In recent years, surface γ/γ′-eutectics have been observed to form continuous layers along the peripheries of the outer surfaces of Ni-based single crystal (SC) castings. This type of solidification defect poses specific concerns for turbine blade production because large surface eutectics are very difficult to completely dissolve during practical heat treatment. In the present work, a third generation SC superalloy was used to cast turbine blades and cylindrical bars. However, no eutectic layer was observed on the outer surface of the blade aerofoils and bars in hundreds of castings. Instead of the lateral eutectic enrichment on the outer side surface, a non-uniform distribution of eutectics along the solidification length was detected. Eutectics were found to accumulate on the upper surface of solidification units such as platforms whereas less eutectics were observed on the bottom surface. Especially on the inclined upper surface at the junctions between platform and airfoil, a large amount of eutectics were accumulated so that only a small part could dissolve during conventional solution heat treatment. This phenomenon, called the eutectic accumulation on the upper surface, could be attributed to the upward solutal flow of eutectics forming elements through diffusion and convection, which is finally stopped beneath the upper surface. The factors influencing the upward eutectic accumulation are the chemical composition of the alloys, the process conditions, and the local geometry of the castings, such as the platform thickness and the inclination of the upper surface. Another surface effect was also observed on the eutectic dissolution during heat treatment process. It was found that the eutectics in the outer surface area were dissolved completely during heat treatment process than that in the internal one. The possible explanation is that at high temperatures, the eutectic forming elements Al and Ti evaporate from the casting surface faster than the other elements, as experimentally confirmed by chemical measurement.
GH4720Li is used for turbine disks in a large number of civil and military propulsion systems because of its excellent mechanical properties and corrosion resistance. GH4720Li turbine disk is mainly manufactured through cast and wrought conventionally, but the addition of a high mass fraction of Ti, Al, and Mo can cause severe element segregation and more difficult microstructure control, which can become more severe as the turbine disk gets larger. Owing to this difficulty, the turbine disk quality cannot be guaranteed if the GH4720Li turbine disk is still in cast and wrought form, and the manufacturing process will be more complex, resulting in increased costs. However, the powder metallurgy method can efficiently eliminate element segregation and produce a more uniform microstructure than the cast and wrought methods. GH4720Li alloys manufactured using the powder metallurgy method are called FGH4720Li alloys. As there has been limited research on FGH4720Li and no report on the microstructure evolution during long-term ageing for FGH4720Li to date, it is necessary to study the microstructure evolution behavior during long-term ageing for FGH4720Li to obtain improved microstructure stability. In this study, field emission scanning electron microscopy and extractive phase analysis were used to investigate the microstructure evolution of FGH4720Li in the temperature range of 600-730oC up to 3000 h. The results showed that primary gamma prime was the most stable; whereas, secondary and tertiary gamma prime microstructure evolutions were comparatively complex. At 600oC, there was no microstructure change. At 650oC, only the tertiary gamma prime grew, but there was no microstructure change for the other gamma primes up to 3000 h. When the ageing temperature increased to 730oC, the tertiary gamma prime grew faster before coarsening rapidly. After 200 h, the secondary gamma prime coarsened noticeably, but the big secondary gamma prime coarsened by Ostwald ripening first, absorbing a large amount of tertiary gamma prime and splitting up between 300 and 500 h, before ageing with further processing. Big secondary gamma prime mainly coarsens by amalgamation; whereas, small secondary gamma prime always coarsens by amalgamation during ageing. The divergence between these two types of secondary gamma prime is related to the distribution characteristics of the tertiary gamma prime.
Ni-based single crystal (SX) superalloys are used for the production of blades in gas turbines and aircraft engines because of their superior mechanical performance at high temperatures. To improve the temperature capabilities of modern SX superalloys, specific refractory elements are added to the alloys. This leads to micro-segregation in alloys, requiring a complex heat treatment process to eliminate γ/γ′ eutectic. Therefore, the dissolution process of γ/γ′ eutectic must be understood. In this study, a second-generation Ni-based SX superalloy was used to investigate the effect of extended holding time at 1290oC and 1300oC on the γ′ phase dissolving temperature (Tγ′) and γ/γ′ eutectic phase-melting temperature (Tγ/γ′), respectively. The method involves measuring the differential heating curves of as-cast and as heat-treated samples using DSC. The results showed that Tγ′ and Tγ/γ′ increased at a holding time of 2 h. However, with an increase in the holding time, the temperature increase was not obvious. The volume fraction of γ/γ′ eutectic decreased with the extended holding at 1300oC, while the volume fraction of γ/γ′ eutectic increased after holding at 1290oC for 8 h. This abnormal phenomenon was confirmed by the metallographic experiments. The analyses showed that the increase in the eutectic volume fraction was due to the incomplete dissolution of coarse γ′ phase at the inter-dendritic region, which resulted in the diffusion of Ta element from dendrite core to the inter-dendritic region, promoting eutectic growth.
During fusion welding of the Cu-containing high-entropy alloy (HEA) of CoCrFeNiCu, Cu precipitates at the grain boundary and reduces the welding quality and mechanical properties. Solid-phase diffusion welding is connected with the diffusion of elements, which reduces the segregation of Cu and solves this problem well. In the present work, the study of diffusion welding of CoCrFeNiCu HEA with 304 stainless steel (304SS) was carried by SEM, EBSD, TEM, and XRD. The properties of the microstructure and crystal type were obtained. The results show that element diffusion can achieve high-quality connection between the two metals. As the temperature increases, the diffusion capacity increases. The pores at the interface disappear. The thickness of the diffusion layer is between 10-31 μm, without forming intermetallic compound, which shows that a solid solution microstructure is formed after diffusion. According to the XRD, this microstructure is mainly as CoCrFeNiCuMn, that leads to an improvement in the mechanical properties. The hardness of the CoCrFeNiCu HEA-diffusion layer-304SS shows a gradient increase. The crystal structure type is mainly as substructured and recrystallized structure. The proportion of low-angle grain boundaries is 93% in the diffusion layer. The EBSD test results show that the grain orientation of the welded joint (diffusion layer) has changed, mostly concentrated on the (111) plane. This also causes anisotropy of the mechanical properties. Tensile performance test showed that the material fractured in the CoCrFeNiCu HEA base material area. These show that diffusion welding achieves a high-quality connection of the two materials.
As a potential high-temperature structural material, the practical application of NiAl intermetallic compound is limited because of its low plasticity and fracture toughness at room temperature and poor strength at high temperature. Adding alloying elements and optimizing the preparation process are effective ways to improve the material's performance. In this study, NiAl-based alloys were prepared through spark plasma sintering using NiAl-28Cr-5.5Mo-0.5Zr (atom fraction, %) prealloyed powders. The influence of sintering temperature and holding time on the density, microstructure, and room-temperature compression properties of the sintered alloys was studied. The microstructures of the sintered alloys were investigated using SEM, and their room-temperature compression properties were tested using an electronic universal testing machine. It is shown that the influence of spark plasma sintering temperature on the density and room-temperature compression properties of sintered alloys is significant while that of the holding time is weaker. Under optimal sintering parameters, i.e., a sintering temperature of 1200oC, holding time of 3 min, and sintering pressure of 50 MPa, the yield strength, compressive strength, and plasticity strain of the sintered alloy were 1321.4 MPa, 2360 MPa, and 0.313, respectively. Additionally, the densification process of NiAl-28Cr-5.5Mo-0.5Zr alloy prepared by spark plasma sintering was studied.
In the engineering design of machines and vehicles, the technologies involving vibration and noise reduction receive considerable attention as they can prevent undesirable fatigue failures and offer more comfort to the users. Compared with other damping alloys, high Mn steel has been intensively studied because its high strength and excellent damp capacity can be achieved simultaneously at a low cost. Particularly, it has a strong potential of circumventing the long-existing trade-off of strength and damping capacity for more applications in the new circumstance. In this study, the microstructures and damping and mechanical properties of a novel hot-rolled Nb-alloyed high Mn steel annealed at the temperature range of 750-1050oC were investigated. Heterogeneous recrystallization occurred during hot rolling, resulting in the formation of recrystallized bands along the rolling direction, in which fine lamellar ε martensite grains were interwoven with austenite; the coarse unrecrystallized ε martensite blocks were located between the bands. The latter was reversely transformed to the coarse austenite with a single orientation, remained unrecrystallized, and transformed back to the coarse highly-dislocated ε martensite blocks during the annealing below 850oC. Conversely, the ε martensite blocks reversely transformed the austenite grains partially and completely recrystallized to form even more refined grains with multi-orientations during annealing at 950 and 1050oC, respectively. Thus, the recrystallized austenite grains transformed to fine ε lamellar martensite, and the austenite grains were retained with multi-orientations, both having a low density of dislocation. Consequently, the higher annealing temperature led to higher damping capacity but lower strength; whereas, the partial recrystallization occurring during annealing at 950oC resulted in the best combination of mechanical and damping properties. Therefore, this indicates that tailoring the extent of recrystallization via the Nb-alloying and annealing process for high Mn steel can be an effective method to achieve different combinations of strength and damping capacity.
High carbon steel is prone to macroscopic and semimacroscopic segregation. Segregation is a phenomenon that results in the nonuniformity of solute element distribution during solidification; segregation severely impacts the quality of the casting billet. Based on qualitative analysis (macroscopic quality rating) and relatively simple quantitative analysis (segregation index, mean square error), it is found that the accuracy of existing technologies is limited for determining the nonuniformity of carbon element distribution in large areas of cast high carbon steel billets; this causes difficulties while applying such existing technologies for the evaluation of large samples under actual steelworks usage conditions. Therefore, it is essential to study the nonuniformity of carbon element distribution to precisely evaluate and optimize the quality of high carbon steel. Based on the grayscale image of the casting blank macrostructure of typical high carbon steels (mass fraction of carbon = 0.7%), the standard deviation, differential box-counting, and moment of inertia are introduced to discuss simple methods for the quantitative characterization of the nonuniformity of the major segregation element (C) in large areas. Then, the validity of the characterization results was verified using statistical homogeneity and the near-equilibrium solidification model, and the similarities and differences of the three characterization methods were discussed. The results showed that the standard deviation, differential box-counting, and moment of inertia can effectively reflect the nonuniformity of carbon element distribution. Furthermore, the mean value of the nonuniformity of carbon element distribution in the equiaxed area was 20.85% higher than that in the columnar area. The standard deviation is mainly based on the statistical characteristics of the grayscale value, while differential box-counting and moment of inertia combine the grayscale value statistical and spatial distribution information on a grayscale image of the casting blank macrostructure. In addition, differential box-counting has scale independence, which is less affected by the size and resolution of the grayscale image; the moment of inertia is sensitive to the variation of the nonuniformity of carbon element distribution in the microregion. Finally, the standard deviation is mainly influenced by large segregation points (e.g., area larger than 1 mm2), while differential box-counting and the moment of inertia are mainly influenced by medium segregation points (e.g., area of 0.1-1 mm2 ). Therefore, the three characterization methods can be combined to comprehensively evaluate the nonuniformity of carbon element distribution in large areas of cast high carbon steel billets. This research can provide a new theoretical reference for comprehensively evaluating the nonuniformity of carbon element distribution in a large area and for the fine quality evaluation of cast high carbon steel billets.
The potentionstatic pulse technique (PPT) has been widely used as a new electrochemical method in research of stainless-steel corrosion. In addition to the susceptibility detection of stainless steel, PPT has also been recently applied in research of pitting corrosion. The influence of PPT parameters on pitting behavior of 317L stainless steel is studied using electrochemical measurements and optical microscopy. This investigation reveals the effect of high potential (Eh) parameters on pitting behavior in samples. The results show that when Eh is in the range of the passivation potential, pits will not occur. When Eh is applied in the pitting potential range, the size and number of pits first increase and then stabilize. When Eh is in the range of the transpassivation potential, the sample can not maintain the passive condition. In addition, potentiodynamic polarization tests show that the pitting potential and re-passivation potential of PPT test samples increase, indicating that the pitting resistance of 317L stainless steel can be enhanced by the PPT test. Therefore, the PPT can be used as a surface modification method to improve pitting corrosion resistance of stainless steel after selecting appropriate parameters.
Metal flow behavior in the stir zone (SZ) is important in friction stir welding (FSW) because it determines the formation of defects, and evolution of microstructure, and affects the mechanical properties of the joint. Applying axial ultrasonic vibration (ultrasonic energy is applied to the stirring tool along the axial direction) during FSW can improve the flowability of SZ metal; however, the reason is unclear. In this study, 6-mm-thick 7N01-T4 alloy plates were welded using FSW and ultrasonic-assisted FSW (UAFSW), using a thin foil of pure aluminum as a marker placed at the butt interface before welding to highlight the actual metal flow during welding. Alongwith the FSW experimental results, the influence of the coupling effect of axial ultrasonic vibration and thread of tool pin on the flow behavior of SZ metal was studied. The results revealed that the macroscopic flow behavior of SZ metal along the welding direction was not affected by axial ultrasonic vibration (e.g., the distance between the arc lines remains unchanged); however, the axial ultrasonic vibration intensified the ring vortex movement of the pin-driven zone (PDZ) metal along the plate-thickness direction. Moreover, the high-frequency forging effect of the shoulder and pin end under the action of ultrasound promoted the flow of metal in the shoulder-driven zone (SDZ) and swirl zone (SWZ). Based on the analysis of the force condition of the plastic metal around the pin, under axial ultrasonic vibration, a microscale sucking-extruding effect model was proposed, and the flowability improvement of SZ metal by axial ultrasonic vibration was explained. The stress superposition and acoustic softening effects induced by ultrasonic vibration are not the only factors affecting the flowability of SZ metal; tool pin geometric features also determine the flow behavior of SZ metal under the action of axial ultrasonic vibration. When a tool with a threaded pin is used for welding, the microscale sucking-extruding effect caused by the coupling of axial ultrasonic vibration and the pin thread improves the SZ metal flowability. When welding using the tool with a smooth pin, axial ultrasonic vibration reduces the shearing effect of the pin on SZ metal, resulting in the weakening of the metal flowability of the SZ, and a high tendency for welding defect formation.
Material genome engineering (MGE) is an advanced research concept that integrates high-throughput computing, high-throughput experiment (synthesis and characterization), and a special database. The purpose of MGE is to adopt an efficient and economical method to accelerate the correlation between the composition, microstructure, and performance. However, low composition, small size, characterization difficulty, and high cost limit its application in the high-throughput synthesis of a bulk metal. A new high-throughput synthesis based on hot isostatic pressing (HIP), which uses a honeycomb structure sleeve made of pure Ni and Ti foils, was proposed. A combinatorial-bulk-alloy with 19 gradient components was HIPed by filling different mixtures of pure Fe, Co, and Ni powders into each cell of the honeycomb sleeve. The high-throughput characterization of the composition and microstructure showed that the composition of each cell was homogeneous and in accordance with the design, the mixed powders diffused to the alloy phases, and the bulk alloy had good density and without macroscopic defects. The mechanism of alloying and its effect on the mechanical behavior was discussed by investigating the microhardness of Fe-Co-Ni combinatorial alloy. Fe- and Ni-based alloys were solid solution and second phase strengthened, whereas Co-based alloy was two-phase solid solution strengthened. The strengthening effect of Ni was better than that of Co for Fe-based alloy. The strengthening effect of Fe was better than that of Co for Ni-based alloy. The strengthening effect of Ni was better than that of Fe for Co-based alloy.
Pinning-controlled Sm2(Co, M)17 (M = Fe, Cu, and Zr) magnets with cellular nanostructures are the strongest high-temperature permanent magnets. The squareness factor of such magnets is smaller than those of nucleation-controlled permanent magnets, leading to a lower-than-ideal maximum energy product. One of the main reasons for this poor squareness is that the pinning strength is weaker at cell edges than at 1:5H cell boundaries. However, the structure of these edges remains a topic of debate. To identify the microstructure of cell edges, electron diffraction, TEM bright/dark field imaging, and HRTEM imaging on a model magnet Sm25Co50.2Fe16.2Cu5.6Zr3.0 (mass fraction, %) were performed using both [100]2:17R and [101]2:17R zone axes. The results revealed a rhombohedral 2:17R' phase at some of the edges, with one faulting basal layer in the 2:17R lattice. Further comparative investigations revealed that all the extra superlattice reflections result from the 2:17R' phase, excluding the previously identified 2:17H or Smn + 1Co5n - 1 or their mixture that can only produce a part of such superlattice reflections. Owing to the 2:17R' phase with a faulted basal plane, the free energy at the cell edges is higher than that of the 2:17R cell interiors, leading to repulsive domain-wall-pinning unfavorable for the squareness factor. This study provides important evidence for understanding the microstructural origin of the poor squareness factor obtained for Sm2(Co, M)17 permanent magnets.
Constraint is the resistance of a specimen or structure against plastic deformation that contains geometric and material costraints. Both can affect the fracture behavior of a material significantly. For a material constraint, most studies focused on the strength mismatch of both sides of a crack, such as over-match and under-match. Nevertheless, the effect range of the material constraint also needs to be clarified. In previous studies, the effect range of a material constraint was demonstrated in different specimens. In this study, the actual and simplified nuclear safe end structures were selected. The J-M curves (where J is the J-integral, which reflects the degree of stress and strain concentration at the crack tip due to a wide range of yield; M is the bending moment), the areas surrounded by the equivalent plastic strain (PEEQ) isoline, and the failure assessment curves of the two structures under different material constraints were calculated to determine the effect range of material constraint in structure. The results show that the effect range of a material constraint exists in actual and simplified nuclear safe end structures. The J-M curves, the areas surrounded by the PEEQ isoline, and the failure assessment curves were unaffected by the material located out of the effect range. Compared to the actual nuclear safe end structure, the simplified structure had a lower geometric constraint, a larger material constraint effect range, and a higher failure assessment curve, possibly producing a non-conservative assessment result. Thus, in the design and structure integrity assessment of a nuclear safety end structure and other strength-mismatched structures, the influence of the material constraint effect range should be considered, particularly in the following two aspects. The first aspect is that in the design process, a material with weak properties should be designed out of the material constraint effect range. This can effectively avoid weakening of the structural properties caused by the weaker material. The second is that in the assessment process, the material out of the material constraint effect range does not need to be taken into account. Only the material in the material constraint effect range should be considered, which will reduce the difficulty and workload of an assessment.
Stress corrosion cracking (SCC) is a major problem in the welded components of austenitic stainless steel in nuclear power plants. High tensile residual stress is an important factor resulting in the SCC of materials. Austenitic stainless steel has a strong tendency for work hardening owing to its fcc crystal structure and low stacking-fault energy. High plastic strain can accumulate during a multipass welding process. On the other hand, accumulated strain hardening can be reduced or even eliminated during the welding thermal cycles owing to dynamic recovery, recrystallization, and grain growth below the melting point, which is called the annealing effect. Influence of strain hardening and annealing effect needs to be investigated to predict the welding-induced residual stresses accurately in austenitic stainless steel joints. In this study, a new time-temperature-dependent annealing model was proposed based on the Johnson-Mehl-Avrami equation. Numerical Satoh tests were performed to clarify the influence of strain-hardening models (i.e., the isotropic strain-hardening model and Chaboche mixed isotropic-kinematic strain-hardening model) and annealing models (i.e., the single-stage annealing model and new time-temperature-dependent annealing model) on the formation of residual stresses and the accumulated plastic strain during multiple thermal cycles. Thermoelastic-plastic finite element (FE) analyses were carried out to predict the welding residual stresses and accumulated plastic strain in a thick-wall 316 stainless steel butt-welded pipe joint with 85 welding passes. The residual stresses of the welded joint were measured by the sectioning method, inherent strain method, and deep-hole drilling method. The simulations of welding residual stresses were compared with the measurements. Annealing effect significantly influences the formation of accumulated plastic strain and welding residual stresses, neglecting which will result in a significant overestimation of FE results. The proposed annealing model showed an excellent match to the experimental data. With the consideration of the annealing effect, the isotropic strain-hardening model overestimated the welding residual stresses slightly, while the FE results of welding residual stresses using the Chaboche mixed strain-hardening model showed better agreement with the measurements. The single-stage annealing model revealed a recommended annealing temperature of 900-1000°C for austenitic stainless steel such as 316 stainless steel.
