This study systematically investigated the influence of the Co/Ni ratio (0.6-2) on the as-cast microstructure of high-W-content Co-Ni-based superalloys, elucidating the underlying mechanisms and correlating the as-cast microstructure with mechanical properties. The results demonstrate that the as-cast microstructure of the experimental alloys, across varying Co/Ni ratios, consists of a gamma matrix, gamma ' phase, Laves phase, beta phase, and TaC carbides. As the Co/Ni ratio increases from 0.6 to 2, significant changes occur in the volume fractions of the precipitated phases: the gamma ' phase decreases from 41.60 % to 25.87 %, while the Laves phase and beta phase increase from 3.09 % and 2.29-5.97 % and 6.92 %, respectively. Concurrently, systematic variations in elemental distribution within the precipitates are observed: the Co content in the gamma ' phase rises, whereas Ni and Al contents decline; the (Co+Ni) and Al concentrations in the beta phase remain relatively constant; and the Cr content in the Laves phase exhibits a decreasing trend. These phenomena can be attributed to the following mechanism: an increased Co/Ni ratio promotes the substitution of Ni by Co at the A-site of the gamma ' phase (A3B-type L12 structure), which compromises its electronic structural stability and weakens the bonding interaction between the A-site and the B-site (Al). This leads to excess Al migrating into interdendritic region, thereby promoting beta-phase formation and inducing a reconfiguration of thermodynamic equilibrium among the precipitate phases. Consequently, spontaneous adjustments occur within the stable regions of the phase diagram, resulting in altered elemental partitioning behavior in the beta and Laves phases and facilitating the attainment of a steady-state microstructural evolution.

This study systematically investigated the effects of Co/Ni ratios (0.6-2.0) on the solidification behavior, as-cast microstructure, and element segregation of Co-Al-W-based superalloys, and elucidated the mechanism of thermodynamic and kinetic synergistic regulation. The results show that increasing the Co/Ni ratio has a negligible effect on the liquidus and solidus temperatures, but it significantly lowers the dissolution temperature of the γ' phase, thereby expanding the alloy's heat treatment window (HTW) from 215 °C to 269 °C. As the Co/Ni ratio increased from 0.6 to 2, the SDAS at the center of the alloy ingot decreased from 112.4 μm to 43.3 μm, resulting in a significant refinement of the as-cast microstructure. The dendritic segregation coefficients for positively segregating elements such as Ta, Hf, and Al, as well as negatively segregating elements such as W, all approached 1 significantly, effectively suppressing microsegregation during solidification. This study reveals the multidimensional synergistic regulation mechanism of the Co/Ni ratio on the non-equilibrium solidification behavior of highly alloyed Co-Al-W-based superalloys and quantitatively elucidates the relationship between the Co/Ni ratio, the microstructural uniformity of as-cast specimens, and the heat treatment process window. For the first time in a highly alloyed multi-component Co-Al-W system, a correlation has been established between the Co/Ni ratio, element segregation, dendrite coarsening coefficient, and heat treatment window.

This study proposes a novel framework for quantifying curved grain boundaries that overcomes key limitations of existing methods. Unlike Fourier-based approaches that require labor-intensive sequential analysis of individual boundaries and selectively represent only high-amplitude regions, or spline-based methods that demand complex parameter selection for interpolation points, the proposed framework integrates curvature variance filtering with U-chord curvature calculation to enable automated, comprehensive, and noise-resistant characterization of grain boundary morphology. The curvature variance filtering adaptively determines smoothing parameters based on local curve properties, while the U-chord curvature method ensures rotational invariance and robustness against digitization errors. Four heat treatment processes were applied to GH4169 alloy, producing distinct grain boundary morphologies with mean curvature (MC) values ranging from 0.0625 to 0.1252. Controlled cooling alone (Process A) yielded predominantly straight boundaries (91.06% straight, 0.12% serrated), while re-dissolution treatment (Process D) produced the highest serration degree (58.81% straight, 3.53% serrated). The quantitative analysis reveals that dispersed δ-phase precipitation creates discrete pinning points, forming serrated boundaries with sharp curvature peaks, whereas dense, parallel δ-phase arrays at specific angles produce coordinated wavy undulations. This framework provides a reliable quantitative tool for optimizing heat treatment protocols to achieve target grain boundary configurations in nickel-based superalloys.

In this study, we investigate the segregation behavior and interactions of stacking faults(SFs) and microtwins in the 718Plus under 650 degrees C/800 MPa and 730 degrees C/500 MPa by atomic-scale Z-contrast (High angle annular dark field image, HAADF) in combination with creep models. Our results reveal that creep deformation is dominated by SFs and microtwins, with pronounced Nb and Co enrichment alongside Al and Ni depletion at SFs sites. Furthermore, gamma ''-Ni3Nb precipitates nucleate preferentially at SFs intersections, suggesting a dynamic interplay between defect evolution and precipitation. A comparative analysis of microtwinning-based creep models and experimental data demonstrates that microtwinning in the gamma' phase provides the dominant strengthening contribution, yielding a slip resistance of approximately 92.3 MPa-accounting for 62% of the total hardening. In stark contrast, gamma ''-induced hardening is negligible (similar to 0.15 MPa). These atomic-scale insights advance our understanding of creep mechanisms and gamma '' precipitation in 718Plus superalloys, providing critical guidance for alloy design.

The 1.5 wt% platinum (Pt) addition on the microstructural stability and gamma ' precipitate evolution in the 718Plus superalloy was revealed in this study. Utilizing scanning electron microscopy (SEM) and atom probe tomography (APT), it is demonstrated that Pt strongly partitions into the gamma ' precipitates with a coefficient of 3.82. This enrichment kinetically suppresses the diffusion of gamma '-forming elements, notably Nb, leading to a higher coarsening activation energy and a reduction in the coarsening rate of gamma ' precipitates. Furthermore, the co-segregation of Pt and Nb at the gamma/gamma ' interface increases the lattice misfit, which thermodynamically drives an earlier morphological transition of the gamma ' precipitates from spheroidal to cuboidal at a smaller particle size. The results conclusively show that Pt changes microstructural stability through dual mechanisms: impeding coarsening kinetics and modulating interfacial strain to guide microstructural evolution.

The eta phase in a newly developed Ni-based superalloy (GH4169D) was characterized by atomic-scale STEM-HAADF. The eta phase adopts a D024 structure with Ni on 6 g/6 h, Nb on 2a, and Al/Ti randomly distributed on 2c sites. The lamellar eta phase follows the Blackburn orientation relationship: {111}(gamma )//(0001)(eta), <1(-)10 > (gamma) // <21(-)1(-)0 > (eta), and <1(-)12 > (gamma) // <011(-)0 > (eta), whereas the plate-like morphology loses this relationship due to dislocation activity and elemental segregation. A gamma'-depleted zone is present between eta precipitates and the gamma matrix, indicating that eta growth consumes the surrounding gamma' phase. These results provide atomic-scale insights into the eta phase configuration and its interfacial characteristics.

Direct energy deposition (DED) of ATI 718Plus is a promising repair technology for impellers and blades. However, unavoidable Laves phases formed in the repaired zone (RZ) during the DED process can deteriorate the performance of the repair component. To maintain the wrought substrate zone (SZ) properties, the repair component can only be treated with a low-temperature heat treatment regime (788 degrees C/8h + 704 degrees C/8h, DA heat treatment), which almost has no help to the eliminate of Laves phase. Therefore, the morphology and content of Laves phase should be regulated during the DED process. In this paper, 50 % SZ + 50 % RZ ATI 718Plus repair joints were fabricated using three laser powers followed by DA heat treatment. The microstructure of RZ and SZ and the mechanical properties of the repaired joint were systematically investigated. In the RZ, reducing the laser power led to the formation of fine, randomly oriented columnar dendrites. Meanwhile, the Laves phase became discrete with decreased volume fraction, whereas the gamma ' phase and eta phase remained largely unaffected. In the SZ, an increase in laser power leads to the dissolution of the rod-shaped eta phase and an increase in the heat-affected zone (HAZ) width and grain size. The mechanical properties showed that the elongation of the 900 W sample (delta =11.8 %) was 2.5 times higher than that of the 1500 W sample while maintaining comparable strength. This work validates the feasibility of optimizing the Laves phase and improving the performance of DA-treated ATI 718 repairs by reducing the laser power.

Objective Laser melting deposition is used to prepare ATI 718Plus samples to study the effects of three heat treatment regimes on their microstructure evolution, hardness, and room temperature tensile properties. These regimes include direct aging heat treatment, solutionizing and aging heat treatment at 982 degree celsius, and high-temperature solutionizing and aging heat treatment at 1020 degree celsius. The aim is to elucidate the phase transformation behavior and mechanical property changes of laser additive ATI 718Plus under different heat treatment regimes and provide guidance for the selection of heat treatment processes used in the laser additive manufacturing of ATI 718Plus. Methods The plasma rotating electrode process is used with ATI 718Plus powder with particle diameter of 45?105 mu m to prepare wrought ATI 718Plus superalloy substrates. The experiments are performed on the laser additive manufacturing system shown in Fig. 2, which consists of a 4000 W continuous wave fiber laser, an inert atmosphere processing chamber, a coaxial nozzle, and a powder feeding device. A well-formed ATI 718Plus sample is prepared using a unidirectional reciprocating scanning method with the following parameters: a laser power of 1200 W, scanning speed of 0.8 m/min, protective gas flow rate of 10 L/min, carrier gas flow rate of 15 L/min, and powder feed rate of 13 g/min. The geometric dimensions of each sample are 50.0 mmx58.0 mmx2.5 mm. Three heat treatment regimes are employed, as shown in Fig. 2(b). The analyzed samples are mechanically ground with SiC paper and polished using diamond suspensions and a colloidal silica suspension to prepare metallographic samples. Then, the polished samples are etched with No.2 waterless Kailing's reagent for optical microscope and scanning electron microscope (SEM) investigations. Uniaxial tensile tests are carried out at room temperature using a universal testing machine with a constant displacement rate of 1 mm/min. Results and Discussions After laser deposition, a large number of Laves phase areas form in the interdendritic region (Fig. 3). This hard and brittle phase deteriorates the mechanical properties of the additive-manufactured ATI 718Plus samples. The as-deposited sample mainly exhibits an epitaxial growth columnar dendritic morphology, with a large number of brittle long-chain Laves phases precipitated between dendrites, which consumes a significant amount of Nb, Mo, and other strengthening elements, severely reducing the mechanical properties of the as-deposited sample. After the direct aging heat treatment, the long-chain Laves phase morphology remains unchanged, and the eta and gamma ' phases precipitate heavily between dendrites. The solution and aging heat treatment system can effectively reduce the size and content of the Laves phase. With an increase in the solution temperature, the size and content of the Laves and eta phases gradually decrease, and the gamma ' phase uniformly precipitates. The hardness significantly increases after heat treatment (Table 2), but the hardness differences between the three heat treatments are relatively small. The room temperature tensile properties are shown in Fig. 8. Compared to the as-deposited sample, after heat treatment the samples exhibit significant increases in both the yield strength and tensile strength, while the elongation at fracture decreases and then increases. The yield and tensile strengths increase by 67.7% and 51. 9% after the direct aging heat treatment, respectively, while the elongation at fracture decreases by 13%. After the solution aging (SA) heat treatment at 982 degree celsius, although the strength improvement is not as significant as that after the direct aging treatment, the yield and tensile strengths still increase by 63.6% and 45.6%, respectively. At the same time, the elongation at fracture increases by 3% compared to that of the as-deposited state. The strength improvement is the smallest after the 1020 degree celsius SA, with a yield strength increase of only 62.0% and tensile strength increase of 34.2%, but the plasticity is significantly improved, with an elongation at fracture increase of 25.8% compared to that of the as-deposited state. Conclusions The strength and hardness values of the ATI 718Plus additive samples significantly increase after heat treatment. The best match between strength and plasticity is obtained after high-temperature solution and aging heat treatment at 1020 degree celsius. Compared with those of the as-deposited state, the tensile strength and elongation at the fracture of the sample increase by 34.2% and 25.8%, respectively, after the 1020 degree celsius solution and aging heat treatment.

The GH4169D superalloy, a novel Nickel-based material widely acknowledged for its exceptional high temperature strength and fatigue resistance properties, plays a pivotal role in the production of critical components of contemporary aero-engines. The material, however, falls under the category of a typical difficult-to-cut metallic material due to the substantial cutting forces and significant tool wear experienced during the machining process. In this paper, a method of longitudinal ultrasonic vibration assisted side milling (LUVM) was developed to extend tool life. The investigation primarily involved an analysis of tool wear behaviors, including milling force, tool life, morphologies of tool wear, and various underlying wear mechanisms in LUVM and conventional milling (CM) of GH4169D superalloy. The results demonstrate that LUVM exhibits the advantage of reducing milling forces and prolonging tool life in comparison to CM. Throughout the entire milling process, the maximum milling force of LUVM is consistently smaller than that of CM, displaying a comparatively slower growth trend. At the conclusion of machining, while CM reaches a maximum milling force of 360.3 N, the maximum milling force of LUVM (195.9 N) is significantly lower. The tool life of LUVM is 28.2 min, representing a 33 % increase compared to CM (21.2 min). Material adhesion, coating delamination, and built-up edge (BUE) phenomena can be observed on both the tool rake face and flank face in CM. LUVM contributes to mitigation of bonding phenomena, prevention of BUE formation, and avoidance of abrasive wear in comparison to CM. However, it is prone to lead to tool tip chipping. For CM, the tool wear mechanisms include adhesive wear, oxidation wear, abrasive wear and diffusion wear; whereas for LUVM, the tool wear mechanisms comprise of adhesive wear, oxidation wear and diffusion wear.

In polycrystalline nickel-base superalloys, the presence of secondary phase precipitation at grain boundaries significantly influences mechanical properties. Numerous studies have revealed that the Ni3(Al/Ti)0.5Nb0.5 phase precipitated at grain boundaries can optimize stress rupture and reduce notch sensitivity, However, there was limited consideration given to the presence of the secondary phase within the grains. Herein, the atomic structure of i- Ni3(Al/Ti)0.5Nb0.5 phase in grain was investigated using atomic scale STEM-HAADF, and the i/gamma interface and interfacial dislocation during tensile and creep process were also discussed. The results indicated that the i/gamma interface exhibited different characteristic features in different stages, and interfacial dislocation interaction occurred during the tensile process, resulting in the formation of a parallel array of dislocations in a nano space. However, dislocations within the i phase and stacking faults (SFs) surrounding the interface are evident for the creep process. STEM-HAADF images also revealed that i/gamma interfacial dislocation interaction coordinated the deformation of two phases and provided the driving force of misfit lattice, which positively influenced the mechanical properties of 718Plus. The crystal structure of the i phase was reconstructed using VESTA software, and its ordered structure was found to be consistent with the corresponding HAADF images for B = [2110] zone axils. This present work provides valuable insights into the atomic-scale coordinated deformation of secondary phases and matrix.

The Laves phase will seriously deteriorate the subsequent hot working and welding performance of cast ATI 718Plus. In traditional homogenization annealing, two-stage treatment is used to prevent the formation of harmful eutectic structures due to the initial melting of the low-melting point Laves phase. This has several obvious disadvantages including high processing temperature, long processing time and complicated operation. Electrical pulsed treatment, as a new efficient, feasible and energy-saving green means, has rapidly achieved the dissolution of Laves phase. The increase in diffusion rate of Nb element in Ni matrix by pulsed electric current is due to the enhancement of defect migration and vacancy defect density, leading to a significant enhancement in the diffusion kinetics and accelerating the dissolution of Laves phase. Laves phase dissolution process under the pulsed electric current is from the aggregated state accompanied by dense needle like η phase to the discrete state accompanied by sparse needle like η phase, finally forming a single long strip and being dissolved into an intermittent state until it disappears. In all, using pulsed electric current instead of traditional heat treatment for annealing cast ingots has the potential to achieve one-step homogenization, completely dissolving the Laves phase while eliminating element segregation.

The burning loss of Al and Ti elements in superalloys during electroslag remelting has become a prevalent issue. And the existing slag system is not suitable for smelting the ATI 718PlusTM alloy. Therefore, it is imperative to develop a new slag system for smelting the ATI 718PlusTM alloy. To mitigate this issue, a thermodynamic model of the oxidation reaction of Al and Ti at the slag and alloy interface was established based on the ion and molecule coexistence theory (IMCT). The thermodynamic model was used to investigate the correlation between the equilibrium content of Al and Ti, slag composition, smelting temperature, and initial Al and Ti content of the electrode. The results indicate that while increasing the smelting temperature can effectively inhibit the burning loss of Al, it will exacerbate the burning loss of Ti. Increasing CaO and Al2O3 contents can inhibit the Al burning loss, while an increase in the TiO2 content can inhibit the Ti burning loss. Although an increase in the MgO content results in the burning loss of Al, its impact on the Al is minimal. The burning loss of Al and Ti was not affected by the change in the CaF2 content. The high Al content in ATI 718PlusTM makes it prone to burning loss of Al during the electroslag remelting. The combustion loss of Al can be reduced by increasing the Ti content in the electrode or adding a suitable amount of aluminum powder to the slag system. The accuracy of the model had been validated through experimental verification.

The paper describes high cycle rotational bending fatigue (HCF) performance and fracture characteristics of 718Plus alloy at 600 degrees C and 700 degrees C, where the relationship between maximum cyclic stress (S) and fatigue cycles (N) at 600 degrees C and 700 degrees C is explored. We drew S-N curves, and established mathematical model, with simulating S-N curve function. The crack initiation of smooth specimens focus on the surface or subsurface, where the stress concentration caused by inclusions, carbides and microstructure defects, leading to tearing, and inclusion is the main form of crack initiation. In addition, the high cycle fatigue life of 718Plus alloy is greatly affected by the microstructure defects. According to the analysis of high cycle fatigue fracture morphology, at 700 degrees C, the alloy fatigue crack growth is faster, and the number of secondary cracks is more than 600 degrees C. Thus, the growth behaviour is greatly affected by temperature.

Laser metal deposition (LMD) is an attractive repaired technology for integral impellers and blades. After maintenance, the LMD usually generates an inhomogeneous microstructure joint consisting of two zones: repaired zone (RZ) is mainly composed of highly textured columnar dendrite and long striped Laves phases in the inter-dendrite, while the substrate zone (SZ) displayed equiaxed grains with rod-like η phases on grain boundaries. RZ is the limiting factor due to the absence of γ′ and the presence of Laves phase. Three specific heat treatment regimes, namely direct aging (DA), solution and aging (SA), and homogenization and aging (HA) were chosen to investigate the microstructure response of heat treatments including the grain size, Laves phase, η phase, and γ′ phase of RZ and SZ. After heat treatment, the mechanical compatibility was highly improved, and the global tensile properties were analyzed based on the microstructure evolution and respective tensile properties. The tensile properties of the DA and SA joint reached the same level of wrought ATI 718plus alloys, whereas the ductility is lower due to the inhomogeneous deformation. Comparatively, the HA sample showed higher ductility and lower strength due to abnormal grain growth.

In this paper, the microstructural evolution and mechanical behaviour of the repaired ATI 718Plus sample were investigated in detail. Results show that the deposited zone (DZ) mainly contained columnar grains inclined along the z axis following the steepest temperature gradient during WAAM. And local Nb segregation leads to that a large amount of Laves, q, gamma' precipitates gathered in the interdendritic region. In contrast, the substrate zone (SZ) consisted of equiaxed grains and uniform distribution of gamma' precipitates. The gamma' precipitates in DZ show an obvious height dependence with a trend of increasing with increasing distance from the top to the bottom. Due to the exposure to elevated temperature, grain growth and the dissolution of q, gamma' phase were observed in HAZ. The hardness shows a gradual increase along the negative Z axis and an abrupt decrease in HAZ. These significant changes make the HAZ a possible vulnerable region. After direct age (DA) treatment, further growth of q precipitates and sufficient precipitation of gamma' phase were observed. The repaired samples always fractured at DZ. The tensile test indicated that UTS in DA condition shows an obvious increase, but the elongation is lower due to the brittle Laves phase acting as stress concentrators in the precipitation-hardened matrix and the fact that only half of the tensile samples experienced obvious deformation.

Wire and arc additive manufacturing technology was applied to repair damaged 718Plus components to save life-cycle costs. In this work, the effects of different solution temperatures on the microstructural evolution and tensile properties of repaired samples were investigated in detail. The results showed that the heat-treated microstructure in the deposited zone (DZ) mainly consisted of columnar grains along the building direction. Laves phase, needle-like η phase and γ′ phase were gathered in the interdendritic region. In contrast, the microstructure of the substrate zone (SZ) contained equiaxed grains with grain boundary (GB) η phase and a uniform distribution of γ′ phase. With the solution temperature increasing, the undesirable Laves phase was gradually dissolved, and its morphology varied from long-striped to granular. The number of η phases was observed to increase initially and then decrease with morphology changing to granular. The resultant granular η phase in SZ lost its pinning effect on GBs, leading to the rapid coarsening of grains in SZ. Tensile tests show that the dissolution of Laves phase and η phase in DZ helps to improve the ductility while the grain growth results in the decrease of tensile stress.

The widely used GH4169 alloy can meet various application through different heat treatment methods. In order to investigate the effect of heat treatment method on crack resistance at elevated temperature, fatigue crack growth tests of direct aging heat-treated GH4169 alloy (DA-GH4169) and standard heat-treated GH4169 alloy (SHT-GH4169) were performed at 650 °C in air. Under 10 Hz sinusoidal waveform, the fatigue crack growth rate of DA-GH4169 is slightly higher than that of SHT-GH4169 at low- $$\Delta K$$ Paris regime. The rates become close as $$\Delta K$$ increased. Under 0.5 Hz triangular waveform, the fatigue crack growth rate of DA-GH4169 is always higher than that of SHT-GH4169 in the tested $$\Delta K$$ scope. The change of fracture morphology from intergranular to transgranular in the Paris regime was observed for two GH4169 alloys. It is attributed to the interaction of cyclic plastic damage and dynamic embrittlement at crack tip, which is influenced by load, microstructure and frequency. Completely different propagation modes were found in the near-threshold region. While intergranular $$\delta $$ phases mildly inhibit grain boundary oxidation in the Paris regime, the transgranular propagation induced by intergranular $$\delta $$ phases in the near-threshold region is responsible for lower threshold.

We describe here the high cycle fatigue (HCF) behavior of a rail steel comprised of multiphase structure of bainite, martensite and film-like retained austenite, which was processed by the quenching and tempering (Q&T) heat treatment process. Conventional mechanical properties of experimental materials were determined, and the high cycle fatigue S-N curve of rail steel was obtained through ultrasonic fatigue test. The fatigue fracture details under cyclic stress were observed and analyzed in detail by scanning electron microscope (SEM). It was observed that the bainite/martensite (B/M) composite structure exhibited good performance, and its fatigue limit was as high as -790 MPa, meanwhile the ratio of the fatigue limit to the tensile strength of rail steel was up to -0.53. Finally, a comparison of the mechanical properties with other high-strength steels was made, which suggested that the rail steel with multiphase structure has both good strength and high fatigue limit. Thus, multiphase structure of steel has broad prospects in engineering applications.

We elucidate here the effect of low oxygen partial pressure on the dwell fatigue crack growth rate (DFCGR) of 718Plus alloy at high temperature. In comparison under the air and vacuum conditions with lower oxygen partial pressure, the DFCGR of 718Plus alloy reduced by three orders of magnitude at 704 ?C at lower oxygen partial pressure than that of in air condition. By analysis of microstructure, oxide, and crack path under different conditions, it was considered that the oxygen-induced dynamic embrittlement (DE) was the dominant impact mechanism for fatigue crack propagation at high-temperature in 718Plus alloy. Meanwhile, the stressaccelerated grain boundary oxidation (SAGBO) mechanism also played a significant role, especially in the long step-wise period. In addition, the types of oxide formed in creep process were directly related to the oxygen partial pressure and oxidation time.

Solidification behaviors of Pt-containing 718Plus superalloyswere studied by scanning electron microscopy (SEM), energy dispersive spectrum (EDS), differential scanning calorimetry (DSC) and simulation calculations. It is found that Pt increases solidification range and decreases solidus temperature of the alloy and precipitation temperature of Laves + γ eutectic phase since Pt enlarges the region of γ phase by increasing Nb solubility. In addition, Pt segregates to the interdendritic region and increases the segregation of Nb and Tiin the interdendriticregion due to the strong attractive interactions between Pt and Nb/Ti. As a result, Pt promotes the precipitation of the Laves + γ eutectic phase and η phase around eutectic phase. The increase of solidification range and segregation degrees of Nb and Al caused by Pt also promotes the precipitation and growth of γ’ + γ” phase around eutectic phase. These results provide experimental bases for understanding the mechanism of Pt in solidification behavior of superalloys.