The second-generation single-crystal superalloy DD6 has a series of merits, such as high-temperature strength, combination properties, structural stability, and better casting performance. It is a good choice for manufacturing turbine blades. A reliable joining technology for the single-crystal superalloy DD6 is important for engineering applications. In this study, a newly designed Ni-based filler alloy with low boron content was used to join the DD6 superalloy. To avoid the formation of brittle borides within the joint, boron was reduced. However, the element Pd was added into the filler alloy as a melting-point depressant. The brazing process can be conducted at 1220oC, which was lower than the solution treatment temperature of the DD6 base material. The effects of the gap size on the joint microstructure and mechanical properties were investigated. After brazing with the new Ni-based filler alloy, the matrix of brazing seam was γ + γ′ dual-phase, which was similar to the DD6 base material. The brittle borides in the joint were increased because of the big gap size, and the morphology of the borides was transformed from the discontinuous strip to coarse fishbone. When prefilling the FGH95 superalloy powder in the brazing seam with gap size of 0.15 mm, borides were refined and dispersed. With the increase in the gap size, the joint strength increased and then decreased. The γ + γ′ dual structure was refined when the gap size increased from 0.05 mm to 0.10 mm. Moreover, the content of elements Al, Ti, and Ta was high in the matrix of the joint with a gap size of 0.10 mm, which can strengthen the γ′ phase. When the gap size was 0.15 mm, the joint strength decreased because of the coarse borides. The highest joint strength was obtained when the gap size was 0.10 mm, and the average joint tensile strength tested at 980oC was 694 MPa. After ageing heat treatment, the morphology of γ + γ′ was modified and the joint tensile strength increased to 807 MPa.
Low carbon microalloyed high-strength pipeline steels processed by the thermomechanical controlled process have a good strength-toughness combination. However, after welding, the microstructure and mechanical properties of the heat-affected zone (HAZ) become deteriorated. Previous studies show that martensite-austenite (M-A) constituent formed in the HAZ is a key factor that lowers the toughness, especially necklace-type M-A constituent formed in the intercritically reheated coarse-grained HAZ (ICCGHAZ). However, the phase transformation mechanism of necklace-type M-A constituent in the ICCGHAZ is unclear. In this study, the crystallography of reverted austenite (γr) during the reversion phase transformation upon heating the ICCGHAZ of a high-strength pipeline steel was studied using Gleeble thermal simulation and electron backscatter diffraction (EBSD) technique. Two thermal cycles with peak temperatures of 1300oC and 760oC/800oC/840oC were conducted to simulate the phase transformation process in the ICCGHAZ. The samples were directly quenched to room temperature after been reheated to the second peak temperature, and the reversion behavior, distribution, and crystallography of γr were studied. The results showed that the volume fraction of γr formed at 760, 800, and 840oC was 4.1%, 8.9%, and 25.2%, respectively. γr preferred to nucleate along prior austenite grain boundaries (PAGB), and posterior the block boundaries within the prior austenite grains. γr preferred to grow to blocky type along the PAGB, and the acicular type γr between bainite laths was suppressed. The crystallographic study showed that the formation of γr at PAGB was not free nucleation. However, γr was formed based on the crystallographic orientation of PAGB alongside prior austenite grain complying with Kurdjuov-Sachs (K-S) relationship, while having non K-S relationship with the prior austenite grain on the other side. After nucleation at PAGB, and at a low second peak temperature (760oC), γr transformed to prior austenite grain with non K-S relationship, and the γr formed in blocky and necklace-type along the PAGB. With an increase in the second peak temperature (800-840oC), γr transformed to the prior austenite grains on both sides. The analysis showed that the reversion behavior and crystallography of γr during the second pass reheating have a big impact on the phase transformation upon cooling and the corresponding microstructure and mechanical properties.
Carburized gear steel has a high-hardness case layer with excellent wear and fatigue resistance and a low-hardness core with high toughness. Such different microstructures imply different susceptibilities to hydrogen embrittlement (HE). However, a few or no studies have explored the HE behavior of carburized gear steel. Herein, the HE behavior of a vacuum-carburized gear steel 20Cr2Ni4A was investigated via an electrochemical hydrogen-charging and slow strain rate tensile test. For comparison, another group of specimens was prepared by a conventional quenched and tempered (QT) treatment. The volume fraction of retained austenite was significantly higher in the case layer of the carburized specimen (13.8%) than in the core and the QT specimen (4.6%). The retained austenite in the case layer showed a mainly irregular block-type morphology with wide size distribution. The room-temperature diffusible hydrogen content in the hydrogen-charged carburized specimen were almost identical to the QT specimen but the nondiffusible hydrogen content was significantly higher in the former than in the latter. Meanwhile, the hydrogen diffusion coefficient was notably lower in the hydrogen-charged carburized specimen than that in the QT sepcimen because the former retained higher fractions of austenite and cementite. The QT specimen exhibited superior strength and ductility. After hydrogen charging, the strength of the QT specimen remained almost unchanged but the total elongation notably decreased, causing the HE index (HEI), as evidenced using the relative total elongation loss, being 54.3%. Relative to the QT specimen, the carburized specimen achieved a higher tensile strength (increase by 34.6%) but a much lower ductility (total elongation and reduction of area reductions by 66.5% and 92.4%, respectively). The carburized specimen underwent premature brittle fracture before yielding, indicating susceptibility to HE. In fact, the HEI was as high as 90.9%. Mixed intergranular and quasi-cleavage fractures were observed in the surface embrittled region of the hydrogen-charged QT specimen. This region roughly corresponded to the maximum hydrogen diffusion distance. Meanwhile, the hydrogen-charged carburized specimen exhibited an embrittled internal-surface region with a certain width of intergranular fracture, and a long crack had propagated along the circumferential direction near the effective case depth. The microstructure, strength level, and residual stress are thought to mainly explain the abovementioned differences between the carburized and QT specimens.
Liquid lead and lead-bismuth eutectic (LBE) are considered primary candidate materials for coolant in advanced lead fast reactors, and also, for coolant and spallation target in accelerator-driven systems because of their favorable thermal-physical and chemical properties. However, liquid lead and LBE exhibit substantial structural material corrosion, which is considered one of the critical challenges in the liquid lead or LBE application. Among the effective methods to reduce the corrosive effect, careful control of the oxygen content dissolved in the liquid introduces a protective oxide layer on the surface of the structural material. Ferritic-martensitic steels with (9%-12%)Cr (mass fraction) have been considered promising structural materials in the advanced lead fast reactor and accelerator-driven system. Oxide scale with duplex structure is formed on the (9%-12%)Cr ferritic-martensitic steels in oxygen-containing LBE, but the oxide scale grows rapidly enough to cause the substrate recession. Recently, pre-oxidation treatment was proposed to further improve the corrosion performance in LBE. As reported, grain refinement promoted the formation of oxide scale without chemical modification. However, grain refinement effects on the oxide scale formation of (9%-12%)Cr ferritic-martensitic steels are not clear, and the effect of oxide scale through pre-oxidation treatment on the corrosion performance in LBE is rarely reported. In this study, the oxide scale formation behavior during pre-oxidation in air at 650oC on the 9Cr2WVTa ferritic-martensitic steel after different cold rotary-swaging deformation rates was analyzed using SEM, XRD, EPMA, and XPS, and the effect of pre-oxide scale on the corrosion performance in stagnant oxygen-saturated LBE was further investigated. The results demonstrated that the high-temperature oxidation resistance in air was enhanced by increasing the diameter reduction. A slight improvement in oxidation resistance was observed after 63% deformation, while significant enhancement in oxidation resistance was present in ultrafine-grained sample fabricated by 94% deformation. The oxide particle size was slightly reduced in the sample produced after 63% deformation during air oxidation compared with the tempered sample, but (Fe, Cr)2O3 oxide particles were formed on both samples. The size of oxide particles was significantly reduced and Mn-riched oxide (MnCr2O4 and Mn2O3) was promoted in the ultrafine-grained sample. The presence of Mn-riched oxide with good stability improved the oxide scale compactness. The compact oxide scale on the ultrafine-grained sample using pre-oxidation treatment at 650oC for 20 h effectively suppressed the corrosive attack of LBE and the outward diffusion of Fe through the pre-oxide scale after exposure for 500 h to stagnant oxygen-saturated LBE at 550oC. The higher solubility of Mn in LBE promoted the dissolution of the Mn-riched pre-oxide scale. The gradual dissolution of Mn in pre-oxide scale and corrosive attack by LBE led to the breakdown of the pre-oxide scale, which was supported by the formation of a continuous corrosion product layer after exposure for 2000 h to stagnant oxygen-saturated LBE.
Magnesium alloys have attracted significant attention due to their excellent characteristics, such as low density, high specific strength, high thermal conductivity, and superior damping ability. The prominent advantages of using magnesium-alloy parts are environmental protection, energy conservation, and emission reduction, especially forged magnesium-alloy parts, which are used to reduce the weight of equipment in transportation and aerospace fields. Presently, there are relatively few investigations on the forged Mg-RE alloys. Based on Mg-Gd binary alloy, a series of Mg-Gd-Er-Zr alloys were developed, exhibiting excellent room-temperature mechanical properties and high-temperature creep resistance. In this study, the Mg-8Gd-1Er-0.5Zr (mass fraction, %) alloy was conducted using a multi-directional forging (MDF) at a high strain rate. The microstructure and texture evolution in various accumulated strains (ΣΔε) were examined, and their effects on the mechanical properties of the alloy were discussed. The results showed that the {101¯2} extension twinning was activated within most grains in the early stage of forging. With an increase in ΣΔε, the area fraction of twin decreases, while the area fraction of recrystallization increases. Continuous dynamic recrystallization (CDRX) was the dominant mechanism, supplemented by discontinuous dynamic recrystallization (DDRX) and twin-induced recrystallization (T-DRX). The grain refinement was attributed to the twin-breaking, and the grain size decreased from 33.0 μm to 13.1 μm when ΣΔε was less than 1.32. However, it was attributed to the dynamic recrystallization, and the grain size was further refined to 4.2 μm when ΣΔε was greater than 1.32. As ΣΔε increases, the texture of the alloy changed from basal to double-peak texture, and its intensity increased. When ΣΔε = 0.66, the tensile strength, yield strength, and elongation at room-temperature of the MDFed-alloy reached 295 MPa, 252 MPa, and 13.8%, respectively, which were 80%, 157%, and 13.1% higher than those of the as-solution state.
Cu-W composites that combine the merits of Cu and W show good electric and heat conductivity, resistance to arc erosion, and high strength, etc., and are good candidates for electric contact materials. Until now, several methods, including the high-temperature liquid phase sintering method and the hot-pressure sintering method, have been developed to fabricate Cu-W composites. However, these methods may cause an uneven distribution of constituents in the material and a relatively low density and poor electric conductivity of the material. In this study, a Cu-W composite with micro-oriented W lamellas was prepared by the infiltration method, and the mechanical and electrical properties were investigated and compared with a commercial Cu-W composite. The results showed that the compressive strength of the studied Cu-W composite with micro-oriented W lamellas was between 300 and 1100 MPa when the W content was between 50% and 90% (mass fraction). The compressive strength of the studied composites presented obvious anisotropy, and the strength along the direction parallel to the W lamellas was higher than that perpendicular to the W lamellas. Compared with commercial Cu-W composites with disordered W frameworks, composites with micro-oriented W lamellas exhibit a higher electrical conductivity and compressive strength along the W lamellar direction, which is mainly related to the regular arrangement of the two phases of Cu and W in the composites. The studied composite is expected to be used as an electrical contact material to significantly improve the effect of electric contracts and prolong their service life while reducing the mass of the components and energy consumption.
The Inconel 718 alloy has become a remarkable candidate material for aerospace jet engines, turbine blades, and some other elevated temperature components owing to its superior tensile strength, anticorrosion and thermal performance. Moreover, TiN ceramic particles with high hardness and chemical stability have been realized to significantly improve the mechanical properties of the alloy matrix at a low content. Accordingly, in this work, the Inconel 718 (IN718) alloy and TiN/IN718 composite were fabricated by the optimized selective laser melting (SLM) process. Further, the microstructures and mechanical properties of the IN718 alloy and TiN/IN718 composite under heat treatments were investigated, respectively. The results show that the TiN particles were highly combined with the matrix, and a transition layer with 0.3 μm was formed in the SLM-fabricated TiN/IN718 specimens. Additionally, the microhardness and tensile strength were significantly improved compared with IN718 alloy (39 HV0.2, 74 MPa, respectively). After the double aging and solution aging (SA) treatments, the number of crack initiation sources was increased owing to the precipitation of the δ phase, which deteriorated the tensile strength of the TiN/IN718 composite. After the homogenization + SA (HSA) treatment, the composite was completely recrystallized, and an appropriate amount of needle- and plate-like δ phases precipitated at the grain boundaries. Hence, the TiN/IN718 composite after the HSA treatment exhibited optimally comprehensive mechanical properties.
The nickel base powder superalloy prepared by modern powder metallurgy (PM) technology is selected because it has the characteristics of compatibility with strength and damage tolerance. Moreover, it is the preferred material for the fabrication of a new generation of aero-engine turbine disks. In this study, experimental techniques, such as FESEM and TEM, are used to systematically evaluate the creep properties of powder metallurgy nickel base superalloys with different Ta contents under the conditions of 750°C and 600 MPa. Additionally, the characteristics of microstructure and defosrmation behavior during creep and the effect of stacking fault energy of the alloy on creep property are also investigated. The results show that with increase in Ta content, the energy associated with alloy stacking fault decreases, demonstrating a nonlinear relationship. The deformation behavior and dislocation configuration changes in each creep deformation stage are closely related to the stacking fault energy. The stacking fault energy of alloys with low Ta content is relatively high, the matrix dislocation a/2<110> is prevented at the γ/γ' interface, and the dislocation is not easy to decompose. Furthermore, it can directly enter the γ' phase to form antiphase boundary or to bypass the γ' phase through the Orowan ring bow bending mode. If the alloy contains a moderate amount of Ta, the stacking fault energy of the alloy is reduced, promoting the decomposition of matrix dislocations at the γ/γ' interface. This results in a/6<112> Shockley incomplete dislocations and starts to shear the γ' phase, forming superlattice stacking faults (superlattice intrinsic stacking faults (SISFs) or superlattice extrinsic stacking faults (SESFs)) and extended stacking faults (ESFs), which are then transformed into deformation twins. Therefore, presenting the co-strengthening effect of stacking faults and deformation twins, which improves the creep property. The stacking fault energy of alloys with high Ta content is very low, which is favorable to the simultaneous formation of wide-sized ESFs on different {111} slip planes. The occurrence of inter-crossing stacking faults inhibits the formation of deformation twins and accelerates the development of creep deformation cracks. These experimental results demonstrate that the addition of an appropriate amount of Ta to the alloy can effectively reduce the stacking fault energy, improve the ability to form both partial dislocation shear γ' phase and micro-twins, increase creep resistance, and effectively improve the alloy creep property.
Performance and fatigue life of coating parts are seriously restricted by their interfacial fatigue property. Herein, TiN films were deposited on W6Mo5Cr4V2 steel substrates by multiarc ion plating. The interfacial fatigue failure mechanisms were studied by the rolling contact fatigue method. The results show that the interfacial fatigue failure mode is mainly film spalling. The fatigue cracks generated initially at the film/substrate interface proceed to the surface, resulting in film spalling. The interfacial maximum shear stress amplitude (Δτinter) is a key factor for controlling interfacial crack initiation and propagation. The evolution model built using Δτinter and critical cycles (N) can be used to determine interfacial fatigue performance and for life forecast. The interfacial fatigue property is determined using a film/substrate interface, and glow discharge cleaning and prefabricated metal layer before coating deposition can improve interface fatigue performance. The evaluation model based on Δτinter-N curves can effectively used to identify the differences in interface states. Selection of the film-spalling area ratios of 5% and 50% and failure probabilities of 30%, 60%, and 90% have little effect on the determination of film/substrate interfacial fatigue performance. The results provide important theoretical references for fatigue performance determination and lifespan prediction of coated bearings and other parts.
H13 steel is one of the most promising materials for molds owing to its outstanding hardenability, high toughness, and thermal cracking resistance. To reinforce the surface performance and extend service life of H13 steel, a WC-Ni matrix cermet composite coating with a Ni or Mo transition layer was prepared by electrospark deposition on an H13 steel substrate. The phase compositions, microstructure, microhardness, and tribological properties of the coating were investigated in detail. The surface of the WC-Ni coating contained accumulated sputtered deposition spots. The cross section of WC-Ni coating is composed of a coating, a transition layer, and a substrate with a clear boundary; the WC hard phases are dispersed in the coating. The Ni/WC-Ni composite coating surface is relatively smooth and flat, and its phase composition is consistent with that of the WC-Ni coating. The WC hard phases show abnormal growth at the interface. The surface of a Mo/WC-Ni composite coating exhibits microcracks and indicates the formation of a new Fe9.7Mo0.3 phase. Hardness values of the composite coatings are greater than that of the WC-Ni coating, and their friction coefficient and wear loss are lower than that of the H13 steel substrate and WC-Ni coating. In addition, the antiabrasive performance of the Mo/WC-Ni composite coating is better than that of the Ni/WC-Ni composite coating.
The phase transformation path is vital to enclosing the macroscopic transport equations for predicting alloy macrosegregation. However, the analytical approximations for micro-segregation, such as the lever rule (LR), are invalid because an actual alloy is a multi-component system with several coexisting solids. The LR only expresses the phase transformation between a single solid phase and a liquid phase and adopts a constant solute partition coefficient, which is insufficient for micro-segregation. In this study, a model combining the thermodynamic phase transformation path calculation with the macroscopic transport was adopted to predict the macrosegregation formation in an Fe-0.1%C (mass fraction) peritectic alloy, which considers the coexistence of multi-solids and the variance of the local partition coefficient at the solid/liquid and solid/solid phase interface with a solidification process. The phase transformation path from the liquid state cooling to room temperature within a certain range of the solute concentrations was obtained using the LR approximation combined with the thermodynamic equilibrium calculation (LR-TEC). By tabulating the phase transformation path and interpolating the local concentration and enthalpy, the corresponding temperature, phase fraction, phase concentration, and phase enthalpy required in the continuum macroscopic transport model were achieved. The latent heat released and the specific heat corresponding to the amounts of the two solid phases at the peritectic or eutectic phase transformation zone were updated along with their dependence on the local concentration and temperature. This method was validated through the benchmark macrosegregation test of the binary Sn-5%Pb alloy. Regarding the Fe-0.1%C alloy, the varied local partition coefficients and the other thermodynamic parameters with multi-solids precipitating during solidification resulted in a more severe macrosegregation profile in the ingot. At the end of the solidification calculation, the predicted minimum relative solute concentration for the Fe-0.1%C alloy was -2.22% at y = 45 mm from the bottom and x = 16 mm from the left wall of the ingot by LR-TEC. In contrast, it was -1.78% using the LR Analytical model near y = 55 mm at the left-side wall. The predicted maximum macrosegregation ratio at the right wall of the ingot by LR-TEC was 1.13% larger than that achieved using the LR Analytical model. Several solids, such as α and γ, α and cementite (CEM), or α, γ, and CEM at the left part (x < 0.0342 m), and δ and γ at the right (x > 0.0858 m), still coexisted in the region at the end of solidification calculation.
Q345 steel is a low-alloy high-strength steel that is widely used in production. As the solid-state phase transition of Q345 steel is very sensitive to temperature, the microstructure and hardness of joints fabricated with this steel are difficult to predict. Therefore, studying the welding metallurgy, residual welding stress, and welding deformation of Q345 steel joints is essential for improving the safety and service life of Q345 welding structures. In this study, the microstructure and distribution of hardness of a Q345-steel tungsten-inert-gas welded and re-melted joint were calculated using four models encoded in the general finite element software, ABAQUS, and FORTRAN language. Three models were based on only one of three welding continuous-cooling transformation curves of the simulated heat-affected zone (SH-CCT) of Q345 steel, with peak temperatures of 1300, 1100, and 900oC (hereafter denoted as SH-CCT1300, SH-CCT1100, and SH-CCT900, respectively). The final model was based on the associated diagram consisting of these curves SH-CCT diagram above. Comparing the simulation and experimental results, the capabilities and accuracies of the prediction methods based on the different models were investigated. The microstructural calculations of the single SH-CCT diagram agreed with the experimental results only in the local heat-affected zone (HAZ) and largely deviated in the other areas. In the model based on SH-CCT900, the relative error of the ferrite volume fraction in the inter-critically HAZ (ICHAZ) was 7.7%. In the model based on SH-CCT1100, the relative errors of the ferrite, bainite, and martensite volume fractions in the fine-grained HAZ (FGHAZ) were 11.3%, 17.3%, and 15.5%, respectively. In the model based on SH-CCT1300, the relative error of the martensite volume fraction in the coarse-grained HAZ (CGHAZ) was 29.6%. In contrast, the microstructural calculations of the associated SH-CCT diagram agreed with the experimental results over the whole HAZ. The results showed that models based on the single SH-CCT diagrams met the tested hardness only in several narrow areas of the HAZ, but the hardness computed by the model, based on the associated diagram, was consistent with the tested hardness, with absolute differences ranging from 0 to 14 HV. The calculated microstructure and hardness in the fusion zone (FZ) greatly deviated from the test results in all the four models, indicating that predictions in the FZ require an accurate SH-CCT diagram of the FZ. In practical welding processes, the corresponding single SH-CCT diagram can adequately predict the microstructure and hardness during the phase transition of one HAZ area (such as the CGHAZ).
