Evolution Of Carbides Research Articles

The evolution of carbides during long-term creep/aging of nickel based superalloy K444

In K444 alloy, the evolution of carbides during stress aging (creep) and stress-free aging at 900 °C for 5000 h, and interrupted at 3000 h were investigated, combining microstructure characterization and quantitative statistics method. After stress-free aging for 5000 h, the volume fraction of intragranular MC and M6C carbides decrease from 0.48 % to 0.66 %–0.26 % and 0.48 %, respectively, while that of M23C6 carbides increase to 0.28 %. The two degradation behaviors can be concluded as MC + γ → M6C + M23C6 + γ′ and M6C + γ → M23C6 + γ′ respectively. The former controlled by the diffusion of Ti occurred earlier, while the latter determined by the diffusion of W and Mo mainly happened later. The applied stress did not change degradation reactions while accelerate the degradation rate slightly. During stress-free aging, the morphology of grain boundaries (GBs) changed from discrete distribution to semi-continuous and then to continuous chain, and the content of MC and M6C carbides decreases while that of M23C6 carbides increases. The applied stress markedly promotes above behaviors. Especially, the completed continuous chain formed before 3000 h. Moreover, the applied stress induced the formation of cavitation on the GBs, which is attributed to the following reasons. First, it can accelerate the degradation of GBs and decline the GBs strength. What's more, dislocations pile up on the GBs and cause a high level of stress concentration. Finally, the new generated phase interfaces have a greater deviation with its adjacent carbides, weakening the binding force of GBs.

Ultra-Smooth Polishing of Single-Crystal Silicon Carbide by Pulsed-Ion-Beam Sputtering of Quantum-Dot Sacrificial Layers.

Single-crystal silicon carbide has excellent electrical, mechanical, and chemical properties. However, due to its high hardness material properties, achieving high-precision manufacturing of single-crystal silicon carbide with an ultra-smooth surface is difficult. In this work, quantum dots were introduced as a sacrificial layer in polishing for pulsed-ion-beam sputtering of single-crystal SiC. The surface of single-crystal silicon carbide with a quantum-dot sacrificial layer was sputtered using a pulsed-ion beam and compared with the surface of single-crystal silicon carbide sputtered directly. The surface roughness evolution of single-crystal silicon carbide etched using a pulsed ion beam was studied, and the mechanism of sacrificial layer sputtering was analyzed theoretically. The results show that direct sputtering of single-crystal silicon carbide will deteriorate the surface quality. On the contrary, the surface roughness of single-crystal silicon carbide with a quantum-dot sacrificial layer added using pulsed-ion-beam sputtering was effectively suppressed, the surface shape accuracy of the Ø120 mm sample was converged to 7.63 nm RMS, and the roughness was reduced to 0.21 nm RMS. Therefore, the single-crystal silicon carbide with the quantum-dot sacrificial layer added via pulsed-ion-beam sputtering can effectively reduce the micro-morphology roughness phenomenon caused by ion-beam sputtering, and it is expected to realize the manufacture of a high-precision ultra-smooth surface of single-crystal silicon carbide.

Thermostructural evolution of boron carbide characterized using in-situ x-ray diffraction

Boron carbide, with a nominal stoichiometry of B4C, is a highly desired ceramic candidate for armor applications due to its high hardness derived from the complex crystal structure. However, stress-induced local amorphization can lead to failure and is a known challenge for this material which must be addressed for applications in ballistic environments. Understanding boron carbide's atomic structural behavior and bonding environment is critical in determining effective strategies to mitigate these issues. In this work, the thermo-structural behavior of B4C has been studied in detail using a conical nozzle levitator system coupled with in-situ synchrotron X-ray diffraction. Lattice expansion and the resulting thermal expansion coefficients (CTEs) were determined from 25-2100 °C. Rietveld refinements showed anisotropic atomic displacement for each of the 4 unique sites as a function of temperature. An exceptionally large z-axis displacement for the boron chain center is linked to bond weakness and may be linked to faα11ster expansion of the α33 relative to CTEs. Thermally induced lattice changes can inform the use of boron carbide at elevated temperatures as well as help develop strategies for mitigating structural failure for armor applications.

Softening Behavior and Hardness Prediction of H13 Hot‐Work Die Steel during Isothermal Tempering

In this study, the microstructural evolution of H13 hot‐work die steel during isothermal tempering at 500 ≈ 650 °C is evaluated, and kinetic equations and a neural network are constructed to describe the softening behavior. The results show that the softening behavior appears as a loss of hardness with increasing isothermal tempering temperature and time. The softening mechanism is ascribed to the decomposition of martensitic laths, annihilation and rearrangement of dislocations, formation of sub‐grains, and precipitation and coarsening of secondary carbides. The evolution of secondary carbides includes the coarsening of V‐rich MC nanoprecipitates, and the precipitation and coarsening of secondary phases (Cr‐rich M23C6 and Mo‐rich M6C carbides) during long‐term isothermal tempering. Regarding the applicability of the model in precipitation evolution and hardness prediction, the Johnson–Mehl–Avrami model is considered to be more suitable than the Hollomon–Jaffe, Lifshitz–Slyozov–Wagner, and back‐propagation neural network models.

Evolution of primary carbides during accelerated solidification process of high-carbon martensitic stainless steels

In order to evaluate the potential application advantage for development of sub-rapid cooling strip casting of high-carbon martensitic stainless steels, the effects of cooling rate on the solidification microstructure, solute element segregation, and primary carbide precipitation in high-carbon martensitic stainless steels were investigated using an association of high-temperature confocal laser scanning microscopy (HT-CLSM), optical microscopy (OM), scanning electron microscopy (SEM) and electro-probe microanalysis (EPMA). As a result, the microstructure becomes more refined and the secondary dendrite arm spacing (SDAS) decreases greatly by 51 μm from 65.9 μm as the increase of cooling rate from 30 to 6000 °C·min−1. The equation of SDAS versus cooling rate was formulated as λ2 = 191.62·RC−0.317. Solute elements are enriched in the inter-dendritic region, leading to the precipitation of primary carbides. The elemental segregation ratio reduces by more than 20 % for all elements, meanwhile, the size of primary carbides decreases significantly and the mean area fraction of the primary carbides decreases from 4.77 % to 0.82 % with increase of cooling rate from 30 to 6000 °C·min−1. Moreover, the reticulated carbides gradually become less continuous. Despite the type of carbide is not sensitive to the cooling rate, the content of Cr and Fe in the primary carbide is sensitive to cooling rate. Compared to the reported methods, sub-rapid cooling of strip casting shows great advantage in control of primary carbides of high-carbon martensitic stainless steels.

Analysis on carbonization evolution behavior of tungsten carbide powders and its application in cutting tools

In this article, the microstructural evolution of tungsten carbide (WC) powder with coarse particle size was studied. The WC powders were prepared by directly carbonizing tungsten powders of 8.5 μm in Fisher sub-sieve size (FSSS) under various conditions. The sequence of chemical reactions was analyzed by calculating the Gibbs free energies, and the phase transition mechanisms of tungsten particles was studied by using characterization techniques including SEM and XRD. The experimental results showed that the phase transition process is dominated by carbon diffusion reaction, and the diffusion rate was studied by analyzing carbonized tungsten particles prepared at specific temperatures. The grain growth process and mechanism were also studied by analyzing the morphology and grain size of WC powders prepared at 1800 °C in different carbonization times, and the results are consistent with the recrystallization grain growth theory. Finally, technique of Electron Backscattered Diffraction (EBSD) was utilized to identify and measure the grain size of WC powders prepared from tungsten powder of 0.6 μm in FSSS. The cemented tungsten carbide substrate prepared from WC powder with higher average grain size exhibited higher performances in metal cutting application of turning and drilling.

Effect of high temperature annealing on the microstructure evolution and hardness behavior of the Inconel 625 superalloy additively manufactured by laser powder bed fusion

Additive manufacturing of Inconel 625 components attracts great interest due to its ability to produce parts with complex geometries that are needed for high-temperature applications in the aerospace, energy, automotive and chemical industries. To take full advantage of the potential of additive manufacturing, an in-depth understanding of the effects of prolonged high-temperature annealing on microstructure and hardness evolution is needed. Previous research in this field has mainly focused on a limited range of temperature and time. This study aims to determine the effect of prolonged high-temperature annealing on the evolution of intermetallic phases and carbides, as well as changes in the dislocation substructure of Inconel 625 superalloy additively manufactured by laser powder bed fusion subjected to stress relief annealing and subsequent isothermal annealing at a temperature up to 800 °C for 5–500 h. The microstructure development is correlated with hardness behaviour. It is determined that the microstructure evolution proceeds in four stages with temperature and time increase. In the initial stress-relieved condition, a cellular microstructure with nano-sized precipitates of the Laves phase and NbC carbides at the cell walls occurs, and hardness is equal to 300 HV10. In the 1st stage of the microstructure evolution, the γ'' phase particles precipitate on the cell walls, which results in hardening up to 383 HV10 in the specimen annealed at 700 °C for 5 h. The 2nd stage involves the precipitation of the γ'' phase both on the cell walls and inside the cells, as well as the formation of dislocation networks, which contribute to the softening effect and hardness drop to 319 HV10. In the 3rd stage, at temperature 700 and 800 °C, the δ phase, M23C6 carbides, and the Laves phase precipitate and grow, and the subgrain boundaries are formed. The hardness is in the range of 340–350 HV10 and is higher than in the 2nd stage. In the 4th stage, as the annealing time is increased at a temperature of 800 °C, the δ phase and M23C6 carbides coagulate, and the Laves phase particles spheroidize or partially dissolve. Very intense precipitation and growth of the hard δ phase particles provide an increase in hardness to 402 HV10. As a result of systematic studies, the various strengthening and softening mechanisms acting during high-temperature annealing are determined.Graphical abstract

From single high-entropy phase to multi-phase transition: Microstructural evolution of multi-component carbide (ZrHfVNbMoW)C realized by adjusting Hf content and temperature

(ZrHfxVNbMoW)C ceramics with different Hf contents are consolidated by hot-pressing sintering at 2100 °C for 1 h. The influence of Hf content and temperature on phase transition are investigated. The phase transition from single high-entropy phase to multi-phase containing (ZrHfNb)C-rich and (VMoW)C-rich intermediate phases occurs with adding Hf, in relation with relevant larger lattice distortion. Remarkable microstructure refinement is observed with Hf content variation, due to the poor sinterability of Hf and the intermediate phases restricting the atomic motion and diffusion. The refined microstructure and severe lattice distortion lead to the improvement in mechanical properties. The annealing at different temperatures further proves the phase stability, sluggish diffusion effect as well as solid solution diffusion in high-entropy and multi-phase carbides.

Evolution of M7C3 carbides near the solidus and the influence of Mn element on the formation of M23C6 carbides in a high carbon martensitic stainless steel 90Cr18MoV

The evolution behavior of M7C3 carbides in a 90Cr18MoV martensite stainless steel during high temperature diffusion annealing treatments at temperatures near the solidus was investigated. Results showed that only M7C3 carbides dissolved, and the dissolution of large-sized M7C3 carbides was limited when the temperature was below the solidus. However, when the temperature was above the solidus, a certain amount of liquid phase appeared around the grain boundaries. Meanwhile, two different types of precipitates appeared within the grains. And these two types of precipitates were eutectic products that mainly consisted of M7C3 carbides and bcc phase, and divorced eutectic products that mainly consisted of M23C6 carbides, respectively. Thermodynamic calculations indicated that with the increase of Mn content, the Gibbs free energy of M23C6 carbides would be lower than that of M7C3 carbides. Thus, the difference in Mn content was the main reason for the formation of those two types of precipitates, i.e., M7C3 carbides were formed in regions with low Mn content and M23C6 carbides were observed in areas with high Mn content.

Segregation and carbide evolution in AISI D2 tool steel produced by curved continuous casting

A promising process with the concept of ‘green, efficient and economic’ was carried out to produce a typical AISI D2 tool steel through curved continuous casting (C3). This study aims to demonstrate the segregation and eutectic carbides homogeneity in the C3 billets, which are the key points. The center porosity and center segregation are 2.0 and 1.0 respectively which are according to the standards GB/T 1979–2001 and YB/T 153–2015, and no shrinkage defects are found in the macrostructure. The segregation zones of C and Cr existed in the core and distributed dispersedly. Furthermore, the carbides showed three morphologies such as rod-like, coarse bulk-like and tortuous, where the rod-like carbides formed at the positions of edge to 3/4, the coarse bulk-like carbides emanated at 1/2 and core positions, and the tortuous carbides appeared somewhere in between. The morphologies of carbides are presumably caused by the inhomogeneous diffusion of C and Cr during the solidification. The maximum size of carbides increased and the area ratio of carbides decreased gradually at the positions of the edge to the core in the billets. The carbides evolved during high-temperature annealing, and it exhibited best dissolution which has less amount segregated at grain boundary with smaller size according to the results of experiments and theoretical calculation.

Two-dimensional structure evolution and formation of silicon carbides and diamonds in a nano-abrasion process

Structural phase transition is often expected when crystalline substrates with high mechanical strength are processed by diamond abrasion at nanoscale. The structural evolution and formation of 3 C-SiC substrates abraded by diamond were explored using molecular dynamics simulation. The evoluted structures were derived by analyzing bond length, bond angle and coordination changes of SiC and diamond abrasives before and after nano-abrasion. Our simulation results explicitly elucidate the stable, graphene-like two-dimensional (2D) structural evolution and formation from SiC and diamond crystalline. This generolized finding of 2D structural formation from three-dimensional (3D) materials in nano-abrasion may shed light on developing new preparation options to make graphene-like 2D-SiC, which is predicted to be a chemically and mechanically stable semicondcuting material with oustanding opto-electro properties.

Effect of Tempering Time on Carbide Evolution and Mechanical Properties of a Nb-V-Ti Micro-Alloyed Steel

The evolution of the microstructure, the precipitation behavior, and the mechanical performances of Nb-V-Ti micro-alloyed steel prepared under different tempering time were studied using transmission electron microscopy (TEM), X-ray diffraction (XRD), and mechanical tests. It was found that the width of the martensite laths increases with the increasing tempering time. Several kinds of carbides, including M3C, M2C, M23C6, M7C3, and MC particles, were identified after tempering. The MC carbides remain stable during tempering, but the transformation behavior of other carbides was identified. The transformation sequence can be summarized as: M3C → M2C → M7C3 → M23C6. The strength decreases and the Charpy impact toughness increases gradually with the increase in the tempering time. The ultimate strength (UTS) decreases from 1231 to 896 MPa, and the yield strength (YS) decreases from 1138 to 835 MPa. The −40 °C Charpy impact toughness increases from 20 to 61 J as the tempering time increases from 10 min to 100 h. The evolution of carbides plays an important role in their mechanical performances.

Effect of precipitation-induced element partitioning during tempering on mechanical properties of hot-rolled 3Mn steel after intercritical annealing

This work explores the carbide evolution behavior and mechanical property evolution mechanisms of intercritical-annealed 3Mn steel during the tempering process by comparing different tempering times (5/10/20 min). The results showed that the elongation of 3Mn steel increased from 19.5% to 26.5% after tempering while maintaining a tensile strength above 1250 MPa. It is also found that carbide precipitation during tempering is first followed by dissolution. The mechanical properties exhibited abnormal degradation at tempering for 10 min, with a tensile strength of 1265 MPa and a total elongation of 16.0%. However, the optimal mechanical properties were achieved at tempering for 20 min, with a tensile strength of 1286 MPa and a total elongation of 26.5%. During the initial stage of tempering, a transformation occurs from M3C to stable M7C3, with the presence of numerous dislocations and grain boundaries promoting the precipitation and growth of abundant carbides. This leads to a reduction in the C and Mn content in the matrix, consequently decreasing the austenite content and stability, resulting in the transformation of austenite to the BCC phase during tempering instead of cooling. The decreased austenite content and stability lead to reduced ductility. Subsequently, dislocations were annihilated during tempering, suppressing carbide precipitation, and the over-precipitated carbides dissolved under thermodynamic driving force, leading to an increase in C and Mn content in the remaining austenite, thus enhancing its stability. The precipitation and dissolution of carbides during tempering, along with the partitioning of elements between carbides and austenite, strengthen austenite stability and improve mechanical properties.

Analysis of wear mechanism and sawing performance of carbide and PCD circular saw blades in machining hard aluminum alloy

High-efficiency sawing is widely applied in primary and secondary machining sectors, but it faces limitations due to saw tooth life and manufacturing constraints. This research investigates the wear mechanism, wear characteristics, and sawing performance of carbide and polycrystalline diamond (PCD) saw teeth when machining hard aluminum alloy. The results show that the sawing forces with PCD saw teeth are less than those of carbide teeth, yielding better surface quality and longer tool life. The wear mechanisms of saw teeth of two materials with different tooth types are classified and characterized finely. Carbide teeth are subject mainly to abrasion and adhesion at the cutting edges, while adhesion, corrosion, and diffusion primarily contribute to flank face wear. PCD teeth mainly suffer from abrasion and micro-chipping on the cutting edges and faces. A comprehensive description of the wear evolution of carbide and PCD teeth is presented, with the cutting heat being the primary cause of carbide tooth wear and the presence of hard particles in the workpiece causing PCD tooth wear by SEM and EDS analysis. The friction coefficient of the PCD teeth is considerably lower than that of the carbide teeth by sliding friction and wear tests, which also proves its more superior thermal stability and wear resistance during wear evolution analysis. Systematic research analyzes and discusses the wear mechanism and wear characteristics of saw teeth in high-speed sawing process, which can further enhance the understanding of the saw tooth wear to provide theoretical guidance for saw tooth design and manufacturing.

Evolution of centerline carbides in a martensitic stainless steel slab during reheating treatment

The formation and evolution of centerline carbides in a martensitic stainless steel slab are investigated using various characterization techniques. The intensive formation of carbides was observed close to the shrinkage cavity at the centerline of the slab. Carbide structures include a complex eutectic-like mixture of δ-ferrite and M7C3 phases with M6C carbides at the δ/M7C3 interface. Nano-sized M7C3 particles also precipitate in the δ holding the orientation relationship of1¯10δ∥100M7C3;111δ∥100M7C3.This complex structure was surrounded by the retained γ. The decomposition of the coarse M7C3 was investigated after the reheating treatment to 1200, 1250, and 1300 °C. It was found that by increasing the reheating temperature, coarse M7C3 carbide completely decomposed, whereas δ-ferrite islands enlarged due to the thermodynamic stability of δ-ferrite at high temperatures. At the same time, the retained austenite reduces because of homogenizing of the segregated solutes. The new formation of M23C6 and the growth of M6C were identified after reheating to 1200 °C, while all carbides were fully decomposed at 1250 and 1300 °C. The effect of reheating temperature on the decomposition process of coarse carbide was further discussed.

Carbide evolution and its effect on the impact toughness of the 9Cr/CrMoV dissimilar welded joint during the aging treatment

The microstructure evolution in the weld metal (WM) and its effect on the impact toughness for the 9Cr/CrMoV dissimilar welded joint with various aging time was systematically investigated. With the aging time increased from 0 h to 3,000 h, the specimen aged at 470 ℃ for 1,000 h revealed the lowest impact energy (13.5 J). Moreover, the impact fracture morphology details were obtained by scanning electron microscopy (SEM). The results showed that the quasi-cleavage fracture appeared for the specimens in the original and aged (aged at 470 °C for 3,000 h) states, while the cleavage fracture could be observed for the specimen aged at 470 °C for 1,000 h. Besides, the microstructure observation exhibited that the main microstructure in the WMs for the specimens in the original and aged states were all the tempered martensite. The columnar and equiaxed grain zones formed owing to the multi-layer and multi-pass techniques. Based on the characterization analysis of the precipitates for the aged specimens, the coarsening of the carbides was determined as the crucial factors to decrease the impact energy. On the contrary, with the aging time increased, the newly precipitated fine carbides could produce the positive effects on the impact toughness. In the original state, the impact energy of the specimens was about 29.2 J. With the aging time increased to 1,000 h and 3,000 h, the impact energy became 13.5 J and 21.0 J, respectively.

Interface-Induced Phase Evolution and Spatial Distribution of Fe-Based Catalysts for Fischer–Tropsch Synthesis

Fe-based catalysts undergo complex reduction, carburization, and phase interconversion as well as oxidation during Fischer–Tropsch synthesis (FTS). Unraveling the evolution of carbides and establishing the dominant factors are essential for rational design and modulation of Fe-based catalysts. In this work, Fe–oxide interfaces were constructed to investigate the effect on phase transformation and FTS performance. By correlating performance to the composition of iron species, the crucial role of Fe2.2C in high CO conversion and C5+ productivity has been confirmed. More importantly, a heterogeneous spatial distribution of iron carbides and oxides induced by Fe–oxide interfaces was observed. Fe3O4 preferentially forms at interfaces with the coexistence of carbon-rich Fe2.2C at subadjacent interfaces, while Fe5C2 locates at the region away from oxides or in the bare Fe catalyst. Further ex/in situ characterizations, including XPS, TPO, in situ XRD, and in situ IR, were conducted for revealing the mechanism. This study provides insights for iron carbides formation and stability by modulating interfaces, which guides the further development of Fe-based FTS catalysts.

Influence of solution temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions

The influence of solution treatment temperature on microstructure and mechanical properties of DZ411 alloys with different Ta compositions (2.72, 3.10, and 4.00 wt.%) is studied in details. With the increase of Ta composition, the average values of the primary dendrite arm spacing (PDAS) in superalloys before heat treatment are found to increase from 145.9 ± 3.7 to 209.8 ± 7.6 μm, the area fractions of carbides increase from 0.59 ± 0.05% to 0.65 ± 0.05%, the area fractions of γ-γ′ eutectics increase from 4.6 ± 0.05% to 8.1 ± 0.05%, and the cubicity of the γ′ phase also increases. After heat treatment, since the γ′ phase shows more superior high temperature performance in alloys with higher Ta composition, the solution temperature of alloys with higher Ta composition should be increased to promote the dissolution of the coarse γ′ phase. The evolution of carbides is mainly divided into two types: the decomposition of MC carbides: MC + γ → M23C6 + γ′ and the precipitation of fine M23C6 carbides. The increase of cubicity of γ′ phase will increase the interfacial energy of two phases, which can lead to the pile-up of dislocation and improve the microhardness of alloys. The fine M23C6 carbides will hinder the initiation and propagation of cracks, and the interaction of fine carbide particles reduces the stress concentration and improve the fracture strength of alloys. Therefore, the addition of Ta increases the mechanical properties including both tensile properties and microhardness of alloys with higher Ta composition increases more obviously after heat treatment at higher temperature.

Laser powder bed fusion of granulation-sintering-deoxygenation WC/18Ni300 composites: anisotropic microstructure characterization and wear properties

PurposeLaser powder bed fusion (LPBF) can be used to fabricate complex extrusion die without the limitation of structures. Layer-by-layer processing leads to differences in microstructures and wear properties. This study aims to investigate the microstructure evolution and effects of tungsten carbide (WC) on the wear properties of LPBF-printed 18Ni300.Design/methodology/approachEconomical spherical granulation-sintering-deoxygenation (GSD) WC-reinforced 18Ni300 steel matrix composites were produced by LPBF from powder mixtures of WC and 18Ni300. The effects of WC contents on anisotropic microstructures and wear properties of the composites were investigated.FindingsThe relative density is more than 99% for all the composites except 25% WC/18Ni300 composite. The grain sizes distributed on the top cross-section are smaller than those on the side cross-section. After adding WC particles, more high-angle grain boundaries and larger Schmid factor generate, and deformed grains decrease. With increasing WC contents, the hardness first decreases and then increases but the wear volume loss decreases. The side cross-section of the composite has higher hardness and better wear resistance. The 18Ni300 exhibits adhesive wear accompanying with abrasive wear, while plowing and fatigue wear are the predominant wear mechanisms of the composites.Originality/valueEconomical spherical GSD WC particles can be used to improve the wear resistance. The novel WC/18Ni300 composites are suitable for the application under the abrasive wear condition with low stress.

Evolution of MC carbides in Ni-Mo-Cr-Ti based alloy for applications in molten salt reactors

Evolution of MC carbides in Ni-Mo-Cr-Ti based alloy for applications in molten salt reactors