Al2O3–Cr2O3/NiCoCrAlYTa coatings were prepared via atmosphere plasma spraying (APS). The microstructure and phase composition of the coatings were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), laser confocal scanning microscopy (LSCM), and transmission electron microscopy (TEM). The dry frictional wear behavior of the coatings at 500°C in static air was investigated and compared with that of 0Cr25Ni20 steel. The results show that the coatings comprise the slatted layers of oxide phases, unmelted particles, and pores. The hot abrasive resistance of the coatings is enhanced compared to that of 0Cr25Ni20, and their mass loss is approximately one-fifteenth that of 0Cr25Ni20 steel. The main wear failure mechanisms of the coatings are abrasive wear, fatigue wear, and adhesive wear.

The fatigue crack propagation behavior of a Ni base superalloy at room temperature with various volume fractions of δ phase was investigated. The result shows that fatigue crack growth rate of the alloy increases as δ phase volume fraction increases, while both the fatigue crack propagation resistance and the strength of the alloy decrease. The characteristic of δ phase on crack propagation behavior is depended on the dislocations slipping blocked by granular δ phase with low length-diameter radio. On the other hand, dislocations easily piled up around long needle-like δ phase which leads to stress concentration. This stress concentration causes δ phase debonding from the matrix and helps the crack easy propagates along long needle-like δ phase surrounded by precipitates free zone (PFZ).

The tensile tests were conducted to investigate the plastic deformation behavior of GH4169 alloy with in-situ electric-pulse current (EPC) at 800 °C, and mechanisms of EPC on plastic deformation during different stage were discussed. The results show that in-situ EPC induces the decreasing of flow stress and the improving of plasticity compared with that without EPC. The flow stress instantaneously increases/decreases without any delay with removing/recovering EPC during deformation. It is found that the thermal vibration of atoms enhances leading to decreasing of Peierls stress with in-situ EPC, which is the essential factor on flow stress decrease. Although, with in-situ EPC, the γ′′ phases are induced to precipitate and then grow up, resulting in changing dislocation motion mode with γ′′ phase.

SA508−III steel was charged with different hydrogen (H) contents using a high-pressure thermal charging method to study the effects of H content on the tensile properties and evaluate the H embrittlement behavior of the steel. The results indicate that the ultimate tensile strength remains nearly unchanged with the addition of H. In contrast, the yielding strength slightly increases, and the elongation significantly decreases with increasing H content, especially at concentrations exceeding 5.6 × 10−6. On the basis of fractographic analysis, it is clear that the addition of H changes the fracture mode from microvoid coalescence to a mixture of river patterns and dimples. Carbides are strong traps for H; thus, the H atoms easily migrate in the form of Cottrell atmosphere toward the carbides following moving dislocations during tensile deformation. In addition, stress-induced H atoms accumulate at the interface between carbides and the matrix after necking under three-dimensional stress, which weakens the interfacial bonding force. Consequently, when the local H concentration reaches a critical value, microcracks occur at the interface, resulting in fracture.

An energy based regression method to estimate critical crack-tip-opening-angle (\(\hbox {CTOA}_\mathrm{C}\)) of high strength and toughness pipeline steels has been established derived from the Martinelli-Venzi ductile fracture model. Key curve method was applied onto the load-displacement curves of standard pressed-notch drop-weight-tear-test specimens to evaluate the dynamic crack extension, providing a way to verify the correlation between the load and remaining ligament width. In the meanwhile, the material based parameter (\(\hbox {A}^{*} \upsigma _\mathrm{f}\)), usually required by other \(\hbox {CTOA}_\mathrm{C}\) estimation algorithm, could also be determined experimentally. As a result, \(\hbox {CTOA}_\mathrm{C}\) of a typical high grade pipeline steel plate was acquired as a constant over steady-state stage during crack propagation, independent of specimen geometry.

The fracture toughness of SA508-III steel was studied in the temperature range from room temperature to 320°C using the J-integral method. The fracture behavior of the steel was also investigated. It was found that the conditional fracture toughness (JQ) of the steel first decreased and then increased with increasing test temperature. The maximum and minimum values of JQ were 517.4 kJ/m2 at 25°C and 304.5 kJ/m2 at 180°C, respectively. Dynamic strain aging (DSA) was also observed to occur when the temperature exceeded 260°C with a certain strain rate. Both the dislocation density and the number of small dislocation cells effectively increased because of the occurrence of DSA; as a consequence, crack propagation was more strongly inhibited in the steel. Simultaneously, an increasing number of fine carbides precipitated under high stress at temperatures greater than 260°C. Thus, the deformation resistance of the steel was improved and the JQ was enhanced.

The hot deformation behavior of IN706 has been investigated by means of hot compression tests in the temperature range of 900–1150 °C and strain rate range of 0.001–1 s−1. The constitutive equation was developed on the basis of experimental data. Power dissipation efficiency (η) and instability parameter (ξ) maps were evaluated using the principles of the dynamic material model. Furthermore, the EBSD microstructure analysis was performed for validation, revealing that η was closely associated with the mechanism of dynamic recrystallization (DRX). Microstructure transition map was composed of contour plots of η, ξ, and DRX. The DRX domain zones and instable zones were identified in the processing map and were classified based on η. In a view of microstructure refinement and workability improvement, the optimum processing should be selected in the temperature range of 970–1025 °C and the strain rate range of 0.08–0.01 s−1.

The hot deformation behaviors of GH4706 alloy were investigated using compression tests in a deformation temperature range from 900°C to 1200°C with a strain rate range of 0.001–1 s−1. Hot processing maps were developed on the basis of the dynamic material model and compression data. A three-dimensional distribution of power dissipation parameter (η) with strain rate and temperature reveals that η decreases in sensitivity with an increase in strain rate and a decrease in temperature. Microstructure studies show that the grain size of GH4706 alloy increases when η is larger than 0.32, and the microstructure exhibits local deformation when η is smaller than 0.23. The hot processing map at the strain of 0.7 exposes a domain peak at η=0.32 for the temperature between 940°C and 970°C with the strain rate from 0.015 s−1 to 0.003 s−1, and these are the optimum parameters for hot working.

The coarsening behaviors of γ′ and γ″ phases in GH4169 alloy aged at 1023 and 1073 K with electric field treatment (EFT) were investigated by transmission electron microscopy (TEM) and positron annihilation lifetime spectroscopy (PALS). It is demonstrated that precipitation coarsening occurs, and the growth activation energies of γ′ and γ″ phases can be decreased to 115.6 and 198.1 kJ·mol−1, respectively, by applying the electric field. The formation of a large number of vacancies in the matrix is induced by EFT. Due to the occurrence of vacancy migration, the diffusion coefficients of Al and Nb atoms are increased to be 1.6–5.0 times larger than those without EFT at 1023 or 1073 K. Furthermore, the formation of vacancy clusters is promoted by EFT, and the increase in strain energy for the coarsening of γ′ and γ″ phases can be counterbalanced by the formation of vacancy clusters.

Electric field treatment (EFT) was applied on GH4169 alloy during aging at 500–800°C to investigate the microstructure and property variation of the alloy under the action of EFT. The results demonstrate that the short-distance diffusion of Al, Ti, and Nb atoms can be accelerated by EFT, which results in the coarsening of γ′ and γ″ phases. Meanwhile, lattice distortion can be caused by the segregation of Fe and Cr atoms, owing to the vacancy flows migrating toward the charged surfaces of the alloy. Therefore, the alloy is hardened by the application of EFT, even if the strength of the alloy is partly reduced, which is caused by precipitation coarsening.

Martensitic stabilization caused by deformation in a TiNi shape memory alloy was studied. Special attention was paid to the deformed microstructures to identify the cause of martensitic stabilization. Martensitic stabilization was demonstrated by differential scanning calorimetry for the tensioned TiNi shape memory alloy. Transmission electron microscopy revealed that antiphase boundaries were formed because of the fourfold dissociation of [110]B19’ super lattice dislocations and were preserved after reverse transformation due to the lattice correspondence. Martensitic stabilization was attributed to dislocations induced by deformation, which reduced the ordering degree of the microstructure, spoiled the reverse path from martensite to parent phase compared with thermoelastic transformation, and imposed resistance on phase transformation through the stress field.

AbstractThe oxidation behaviors of the Ni-22Cr-14W-2Mo superalloys with various La contents were investigated at 1 000 °C and 1 100 °C up to 500 h. The experimental results show that the oxidation resistance is improved by La addition. The mechanism of the reactive-element effect (REE) exhibits some difference at various oxidation temperatures. The REE is dependent on the solutionized La content when the alloy is oxidized at 1 000 °C. The dispersed LaCrO3 segregates on the grain boundaries of Cr-oxides, and hence the diffusion of metallic ions, e.g., Mn2+ and Cr3+, are prevented. The oxidation resistance decreases with the increase of La content. However, the REE ascribes not only to the solutionized La, but also to the La-Ni intermetallic phases, when the alloy is oxidized at 1 100 °C. The oxidation resistance is influenced by two effects: firstly, the formation of the dispersed La2O3 from the solutionized La on the grain boundary of Cr-oxides reduces the oxidation rate, and secondly, the aggregation of the La-O oxides from the large amount of La-Ni phases accelerates the oxidation process. The alloy with 0.026% La (mass fraction) exhibited excellent oxidation resistance at 1 100 °C.

The microstructure and precipitation characteristics of 22Cr-14W-2Mo superalloys added with various La contents have been studied by SEM, TEM and EDX. It is found that the carbides are refined by adding La. M23C6 carbides inside the grain in a diameter of 50 nm have a cube-cube orientation relationship with the matrix, and these precipitates on {111} planes align along 〈110〉 orientation. Since the large dissolved La atom will be a barrier to atom diffusion, it leads to an inhibiting effect on the carbides growing. M6C carbides are influenced by La-S phases. La-S phases formed in the center of M6C seem to be the heterogeneous nucleus of M6C to increase the nucleation ratio. It is also noticed that finer M6C carbides precipitated on the {111} planes of the matrix are around large M6C. The finer M6C carbides are caused by the special absorption effect of La-S phases.

•A 40 kJ pendulum machine for DWTT tests with well traceability and reproducibility.•Analytical regression method to estimate CTOA over stable cracking of specimens.•Key-curve method to monitor the dynamic crack extension of standard DWTT samples.•Experimental solution to obtain material parameters (A*σf) for DWTT-like specimens.A regression based algorithm, load-line displacement method has been developed from the Martinelli–Venzi ductile fracture model to measure the crack-tip-opening-angle (CTOA) of high strength and toughness pipeline steels. Key-curve method is employed on the instrument force–displacement curve to estimate the dynamic cracking behavior of a standard pressed-notch DWTT specimen, also conveniently to identify the stable fracture propagation. From the above laboratory procedure, the critical CTOA is found to be constant over the steady-state stage, independence of the thickness in the range 19–31 mm as studied.