M23C6 Carbides Research Articles

Microalloying effects of Y on performance of cast nickel-based superalloy IN713C

Microalloying is a cost-effective strategy to develop high performance nickel-based superalloys. Here, the mechanisms of rare earth Y in IN713C on purification, mechanical property improvement, and microstructure modification were systematically investigated. Via melt purification of Y, the O, N, and S elements are reduced to 5 ppm in the IN713C alloy. By adding Y to 74 ppm, the yield strength, tensile strength, and elongation of the IN713C alloy at 650 °C are increased by 12.2%, 14.5%, and 66%, respectively. Microstructure analysis indicated that the strength of the alloy is improved by refining the size and increasing the volume fraction of the γ’ phase. The plasticity of the alloy is improved by modifying the morphology and distribution of MC carbides. The modification of carbide morphology can be attributed to the enrichment of Y at the surface of carbides and the change of segregation of Nb, which is the main carbide-formation element.

Oxidation behavior and corrosion properties of 2-GPa grade Co-reduced ultrahigh-strength stainless steel

This study investigated the oxidation behavior and corrosion resistance of 2-GPa grade ultrahigh-strength stainless steel. Results revealed that the addition of V to Co-reduced FerriumS53 (7Co1V) accelerated the precipitation of MC carbide during aging, while inhibiting the precipitation of Cr-rich M23C6 carbide. A significant amount of Cr was predicted in the microstructure of the alloys, and this microstructure increased the partitioning of Cr to the alloy surface and provided enhanced resistance against corrosion pitting. These findings suggest that the addition of V into Co-reduced stainless secondary hardening steels can enhance their oxidation and corrosion resistance without compromising their mechanical properties.

Evolution Behavior and Fracture Criterion of Carbides in D2 Cold‐Work Die Steel During Hot Working Process

Investigating the fracture mechanisms of carbides is of paramount importance for optimizing hot working processes in die steels. This work establishes a comprehensive criterion equation to predict the behavior of carbide fracture in D2 cold‐work die steel during hot working process. High‐temperature laser confocal microscope is employed to prepare samples with varying solidification rates. Subsequently, these samples are subjected to a range of heating temperatures spanning from 1000 °C and 1100 °C. The samples are analyzed utilizing scanning electron microscopy energy spectrum analysis and X‐ray diffraction. The activation energy of M7C3 carbides is calculated through the utilization of kinetic equations and the Arrhenius law. Additionally, a criterion model for the fracture behavior of M7C3 carbides in D2 cold‐work die steel is established. The results reveal that thin hollow rod‐shaped carbide is easy to fracture. Moreover, the activation energy for phase transition of M7C3 carbides in D2 cold‐work die steels is determined to be 226.974 KJ mol−1. Furthermore, the fracture behavior of carbides in D2 cold‐work steel is found to be influenced by various factors such as heating temperature, deformation rate, and peak stress. Finally, the criterion model demonstrates substantial agreement with the experimental observations gathered, affirming its reliability and accuracy.

The microstructure and mechanical performance optimization of a new Fe-Ni-based superalloy for Gen IV nuclear reactor: The critical role of Nb alloying strategy

The effects of Nb alloying on microstructural evolution and mechanical performance of a newly developed solid-solution strengthened Fe-Ni-based superalloy, considered as the candidate material for high-temperature components manufacturing in fourth Generation (Gen IV) advanced nuclear reactor, were systematically investigated. The results show that the increase in the Nb content obviously increased the volume fraction and average size of MC carbides, and the MC carbides more preferentially distributed at grain boundaries. The average grain size and low Σ coincident site lattice (CSL) boundary exhibited a gradually decreased trend with increasing Nb content. Meanwhile, the thermal stability (grain size and grain boundary precipitation) of the alloy can be improved notably via Nb alloying. The tensile strength of Nb-alloyed alloys at 750 °C was enhanced significantly compared to that of Nb-free alloy, mainly due to the synergistic effect of solid-solution strengthening and fine grain strengthening. It was impressive that the variation of Nb content had no obvious influence on the yield strength, but gradually decreased the ultimate tensile strength. The detrimental effects of the decrease in Σ3 boundaries and intergranular precipitates were assumed to restrict the strengthening factors. The ductility of Nb-alloyed alloys decreased remarkably due to the changed dominant deformation mechanism from planar slip to wavy slip; however, it showed an increasing trend in high Nb-content alloys due to the occurrence of dynamic recrystallization and grain rotation. The present work establishes Nb alloying as an effective method to optimize the microstructure and mechanical properties of solid-solution strengthened Fe-Ni-based superalloys, and the appropriate Nb content is recommended to be controlled below 0.5 wt% for this alloy.

Corrosion Behavior of SiMo Ductile Cast Iron in Different Corrosive Environments

Standard SiMo ductile cast iron automotive alloys are typically subjected to various hostile environmental conditions. For the current investigation, SiMo with a silicon content of 5% and molybdenum contents ranging from 0 to 1.5% was used. In two corrosive environments of 0.6 M NaCl and 0.5 M H2SO4, the study intends to present the corrosion behavior of high silicon molybdenum ductile cast iron (SiMo). Moreover, the impact of changing Mo on the microstructural characteristics has been investigated. The SEM-EDX examinations revealed that the nodule counts, M6C carbides, and lamellar pearlite increased while the ferritic matrix decreased with increasing Mo contents. It is clear that the addition of Mo reduces cast iron's rate of corrosion and thus increases its corrosion resistance. The results showed that SiMo cast iron alloy with 1.0 wt% Mo had a lower corrosion current (Icorr) in 0.6 M NaCl solution while lower corrosion current (Icorr) with the cast iron containing 1.5 wt% Mo in 0.5 M H2SO4 solution, which resulted in the lowest corrosion rate. The occurrence of a galvanic couple between the alloy matrix and the graphite nodules results in electrochemical corrosion, with the largest corrosion rates taking place at Mo-free alloy in both media.

Reactivity of PVD cathodic arc coated hardmetal tools with Inconel 718: Correlation between diffusion couples and tool wear in drilling tests

The reactivity of Al0.66Ti0.33N, Al0.31Ti0.62Si0.07N, Al0.60Cr0.40N, and Al0.38Cr0.54Si0.07N cathodic arc-coated hardmetal tools with Inconel 718 has been evaluated by using diffusion couple experiments at 900 °C and 1100 °C. As expected, a strong reaction occurs at these temperatures between Inconel 718 and uncoated hardmetal samples leading to the formation of η phases; mainly M12C and M6C carbides. PVD coatings act as diffusion barriers limiting the presence of such reactions as long as they maintain their structure in contact with Inconel 718 at high temperatures. Diffusion couple experiments confirm that Cr-containing coatings are significantly more unstable than Al0.66Ti0.33N or Al0.31Ti0.62Si0.07N materials, thus leading to notable differences in cutting performance. The relation between diffusion couple results and drill wear mechanisms in drilling Inconel 718 with different coating materials are also analysed. The best results are obtained either with Al0.66Ti0.33N or Al0.31Ti0.62Si0.07N coated drills, where the coating remains longer time at the cutting edge leading to more efficient protection, especially at the drill corner.

Scanning precession electron diffraction study of carbide precipitation sequence in low alloy martensitic Cr-Mo-V tool steel

Precipitation of carbides after tempering of a medium carbon low alloyed Cr-Mo-V tool steel at 600°C was studied with scanning precession electron diffraction (SPED) in a transmission electron microscope (TEM). The precipitation sequence was evaluated by mapping the carbide distribution on carbon extraction replicas prepared from samples tempered for different durations of up to 24 h. The SPED results were supplemented by equilibrium calculations and energy dispersive x-ray spectroscopy (EDX) measurements in a TEM. It was found that ε-Fe2C precipitates within martensite laths during quenching via auto-tempering. During reheating to tempering temperature ε-Fe2C was dissolved and replaced by cementite, θ-M3C, which predominately form on martensite boundaries. It was further found that the small carbides in the early stage of tempering have predominantly a cubic MC structure, even if the V/Mo-ratio of the studied steel was only 0.12, and it is known from literature that V and Mo form cubic MC or hexagonal M2C carbides, respectively. Later during tempering more stable carbides, such as M7C3 and M23C6, also form, and it was concluded that the M7C3 form both by separate nucleation and precipitation on the cementite/matrix interface. The latter phenomenon was seen as particles with a core of cementite and a shell of M7C3 after 24 h of tempering.

Effect of austenitizing temperature and cooling rate on Ms temperature of Fe–Ni–Cr cast iron

The effect of Ms temperature variation in Fe–Ni–Cr cast iron has been investigated in this paper. The heating and cooling processes were carefully compared and analysed through in-situ observation. The results showed that the size of austenite grains remained relatively constant throughout the austenitizing process. Moreover, higher austenitizing temperatures were found to facilitate the transfer of carbon from graphite to austenite, which hindered the martensite transformation. The M3C and M23C6 carbide generated during heating and cooling diminished the concentration of carbon and alloying elements in the austenite and enhanced the Ms temperature. The acceleration of cooling rate could impede the precipitation of carbide and result in more significant lattice distortion of austenite, leading to a notable reduction in Ms temperature and an increment in residual austenite content. The augmentation in austenitizing temperature and cooling rate amplified the non-chemical driving force, culminating in a more arduous martensitic transformation.

Effect of normalizing temperature on the strengthening mechanisms of the advanced Re-containing 10 %Cr steel with low N and high B contents

The effect of different normalizing temperature ranging from 1050 to 1200 °C on the structure and tensile properties of the Re-containing 10 %Cr steel was investigated. 1100 °C normalizing with following tempering at 770 °C provides + 10% increments in both yield strength and ultimate tensile strength compared with other normalizing temperatures. Tempering equalizes strengthening contributions due to laths, dislocations and alloying elements in matrix after all normalizing temperatures. Improvement in tensile strength after 1100 °C normalizing is caused by change in the dispersion of M23C6 carbides and NbX carbonitrides that increases particle strengthening by 29%.

Thermal cycling to improve the impact toughness of 1Cr11Ni2W2MoV steel with banded microstructure

Microscopic segregation of chemical elements results in the banded microstructure of finished steel products, and the harmful effect of banded microstructure on toughness is well recognized. However, typical industrial hot work processing, such as hot rolling and forging, cannot erase the banding, and eliminating it with homogenization heat treatment is also challenging and uneconomical. In this study, the impact energy of a 12%Cr tempered martensitic steel with the banded microstructure was significantly improved from 74 J to 154 J by employing thermal cycling and tempering as the final heat treatment to meet the service performance requirement. Compared to conventional quenching and tempering (Q + T), thermal cycling and tempering (TC + T) simultaneously refined the effective grain size of tempered martensite matrix and coarse M23C6 carbides that were continuously distributed along the δ-ferrite/matrix phase boundaries. As a result, more high angle grain boundaries (HAGBs) were introduced and acted as obstacles in retarding the propagation of microcracks. Additionally, finer carbides precipitated at the banding boundary during tempering, owing to the increasing triple junctions between δ-ferrite and tempered martensite. These finer carbides prevent the initiation of microcracks. The results demonstrate the potential of TC + T as a final heat treatment to remove the detrimental effect of banded microstructure on impact toughness.

On the normal and inverse Portevin–Le Chatelier behavior in hot rolled Inconel 617 superalloy

In the present study the effect of hot rolling process on the normal and inverse Portevin-Le Chatelier behavior was investigated. The hot rolling process was carried out at 950 °C, 1050 °C, and 1150 °C to capture the effect of thermomechanical process on elimination or postponing of dynamic strain aging phenomenon in Inconel 617 during subsequent deformation process. The tensile test was performed within the temperature range between 350 °C and 750 °C at a constant strain rate. The microstructure evaluations were carried out by optical microscope and scanning electron microscope. The serrated flow was observed in the stress-strain curves at specific temperatures. The type of serrations changed from type B to type C at the strain rate of 5 × 10−4 s−1 as the tensile test temperature was enhanced. The change of normal PLC effect to the inverse PLC effect was intensified when the hot rolling temperature was increased; Hence, the inverse PLC effect was observed at 750 °C in the sample which had rolled at 1150 °C. Whilst the serrated flow was not seen at 750 °C in the samples which had rolled at 950 °C and 1050 °C. The critical strain, for the initiation of the serrated flow was found to reduce with the increasing temperature of hot rolling from 950 °C to 1150 °C. After tensile test, the microstructural evaluations revealed the proven substantial role of precipitation of γ′ and the Cr-rich M23C6 carbides in pinning of the mobile dislocations, which led to the inverse PLC effect.

Formation and evolution behavior of M6C carbide in a Ni-W-Cr superalloy

The carbide formation and evolution of a new Ni-26W-6Cr (wt.%) superalloy for the 800 °C molten salt reactor (MSR) was investigated by homogenization heat treatment tests and the effect of carbide characteristics on the plastic deformation behavior was further investigated by isothermal simulated tensile tests. The plastic deformation properties of Ni-26W-6Cr superalloys are associated with the morphological characteristics and distribution of carbides. During homogenization at 1220 °C, a unique phenomenon was discovered, that is, the precipitation of the acicular carbide. The XRD, TEM and EBSD results show that this acicular carbide is distributed along the particular crystal plane of the γ matrix. Based on crystallographic theory, there is no obvious orientation relationship between the carbide and the γ matrix, which is prone to cracking along the carbide/γ interface during deformation. After homogenization at 1220 °C for 48 h, the acicular M6C carbides dissolved and the discrete granular M6C carbides distributed diffusely at the grain boundaries can relieve stress concentration and strengthen the grain boundaries, reducing the deformation resistance of the alloy, effectively inhibiting grain boundary slip and thus inhibiting crack propagation, and improving the tensile strength and plasticity of the material.

Analysis of the As-Cast Microstructure and Properties of the Ni-Based Superalloy MAR-M247® Produced Via Directional Solidification

The presented research investigates MAR-M247® Ni-based superalloy castings produced via directional solidification at various mold preheating temperatures (1510, 1566 °C) and withdrawal rates (3.4, 5.0 mm/min). Casting analyses were carried out via thermodynamic simulations, differential scanning calorimetry (DSC), X-ray diffraction (XRD), light microscopy (LM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), scanning transmission electron microscopy (STEM), and tensile testing. On DSC curve, four effects have been registered during cooling: liquidus (1337 °C), formation of eutectic γ − γ′ (1315 °C), precipitation of Ni7(Hf, Zr)2 (1244 °C), and M5B3 borides (1201 °C). The castings’ primary and secondary dendrite arm spacing decreases with increasing withdrawal rates for both shell mold temperatures. The dendritic regions of the castings are characterized by a relatively homogenous microstructure, consisting of γ′ precipitates surrounded by the matrix, with a mean size in the range of 0.437 to 0.481 μm, depending on the casting parameters. In the interdendritic spaces, γ − γ′ eutectic, MC carbides, M5B3, Ni7(Hf, Zr)2, and M3C2 phases were identified. The ultimate tensile strength of the produced castings was in the range of 970 to 1088 MPa.

Ti and Nb alloying effect on the mechanical property of M7C3 carbide in 9Cr18Mo stainless steel: First-principles calculation

The effect of Ti and Nb alloying on the mechanical property of M7C3 carbide has not been thoroughly explored till date. Based on first-principle calculations, several parameters such as binding energy, mechanical properties and electronic structure were investigated and calculated systematically in this work. The results show that the Ti and Nb preferably substitute the Cr sites instead of Fe, due to the smaller formation enthalpy associates with Cr sites. Also, the substitution of Cr with Ti and Nb atoms lead to improve ductility of Fe3Cr4C3 and Cr3Fe4C3 phases. The ductility of the carbide phases could be improved as the G/B ratio decreases by alloying M7C3 carbide with Ti and Nb. The Cauchy pressures involved in substitution of Cr by Nb or Ti are larger than the pressures associated with the substitution of Fe in Cr3Fe4C3 or Fe3Cr4C3 phases. Furthermore, the initial anisotropy degree A increases to 0.9–1.1 after alloying with Ti or Nb. Mulliken population shows the presence of large amount of metallic bonds when the Cr is substituted with Ti or Nb elements in M7C3 carbide. The DOS shows a drop in intensity peaks after Ti or Nb alloying process. Thus the improved ductility associated with the Ti or Nb alloying process results from the interactions between Ti–Cr or Nb–Cr metallic bonds.

Corrosion Resistance Enhancement of Selective Laser Melted Cost‐Effective Interstitial Medium‐Entropy Alloy via Deep Cryogenic Treatment

An economically interstitial FeMnCrNiCu medium‐entropy alloy (MEA), which exhibits preferable corrosion resistance in 3.5 wt% NaCl solution, is fabricated by selective laser melting (SLM). Compared with annealing, which is one of the posttreatments for additive manufacturing, deep cryogenic treatment (DCT) also contributes significantly to the alloy properties and is expected to improve corrosion resistance. Herein, the corrosion behavior of SLM‐MEA after DCT in 3.5 wt% NaCl solution is investigated. DCT improves corrosion resistance by promoting the uniform distribution of precipitates in SLM‐MEA and changing the cation ratio in the passive film. After DCT, the corrosion current density of the SLM‐MEA in original and annealed state decreases by 27.07% and 62.69%, respectively. The annealed SLM‐MEA exhibits the worst corrosion resistance as a result of the precipitation of continuous Cr‐rich M23C6 carbides at grain boundaries. The face‐centered cubic structure of MEA does not change after annealing or DCT. Significantly, the number, size, and distribution of the precipitates are greatly affected. DCT after annealing promotes the uniform diffuse distribution of M23C6 carbides and nanoscale Cu‐rich particles while increasing the Cr+Ni/Fe+Mn ratio in the passive film, resulting in a significant improvement in corrosion resistance.

Effect of C content on microstructure and hot deformation behavior of a Ni-based superalloy

In this work, isothermal compression tests of a Ni-based superalloy were carried out at γ′ sub-solvus (1140 ℃) and γ′ super-solvus (1170 ℃) temperatures under a true strain 0.693. The effect of C contents (0.01 wt%, 0.04 wt% and 0.08 wt%) on the microstructural evolution and hot deformation behavior of the alloy was investigated. The formation of MC carbide is closely related to C content. Besides, the initial grain size decreased slightly with the increase of C content due to the effect MC carbides. During compression deformation at 1140 ℃, the peak stress increased with the increase of C content. When deformed at 1170 ℃, the 0.04 wt% C alloy has the lowest peak stress, because of the acceleration effect on dynamic recrystallization DRX by MC carbides. However, the 0.08 wt% C alloy has the highest peak stress, the dislocation density also increases with the number of MC carbides increase，which caused high working hardening rate and increased the peak stress in 0.08 wt% C alloy. The discontinuous dynamic recrystallization (DDRX) is the dominant mechanism during deformation at 1140 ℃, while the DDRX and continuous dynamic recrystallization (CDRX) occurred at the same time when deformed at 1170 ℃.

Comparative study on microstructure and mechanical properties of a novel nano-composite strengthening heat-resistant steel and two typical heat-resistant steels

In this study, a new nano-composite strengthening austenitic heat-resistant steel designed for ultra-super critical (USC) thermal power plants was analyzed in comparison with the traditional H3RC and S30432 heat-resistant steels to assess its reliability. The microstructure, precipitates, hardness, tensile property and low cycle fatigue behaviors of the three steels at room temperature and service temperature were characterized. The results show that the SP2215 has finer grains, higher twin density, more primary MX, NbCrN and M23C6 precipitates compared with the HR3C and S30432 steels. The main precipitates in the steels after solution treatment are secondary (Nb, V)(C, N) phase (MX). The subsequent aging treatment of the SP2215 steel results in precipitation of the M23C6 carbides at the grain boundaries and Cu-rich phase inside the grains. As a result, the SP2215 steel shows higher Vickers hardness, tensile and fatigue strength (400 MPa) at room temperature than the other two steels. Transgranular fracture is the dominant tensile fracture mechanism of the three steels at room temperature, while intergranular cracking becomes the main fracture mode at high temperature due to precipitation of the continuous M23C6 at the grain boundaries. At the temperature of 700 ºC, fatigue strength of the SP2215 under a peak stress lower than 450 MPa decreases only a little.

Microstructural study of keyhole TIG welded nickel-based superalloy G27

The weld fusion zone (FZ) microstructure obtained after keyhole tungsten inert gas welding and post-weld solution heat treatments (PWSHTs) of a new nickel (Ni)-based superalloy called G27 is studied, and the grain growth behavior in the base material (BM) during PWSHTs is characterized. Microsegregation-induced interdendritic microconstituents in the FZ of as-welded G27 are identified by analytical (scanning) transmission electron microscopy ((S)TEM) as niobium (Nb)-rich MC carbides, Nb-rich Laves eutectic constituents, γ’ and η phases. The Laves eutectics are generally considered brittle and can adversely affect the mechanical properties of the weldment; thus, an hour PWSHTs were performed at 954 °C–1060 °C to eliminate the γ/Laves eutectics. PWSHT up to 1010 °C results in only partial removal of Laves eutectics with an excessive formation of η phase surrounding the Laves phase. Complete dissolution of Laves eutectics with no η phase formation is achieved after a PWSHT is performed at 1060 °C. Relative to INCONEL® alloy 718, the complete elimination of the γ/Laves eutectic constituents in the FZ of G27 through a PWSHT was proven to be achieved without causing excessive grain growth in the BM, which could be due to the pinning effect of the fine molybdenum (Mo)-rich precipitates, that are formed during solution heat treatment and identified as hexagonal close-packed phase particles through extensive (S)TEM analyses.

Effect of Heat Treatment Schedules on Creep Performance of Ni-Based Superalloy Mar-M247 at 871 °C and 250 Mpa

The effects of heat treatment (H1 and H2) on the creep behavior and microstructures of Mar-M247 at 871 °C/250 MPa are studied. The results show that the as-cast microstructure is composed of eutectics, γ phase, γ’ phase, MC and M23C6 carbides, while new M6C appears in heat-treated microstructures, indicating the transformation of carbides after heat treatments. The Mar-M247 is excellent, with over 1500 h of creep life, and H1 is 48% higher than H2. The addition of post-brazing and diffusion heat treatment in H2 is detrimental to creep resistance; the two steps promoted the transformation of MC into M23C6 in advance. The increase and coarsening of M23C6 would consume more γ-phase-forming elements, weakening the solution strengthening at grain boundaries. As a result, the resistance of the grain boundary and γ/γ’ interface to dislocation motion is significantly reduced, leading to the cracks’ initiation and propagation along the grain boundaries.

Microstructures, mechanical properties, and oxidation behavior of in-situ ultra-fine duplex carbides reinforced Ni-based superalloy composites

A Ni-based superalloy composite (FTSC) was successfully prepared by spark plasma sintering with Ti3SiC2 (15 vol%) and FGH97 alloy powders as precursors. During sintering, the weakly bonded Si layers de-intercalate from Ti3SiC2 and the remaining TiC is eroded by the elements of the matrix, which promoted in-situ precipitation of MC and M6C carbides. MC with core-shell structure builds up a semi-coherent interface with the alloy matrix: (1¯11¯)γ/γ'‖(11¯1)MC with [110]γ/γ'‖[110]MC. The Zener pinning effect of ultra-fine duplex carbides results in refinement of alloy grains and notably improvement in mechanical strength. Comparing to the alloy FGH97, the composite FTSC possesses 55% higher microhardness, 100% higher compressive yield strength and 92% higher flexural strength. On the other hand, the refined grains ensured fast formation of a protective alumina scale at surface, which provides high oxidation resistance for the composite.