MC-type Carbides Research Articles

M7C3: The story of a misunderstood carbide

Since the discovery of the M7C3-type carbide in 1935, it has been the subject of many studies owing to it extensive usage in wear sensitive applications. Despite the technological importance of (Cr,Fe)7C3 carbides, attempts at using computational methods for tailoring its growth characteristics, stability and mechanical properties are hindered by the ambiguity associated with its atomisitic structure. The presented work aims at removing the former ambiguity regarding the atomistic structure of M7C3-type carbides. To this end we utilized EBSD (electron backscattered diffraction), TEM (transmission electron microscopy) diffractions and high-resolution STEM (scanning transmission electron microscopy) imaging, gathered from 9 zone-axis of the (Cr,Fe)7C3 phase formed in the as-cast AlCrFe2Ni2 high entropy alloy. Our experimental findings, supplemented by DFT calculations, indicates that only the hexagonal structure is a viable option. Additionally, we show that a carbon atom has to populate the center of a octahedral complex, a cite which was previously considered vacant. Furthermore, we demonstrate the co-existence of two variants of the newly revealed atomic structure which we believe were mistakenly identified as stacking faults in previous studies. The higher structural complexity of the newly identified structure, strengthened by the existence of spatially alternating variants, is suggested to partially justify the preference of this structure over the orthorhombic one. We believe that this study, and its like, are vital for aiding computational efforts aimed at controlling the growth, stability and performance of (Cr,Fe)7C3 carbides.

Effect of carbon alloying on microstructure and mechanical behaviors of Fe35Ni35Cr20Mn10 multi-component alloy

The effect of carbon-alloying on the microstructure and mechanical behaviors of Fe35Ni35Cr20Mn10 multi-component alloy (MCA) was studied by microstructure characterization and tensile test. In as-cast (Fe35Ni35Cr20Mn10)98.5C1.5 MCA, M23C6-type carbide (M = Cr, Fe, Ni, Mn; more Cr contained) formed at the grain boundaries has a coherent relationship with the FCC matrix. After a homogenization treatment at 1000 °C for 12 h, the carbide phase formed during the solidification course becomes coarsened and rounded by partially re-dissolving, and, at the same time, a great number of blocky M23C6-type particles (about 50–200 nm) are coherently precipitated in a large area around the grain boundaries. In as-homogenized state, carbon-alloying induced fine-grain strengthening, solution strengthening and coherent precipitation hardening of nano carbide particles greatly increase the yield strength from 198 MPa to 480 MPa. The carbon-alloying induced grain refinement and coherent precipitation of nano carbide particles and the formation of dislocation cells and deformation twins during the tensile deformation are thought to be responsible for the high plasticity of (Fe35Ni35Cr20Mn10)98.5C1.5 MCA (28.3% in elongation). Microstructure evolution at different strain stages and the working hardening rate curves demonstrate that addition of carbon in Fe35Ni35Cr20Mn10 MCA system increases the critical stress for the formation of deformation twinning and thus increases the stacking fault energy.

Tensile deformation behavior and generalized stacking fault energy surface of γ-Fe23C6 by atomistic modelling

To investigate the fatigue behavior of steel or Ni-based superalloy, it is necessary to identify the mechanism of crack initiation and non-uniform deformation by studying the mechanical properties of M23C6 carbides. The effects of temperature, vacancies, and void on the mechanical properties of M23C6 (M = Fe) carbides are investigated by the molecular dynamics method. Furthermore, in order to identify the slip system which is the most conductive to dislocation activity for M23C6 type carbides, based on the generalized stacking fault energy (GSFE) model of Fe23C6, the three-dimensional GSFE surface is calculated. The results show that Fe23C6 is more prone to cleavage fracture along the [110] direction than along the [100] and the [111] directions at the same temperature; the void size has a great effect on the tensile stress curve of Fe23C6 along the [100] direction but has little effect on the tensile stress curve along [110] and [111] directions. The slip of Fe23C6 on the (111) plane tends to follow the [11 2‾] direction, and Fe23C6 is more likely to produce the dislocation structure on the (111) plane than on the (110) plane.

Hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening of the micro-alloyed high entropy alloy

We report the annealing time-dependent microstructures and deformation mechanisms of the novel face-centered cubic Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 HEA. Three types of precipitates, σ-phase, Cr-rich MC-type carbides, and nano-scale (Ti, V, Mo)C, are present after cold-rolling and annealing at 600 °C. Such hierarchical precipitates could lead to sluggish recrystallization and grain growth upon annealing. The partially recrystallized microstructures and hierarchical precipitates could lead to a high yield strength even for prolonged annealing conditions. Deformation mechanisms change with annealing time. The materials annealed for short times (< 2 h) are deformed by dislocation glide, deformation twinning, and deformation-induced ε phase. A longer annealing time (> 10 h) triggers a multi-variant ε phase, reverse transformation from ε to γ, and the multi-step sequential transformation, γ → ε → reverse transformed γ from ε → ε transformed from the reverse transformed γ. Further, materials annealed for longer times shows a higher contribution of back stress strengthening, which could be attributed to the increase in γ/ε and γ/σ interfaces. The activation of various deformation mechanisms and high back stress strengthening could lead to a superior strain hardening capacity and strength-ductility combination (YS: 699 MPa, UTS: 1041 MPa, TE: 45%) of the material annealed for 10 h. The present work provides the novel microstructure design solution of the metastable high entropy alloys with exceptional mechanical properties, utilizing hierarchical precipitates, sequential deformation-induced phase transformation, and enhanced back stress strengthening.

In-situ radiation response of additively manufactured modified Inconel 718 alloys

In this study, a novel alloy of modified Inconel 718 produced by laser powder bed fusion is studied before and after in-situ Kr irradiation up to 3 dpa at 200 and 450 °C. Before irradiation, the microstructure consists of dislocation cells having a misorientation angle less than 5° and with an average size of ~500 nm. There are also second phase particles of MC type carbides, Laves phase and oxides such as Y-O, Y-(Ti)-Al-O. While the microstructure consists of stacking fault tetrahedra, faulted and perfect loops after irradiation at 200 °C, dislocation loops are the primary defects at 450 °C. With increasing dose, the size of the defects remains similar at 200 °C while it increases at 450 °C. This has been attributed to the existence of vacancy type defects at 200 °C and the different defect transport mechanisms at different temperatures. Moreover, matrix and second phase particle compositions seem to be similar after irradiation. The sink strengths of the structures have been calculated and superior radiation resistance of this alloy has been attributed to the existence of fine cell boundaries stabilized by the second phase particles produced by additive manufacturing.

Effect of progressive solid-solution treatment on microstructures, mechanical properties and impact abrasive wear behavior of alloyed high manganese steel

Alloyed high manganese steel is a general type of wear-resistant steel, and its precipitates and austenite matrix grain sizes play an important role in impact abrasive wear behavior. Solid-solution treatment is an effective method to balance the size of the austenite grains and precipitates. This work attempts to find a new progressive solid-solution treatment to make alloyed high manganese steel castings refine the microstructure and balance the wear resistance under impact abrasive wear conditions. Compared with traditional solid-solution treatment, the MC- and M23C6-type carbide precipitates resulting from progressive solid-solution treatment decrease, and the tensile strength and impact toughness of steel are significantly increased. After the solid-solution treatment at different heating rates, the grain refinement effect is remarkable after shortening the holding time at 1100 °C and increasing the heating rate to 150 °C h−1. The impact abrasive wear test results show that, compared with the traditional solid-solution treatment at 1100 °C for 4 h, the wear resistance of high manganese steel has a 23.1% improvement after progressive solution treatment at 1100 °C for 2 h, and the wear failure forms of all the experimental steels are microcutting, oxidation wear and plastic deformation.

Effects of a modified heat-treatment on microstructure and mechanical properties of additively manufactured Inconel 718

The effectiveness of a modified heat treatment on the microstructure and mechanical properties of Inconel 718 (IN718) fabricated by the laser powder bed fusion (LPBF) technique was investigated in this work. A modified solution heat treatment (MSHT) conducted at 1160 °C for 4 h was applied to both a specimen printed in the X–Y orientation as well as another printed in the X–Z orientation. Following MSHT, the specimens were treated in two stages at 720 °C for 8 h and then again at 620 °C for 8 h. Microstructural data such as grain morphology, size, recrystallization, and texture were studied along with mechanical properties. The MSHT treatment was successful in removing the Laves phase, cellular-dendritic structure, and segregations but the metal carbides (MC-type carbides) remained around grain boundaries. Additionally, recrystallization and grain growth led to an equiaxed microstructure for both specimen orientations. Precipitated γ′ and γ″ phases were revealed within the microstructure via transmission electron microscopy (TEM) following the two-stage aging process. Micro and nanoindentation measurements were performed to validate mechanical property evolution. Hardness values decreased following the MSHT treatment but greatly improved following the aging procedure which is attributed to the dissolution of microstructural inhomogeneities during MSHT and subsequent precipitation of uniformly distributed fine γ′ and γ″ particles following the aging stages. The microstructure and mechanical properties of additively manufactured IN718 can be significantly improved through use of proper solutionizing and aging heat-treatments.

Numerical simulation and experimental study on production of high-speed steel powder by high-frequency induction melting gas atomization

A new method (electrode induction gas atomization, EIGA) of producing high-speed steel powder was preliminarily studied by a combination of numerical simulation and experiment. Based on COMSOL Multiphysics® software, the effect of various parameters including coil angle, output frequency and power of the electrical source on flux density, induced current, temperature field and phase field was simulated. Meanwhile, the experiment was carried out on the EIGA device to produce high-speed steel powder. The results of FEM simulation indicate that when the coil angle is 30°, there is the highest thermal efficiency on the electrode cone, and the induced current and temperature will increase as the output frequency and power of the electrical source increase. In addition, the powder experimentally obtained by the EIGA method exhibits good particle sphericity regardless of the size, with a median diameter (D50) of 71.4 μm and a low oxygen content of 81 μg/g. The phase composition of the powder is mainly composed of γ-Fe and α-Fe structures and MC-type carbides. Due to the faster cooling rate, a solidification microstructure consists of fine cellular crystals and dendrites, and no coarse eutectic carbide network is observed, also confirmed by EDX elemental mapping.

Effect of MC-type Carbide on Belag Formation of High Speed Tool Steel

Effect of MC-type Carbide on Belag Formation of High Speed Tool Steel

Exploration of the competing failure during the stress rupture process for 9Cr-CrMoV dissimilar welded joint

The competing failure mechanism between the heat affected zone (HAZ) of CrMoV steel and weld metal (WM) was found by the stress rupture tests for the 9Cr-CrMoV dissimilar welded joints. The stress rupture occurred in the HAZ of CrMoV steel for the welded joint by the 9Cr steel without element W, yet the failure location shifted to the WM for the welded joint by the 9Cr steel with element W. The precipitates in the WM were mainly the Cr- and Mo-rich M23C6 type carbides. While the W-rich particles were detected in the region of the WM near the fusion line. The partially melted BM2 and its movement into the WM during welding could trigger the formation of the W-rich particles. Thus, the formation of the harder and more brittle W-rich particles in the WM near the fusion line is considered as the crucial factor to result in the failure transition.

Microstructure and phases structure in nickel-based superalloy IN713C after solidification

The effects of cooling rate on microstructure and microtexture development during solidification of IN713C nickel-based superalloy have been studied with a range of cooling rate from 0.3 °C/s to 4 °C/s (air cooled). The results showed an increase in grain size (from 140 μm to 2 mm) and carbides volume fraction (1.3% to 2%) with increasing cooling rate. It was also found that primary γ′ size decreased with increasing cooling rate. Furthermore, it was demonstrated that the dendrite structure was weakened with decreasing cooling rate. The dendrite structure was nearly eliminated at slowest cooling rate applied here (0.3 °C/s). Eutectic clusters that mainly consisted of large γ′, Ni7Zr2, M3B2 type boride and/or MC-type carbide precipitates were identified in air cooled samples. These undesirable clusters presented specific morphologies and their density (volume fraction) was found to decrease with decreasing cooling rate. Moreover, predominant phase separation of γ′ is observed in the slowest cooling sample and in interdendritic areas in the fastest cooling sample. Hardness test results showed a slight decrease in hardness with decreasing cooling rate. This may result from the combined effects of detrimental effect of larger γ′ and beneficial effect of smaller grain size in slowest cooling sample. The observations demonstrated here can contribute greatly to the modification of standard investment casting parameters that applied during IN713C turbine wheels production for turbochargers.

Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

Effect of MC Type Carbides on Wear Resistance of High Wear Resistant Cast Iron Rolls Developed for Work Rolls of Hot Strip Mills

Structural evolution of contact parts of the friction stir processing heat-resistant nickel alloy tool used for multi-pass processing of Ti6Al4V/(Cu+Al) system

Interaction between nickel base superalloy ZhS6U used as a friction processing tool for in-situ synthesis of intermetallics in Ti6Al4V and this titanium alloy metal has been studied. Severe abrasion-adhesion wear was localized on the tool’ pin areas while shoulder was protected by mechanically mixed layer. Generation of titanium enriched mechanically mixed layers (MML) on the tool's surface was observed whose chemical and phase compositions corresponded to those of Ti2Ni and Ni4Ti3 intermetallic compounds. Fine-crystalline transition layer (TL) was found below the MML which contained high amount of tungsten in the form of fine MC carbide network. Dissolution of initial M6C carbides and diffusion of their components to the MML where new MC-type carbide network formed after stopping the FSP. The MML fragments detached from the tool surface and then were intermixed with the titanium alloy stir zone.

Metastability engineering of partially recrystallized C-doped non-equiatomic CoCrFeNiMo medium-entropy alloy

Ferrous medium-entropy alloys (FeMEAs) are coming into attention these days for their excellent mechanical properties. Most of the FeMEAs developed so far form metastable face-centered cubic (FCC) matrix, and “metastability engineering” that utilizes deformation-induced martensitic transformation (DIMT) from FCC to body-centered cubic (BCC) as a method to enhance work hardenability has been the key to the exceptional mechanical behaviors. However, the FeMEAs have a significant weakness: low yield strength compared with high tensile strength and ductility. In this study, partial recrystallization is presented as a solution to the current drawback of the FeMEAs. A Co18.5Cr12Fe55Ni9Mo3.5C2 (at. %) FeMEA was annealed at 800 °C for 10 and 30 min and partially recrystallized microstructures with relatively coarse non-recrystallized grains that contain profuse mechanical twins and ultrafine recrystallized grains were attained. In addition, nanosized Cr-rich M23C6-type and Mo-rich M6C-type carbides were precipitated during the annealing. The partially recrystallized FeMEA showed a yield strength of ∼1.07 GPa, significantly enhanced from ∼600 MPa of the recrystallized counterpart. Dislocation strengthening, precipitation strengthening, grain boundary strengthening, and twin boundary strengthening led to the improved strength of the partially recrystallized FeMEA. Back stress hardening owing to the heterogeneity also contributed to the high strength and work hardenability. Moreover, the transformation-induced plasticity effect from the FCC-to-BCC DIMT activated by BCC nucleation at defects within the non-recrystallized grains effectively enhanced the work hardenability, leading to ∼1.34 GPa of tensile strength and ∼30% of elongation. This study provides an insight to optimize the microstructure and corresponding mechanical properties of metastable metallic materials.

Mechanism of M23C6 → M7C3 carbides reaction of Cr35Ni45Nb type alloy during carburization

The carbide transformation of Cr35Ni45Nb type alloy during service has been investigated in the present work. The primary carbide of the as-cast Cr35Ni45Nb type alloy is M7C3(M is mainly Cr element) and NbC, which transformed into M23C6 type carbides (M is mainly Cr element) and G phase(Ni16Nb6Si7) after service, respectively. The G phase and M23C6 carbides mix together and grow up with each other during service. After carburization, the crystal structure of M23C6 type carbide changes again and transforms into M7C3 type carbide. The mechanism of M23C6 → M7C3 carbide reaction is an in situ transformation; the M7C3 type carbide preferentially nucleates at the interface of M23C6/γ and grows toward the M23C6 type carbide interior. With the diffusion of free carbon atoms into the M23C6 type carbides, the M23C6 type carbides are gradually surrounded by the M7C3 type carbides until they are completely changed into M7C3 type carbides.

Investigation of the fatigue damage mechanism of Inconel 617 at elevated temperatures obtained by in situ SEM fatigue tests

In the present study, fatigue damage evolution of Inconel 617 at 600 °C and 700 °C was investigated by using an in situ scanning electron microscope (SEM) fatigue test. The fatigue crack initiation and propagation processes were directly observed and recorded online. Fractography and microstructure analyses were performed to further reveal the mechanism of high-temperature failure. Experimental results showed that Inconel 617 was not sensitive to an artificial notch even if the curvature reached approximately a hundred microns, and it was more vulnerable to intergranular fracture. This alloy showed multiple crack initiation and propagation behavior at high temperatures. Multiple crack initiation sites did not occur in the same planes, and the number of crack initiation sites increased with increasing environmental temperature. Intergranular cracking behavior was observed in the near surface region, and transgranular crack initiation and propagation were mainly observed in the interior. Coarsening and discrete M23C6-type carbides formed at grain boundaries were the main factors responsible for intergranular cracking. The twin boundary (TB) was directly observed as the crack initiation site, and it retarded crack propagation along the TB and changed the crack propagation mode.

Microstructure and Mechanical Properties of Carbides Reinforced Nickel Matrix Alloy Prepared by Selective Laser Melting.

Selective laser melting was used to prepare the ceramic particles reinforced nickel alloy owing to its high designability, high working flexibility and high efficiency. In this paper, a carbides particles reinforced Haynes 230 alloy was prepared using SLM technology to further strengthen the alloy. Microstructures of the carbide particles reinforced Haynes 230 alloy were investigated using electron microscopy (SEM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). Meanwhile, the tensile tests were carried out to determine the strengths of the composite. The results show that the microstructure of the composite consisted of uniformly distributed M23C6 and M6C type carbides and the strengths of the alloy were higher than the matrix alloy Haynes 230. The increased strengths of the carbide reinforced Haynes 230 alloy (room temperature yield strength 113 MPa increased, ~ 33.2%) can be attributed to the synergy strengthening including refined grain strengthening, Orowan strengthening and dislocation strengthening.

Suppression of liquation cracking susceptibility via pre-weld heat treatment for manufacturing of CM247LC superalloy turbine blade welds

In this study, the weldability of as-cast CM247LC superalloy was metallurgically evaluated for turbine blade applications in terms of its hot cracking behavior and susceptibility. For this purpose, a real blade was manufactured using a directional solidification casting process, and gas tungsten arc welding was performed at the tip and cavity of the upper blade. Hot cracking was confirmed in the heat-affected zone of the gas tungsten arc welds, and the cracks were characterized as liquation cracks. Metallurgical solutions to suppress the liquation cracking susceptibility were derived via visualization-based spot-Varestraint test. The alloy subjected to aging treatment exhibited the lowest liquation cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC welds were elucidated via microstructural analyses and thermodynamic calculations. The suppressed liquation cracking susceptibility of the aged CM247LC could be explained as follows: (i) reduced MC-type carbide, which lowered the local solidus temperature compared with the equilibrium solidus temperature of CM247LC and (ii) increased solidus temperature owing to γ’ precipitation γ’ within the γ matrix. To validate the suppressed liquation cracking susceptibility of the aged CM247LC specimen in real welding, it was subjected to gas tungsten arc welding. The results indicated considerable suppression of liquation cracking compared with as-cast welds; in particular, the reduced liquation cracking temperature range was well-reflected in the welding.

Microstructure and Properties of Two Cr8 Cold Working Die Steel

In this paper, we studied the effect on microstructure and properties during low tempering and high tempering temperature of HNC53 and KD11max Cr8 cold working die steel by means of hardness tester, universal tensile testing machine, pendulum impact tester, metallographic microscope, SEM and EDS. The results showed that the microstructure of the two materials were spheroidized pearlite and large eutectic carbide. After quenching then tempering at high temperature 520°C and low temperature 200°C, the microstructure was martensite, eutectic carbide with large particles and small secondary precipitated carbide which was M7C3 type carbide. When KD11max was tempered at high temperature of 520°C, more nano-level secondary carbonization precipitated, and secondary hardening phenomenon was more obvious. The increase of Si and decrease of C could promote secondary hardening and improve the impact toughness of materials. KD11max impact toughness was better than HNC53.

Evaluation and Control of Liquation Cracking Susceptibility for CM247LC Superalloy Weld Heat-Affected Zone via Visualization-Based Varestraint Test

The metallurgical aspects of weld cracking in Ni-based superalloys remain relatively unexplored in existing research. The present study performed comprehensive metallurgical and manufactural investigations into the weldability of an Ni-based superalloy, CM247LC, from the viewpoint of the liquation cracking behavior and its susceptibility. Metallurgical solutions to suppress the liquation-cracking susceptibility were derived via the visualization-based Varestraint test, and the possibility of liquation crack-free welding was explored by employing pre-weld heat treatments and laser beam welding. The alloy that was subjected to aging treatment exhibited the lowest liquation-cracking susceptibility (liquation cracking temperature range: 66 K), while the as-cast alloy specimen exhibited the highest liquation-cracking susceptibility (liquation cracking temperature range: 620 K). The metallurgical mechanisms of the liquation cracking susceptibility of as-cast CM247LC weld were elucidated via microstructural analyses and thermodynamic calculations. The suppressed liquation cracking susceptibility of the aged CM247LC can be attributed to the MC-type carbide fraction and homogenized matrix phase, as compared with those of as-cast CM247LC. The aged CM247LC specimen was subjected to gas tungsten arc welding to validate its minimal liquation-cracking susceptibility. The results confirmed the suppression of liquation cracking, due to the low susceptibility of the specimen. However, crackfree welds could not be obtained. Finally, metallurgically sound welds without liquation cracks were successfully obtained via laser beam welding. The outcomes of the present study will facilitate the generation of electric power from fossil fuels via a clean and efficient gas turbine-based power generation cycle.