

Precipitation strengthened austenitic stainless steels are widely used in hydrogen related applications. However, their applications may face hydrogen damage resulting in hydrogen-induced delayed failure. Results show that the weld is more sensitive to fracture and hydrogen-induced failure than the matrix. High density curved dislocations, abundant of large size precipitates and considerable γ′ precipitates coarsening are found in the weld. Large size precipitates are found to be major hydrogen traps and preferential microcrack nucleation sites. The γ′ precipitates coarsening make the weld more ductile than the matrix. With the decrease of the applied stress, hydrogen-induced cracking mechanism in the weld changes from brittle transgranular fracture to brittle intergranular fracture.

The deformation behaviour of a newly developed alumina-forming austenitic (AFA) stainless steel at elevated temperatures was investigated. It was found that the steady-state data could be well described by a stress power law that considered the threshold stress and temperature dependence of the shear modulus and self-diffusion coefficient, suggesting that the deformation of the austenite matrix was controlled by the lattice self-diffusion. The threshold stress was temperature-dependent and appeared to be caused by Orowan bowing stress. At temperatures below 1023 K, the secondary NbC phase was the major hardening precipitate, but at temperatures above 1023 K, the Laves Fe2Nb phase became dominant. The apparent interaction energy required for mobile dislocations to overcome the particle obstacles decreased from 163 to 34 kJ/mol with increasing temperature, which was most likely associated with the formation of the two different precipitates.

The fatigue properties of sintered Ti–6Al–4V were investigated as a function of sintered density and pore morphology. A quantitative analysis of microstructure was conducted to correlate with tensile and fatigue properties to understand the influence of sintered density and pore morphology. Low cycle fatigue (LCF) and high cycle fatigue (HCF) tests were conducted using two alloys with different sintered densities (95% and 99%). It was found that there was no significant difference in tensile strength but the fatigue endurance limit was increased by 20% when the sintered density was increased from 95% to 99%. Fracture surface analysis indicated that the fatigue cracks initiated from clusters of pores either at the surface or at the near sub-surface. The effect of pore morphology and porosity on mechanical behavior and fatigue fracture of P/M Ti–6Al–4V was quantified using the pore shape factor. It was found that specimens with a higher pore shape factor had a higher fatigue endurance limit.

A block-on-slip ring-type wear tester was used to investigate the tribological behavior of copper-impregnated metallized carbon against a Cu–Cr–Zr alloy under 2 to 6 N applied load and 0 to 20 A electrical current. The sliding speed was maintained at 25 km/h. The wear loss of copper-impregnated metallized carbon increased with greater electrical current. Under a certain applied load, the wear loss with electrical current was minimized. The tribo-layer had an apparent effect on the friction coefficient. The wear mechanisms were complex, consisting of adhesive wear, abrasive wear and arc erosion.

The fracture behaviour of 8%Ni 980 MPa grade high strength steel is investigated by combining experimental results of crack opening displacement (COD) tests at various temperatures with detailed microscopic observations of fracture surfaces and crack configurations in unloaded specimens. The results reveal that this high strength steel possesses high toughness with a transition temperature around −150°C. Even though at a very low temperature (−196°C), cleavage cracking dominates the fracture process and the crack does not propagate immediately through the entire ligament: a ‘pop-in’ extension is observed in macroscopic tests, and the microscopic fracture mode is quasi-cleavage. It is found that resistance to crack propagation is provided by three barriers: original austenite grain boundaries, bainite colony boundaries and interlayers between bainite laths. These barriers manifest themselves by tear ridges with dimples on the fracture surfaces. At higher temperatures, the fracture mechanism is domi...

Based on the results of four-point notched bend tests together with detailed microscopic observations of fracture surfaces and crack configurations below the unbroken notch roots of double notch specimens, the fracture mechanisms in notched specimens of 8%Ni high strength (980 MPa) steel have been observed to be as follows. A fibrous crack initiates in the bainitic matrix at the notch root and then develops into a cleavage crack at a critical length. The cleavage crack propagates in an unstable manner and causes the final fracture of specimen. The critical event controlling the cleavage fracture is the propagation of the bainitic packet-sized crack, and the local fracture stress is measured as around 1845–2200 MPa.

Fracture behavior of a 9% nickel 1000 MPa grade high-strength steel was investigated with tensile tests at various temperatures. Four critical stresses were found, which determined the fracture behaviors at various temperatures. Various fracture behaviors could be classified into three categories: (1) at −196 °C, a longitudinal crack initiated from the center of the necking region and propagated along the tensile direction to the regions close to both ends of the necking, it then changed the orientation and developed into two transverse cracks which propagated into opposite directions on two separated cross-sections. (2) In the range of −30 °C to 20 °C, the fracture surfaces were composed of typical center-fibrous-initiation region, middle shear-radical region and outer shear-lip region. (3) In the range of −150 °C to −60 °C, the middle shear-radiation region showed a very rough pattern with several convex ridges. Fracture mechanisms were analyzed by combining various fracture morphologies with FEM-calculated results of stress and strain.

Ag/TiO 2 nanocomposites were prepared through combination of a sol–gel process of a titanium precursor in inverse miniemulsions and in situ reduction of silver ions in the “nanoreactors”. The morphological investigation shows that Ag nanoparticles are mainly located on the surface of TiO 2 nano-supports because of the fast reduction rate of Ag ions by hydrazine. Ag/TiO 2 nanocomposites with amorphous or anatase TiO 2 phase displayed high visible-light catalytic activity for degradation of Rhodamine B. The photoactivity of Ag/anatase TiO 2 nanocomposites could be influenced by the Ag content that could be conveniently tuned by the loading of silver salts. The influence of the loading of silver salts on the particle properties of the Ag/TiO 2 nanocomposites was investigated systematically.

Nanogel (hydrophilic polymer nanoparticles) core-silica shell nanoparticles were successfully fabricated via hydrolysis and condensation reaction of tetraethoxysilane (TEOS). Transition-metal tetrafluoroborate-containing nanogels were used as templates for fabrication in inverse systems (cyclohexane as continuous phase). Magnetic, hollow silica particles were subsequently formed by removing the polymer core and converting iron salts to iron oxides via heat treatment. We propose that the formation of the core-shell morphology is induced by the promoted precipitation of silica species at the surface of nanogels due to the interaction between silica species and transition-metal tetrafluoroborate. The influence of the synthesis parameters (type and amount of salts, pH of the nanogels, and amount of TEOS) on the particle morphology was systematically investigated. The pore properties and specific surface area of the hollow silica particles could be modified by the varying the amount of salt.