MC-type Carbides Research Articles

Grain boundary migration hysteresis during recrystallization of IN738LC alloy fabricated via laser powder bed fusion

The metastable microstructure formed during laser powder bed fusion (LPBF) significantly influences recrystallization behavior, which is crucial for flexible control of microstructure. To investigate these effects, various IN738LC alloys were fabricated by adjusting the volume energy density (VED) and then subjected to the same hot isostatic pressing (HIP) treatment. The microstructures before and after heat treatment were systematically analyzed using multi-scales characterization methods. The results indicate that the recrystallization fraction in HIP-treated samples decreased from 75% to 24% as the VED increased from 53 J/mm3 to 125 J/mm3. This reduction is primarily attributed to the increased grain boundary migration hysteresis caused by the higher density and larger diameter of MC-type carbide particles (i.e., MC particles). Subsequently, these HIP-treated samples were further processed using standard heat treatment (SHT) to evaluate their tensile properties at 23 °C and 900 °C. At 23 °C, the sample with the lowest recrystallization fraction (VED = 125 J/mm3) exhibited the best combination of strength and ductility, with an ultimate tensile strength (UTS) of 1422 MPa, yield strength (YS) of 932 MPa, and elongation of 21%. At 900 °C, the sample with a medium recrystallization fraction (VED = 75 J/mm3) demonstrated the best combination of strength and ductility, with an UTS of 626 MPa, YS of 420 MPa, and elongation of 15.5%.

Effect of rare earth Ce treatment on the microstructure and properties of 430 ferritic stainless steel bearing Sn and Cu

The effects of rare earth Ce on purifying molten steel, modifying inclusions and micro-alloying in 430 ferritic stainless-steel bearings with Sn and Cu has been studied in this paper. The results demonstrate that Ce can purify molten steel, modify inclusions, enhance the mechanical properties and improve the pitting and corrosion resistance of the experimental steel. Ce effectively purifies the molten steel, reducing the total oxygen (T. O) content to as low as 0.00086 wt.% and sulphur (S) content to 0.0020 wt.%. The transformation of inclusions in the steel follows the sequence of MnS·Al2O3 → CeAlO3 → Ce2O2S → Ce2O3 with increasing Ce content. The number of inclusions initially decreases and then increases, with the experimental steel containing 0.012 wt.% Ce showing the best cleaning effect. Additionally, Ce treatment only slightly improves the mechanical properties of the experimental steel, but significantly enhances its pitting corrosion resistance and intergranular corrosion resistance by changing the enrichment state of carbides and impurity elements at the grain boundaries. In detail, Ce can partially inhibit the precipitation of M23C6 type carbides at grain boundaries, thereby improving the intergranular corrosion resistance of the experimental steel.

Preparation and properties of ZTA/alloyed high manganese steel composites

In the long-term use of high manganese steels, it is not desirable to achieve wear resistance by austenite deformation strengthening alone. The wear resistance of high manganese steel is much lower than that of ordinary wear-resistant steel under medium and low stress conditions. Improving the instability of wear resistance caused by defects in high manganese steels is an important problem to be solved in current industrial production. In this study, a new type of high manganese alloy with high hardness and high wear resistance was prepared by multi-alloying with Cr-W-Mo elements on the basis of 20Mn1.2C. ZTA ceramic particles reinforced high manganese steel composites were prepared by selecting high manganese steel materials with excellent mechanical properties as the metal matrix of ceramic reinforced metal matrix composites. The Cr-W-Mo multi-alloyed high manganese alloys are mainly composed of a mixed austenite and ferrite matrix, M3C-type cementite and M6C-type carbides. The Cr2W6Mo6 sample has the best overall mechanical properties. Its hardness reached 49.8 HRC, which is 1.20 times higher than that of the Cr2 sample. The mass loss was 0.2571 g, which is 79.92% of the Cr16 sample (0.3217 g). The ZTA ceramic particles were well composited with the alloyed high manganese steel matrix, and the thickness of the interfacial layer was 20-30 μm, with the formation of Mn2AlO4 and (Fe, Mn)2SiO4, and (Al, Cr)2O3 in the interfacial layer. The best abrasion resistance was obtained at a ZTA content of 40 vol.%. The mass loss was 0.1669 g, which is 51.88% of the Cr16 material (0.3217 g).

Cu-bearing HHCCI interface combined with first principles calculation and corrosive wear resistance

The corrosive wear resistance of copper-bearing HHCCI was tested through experiments and HRTEM, combined with first-principles calculations to study the atomic structure, interface fracture work, thermodynamic stability, electronic structure and bonding structure of six Fe3Cr4C3(01 1‾ 0)/γ-Fe(101) interface models with different termination methods. The HRTEM results show that the interface formed by (01 1‾ 0) of M7C3-type carbide and (101) of the austenite matrix is a coherent interface. The first principles calculation results show that the interface formed by the Fe3Cr4C3(01 1‾ 0)-Cr termination model and the γ-Fe (101)-2 termination model has the highest interface bonding strength. The Fe-end/7Fe-2 interface model is the most stable. Both the interfacial chemical energy and the interfacial elastic energy will affect the overall thermodynamic stability of the Fe3Cr4C3(01 1‾ 0)/γ-Fe (101) interface. The fracture work of Fe3Cr4C3(01 1‾ 0) and γ-Fe (101) is greater than the corresponding interface adhesion work. Moreover, the fracture work on each terminal end of M7C3 along the (01 1‾ 0) surface is higher than that on each terminal end of γ-Fe along the (101) surface. The failure of HHCCI during corrosive wear may mainly occur in the interface area or the side close to the γ-Fe matrix. The chemical bonds in the Fe-end/7Fe interface are mainly Fe-Fe metal bonds, Fe-Cr metal bonds and some Fe-C polar covalent bonds. The chemical bonds in the Cr-end/7Fe interface are mainly Cr-Fe metal bonds and some Cr-C polar covalent bonds. The chemical bonds in the C-end/7Fe interface are mainly composed of Cr-Fe metal bonds, C-Fe polar covalent bonds and part of C-Cr polar covalent bonds, as a result, each interface exhibits different corrosive wear resistance potentials.

Unraveling dynamic recrystallization mechanisms in Ni-based wrought superalloy GH4698 through comprehensive EBSD analysis

A comprehensive examination of dynamic recrystallization (DRX) mechanisms in wrought superalloy GH4698 has been conducted, leveraging Gleeble thermal simulation experiments and advanced Electron Backscatter Diffraction (EBSD) characterization techniques. Rigorous analysis elucidates four distinct recrystallization processes: Discontinuous Dynamic Recrystallization (DDRX), instigated by strain-induced boundary migration (SIBM); Continuous Dynamic Recrystallization (CDRX), characterized by progressive lattice rotation; Particle Stimulated Nucleation (PSN), driven by the Zener pinning effect of MC-type carbides; and Twin-induced Dynamic Recrystallization (TDRX), facilitated by the interaction between twin boundaries and existing dislocations.

Study of mechanical and metallurgical properties of wire arc additively manufactured inconel 718 alloy

The current research work deals with the investigation of mechanical, wetting transition on grain boundary (GB) and metallurgical properties of Inconel 718 alloy used in aerospace industrial applications. The fabrication of thin-wall structure (multilayer structure) is fabricated using wire arc additive manufacturing (WAAM) technique. The novelty of this method is that gas metal arc welding (GMAW) is used as energy source and cold metal transfer (CMT) is used as mode of droplet transfer. Four tensile samples are extracted asper ASTM E8/E8M standards and three microstructure samples are extracted from thin wall structure. Optical microscopy (OM), SEM, EDAX, and EBSD analysis were carried out. In addition, Hardness and tensile tests, as well as fractography analysis were conducted. The grain boundary wetting transition (GBWT) analysis is also carried out. SEM analysis reveals that Nb-rich MC-type carbide contains grain boundaries of droplets, indicating incomplete wetting. Laves and delta (δ) phases are also identified as liquids of a continuous layer separated by Ni elements, indicating complete wetting. EBSD analysis indicates the average grain size as 251.2 μm. The average value of micro hardness as 417.3 HV and UTS as 609.75 MPa are achieved.

Microstructure, hardness, and wear resistance of high-carbon hypereutectic high chromium irons containing W for use as roller sleeves

High chromium irons (HCI) have rarely been investigated the impact of varying W content on the microstructure and performance of high-carbon irons by scholars. In this study, after calculating by Java-based Materials Properties software (JMatPro), the casting alloying method was applied to explore the effect of adding W (0-15 wt%), Mo 3 wt%, and C 6 wt% on HCI to prepare highly wear-resistant and hard materials. The results showed that the maximum volume fraction of carbide was 38% at C 6 wt%. The addition of W promoted the formation of M6C with a maximum volume fraction of 21%, which increased the hardness to 64.5 HRC and reduced the weight loss to 0.062 g. Since W10 (the iron with W 10 wt%) and W15 (the iron with W 15 wt%) exhibited similar wear resistance but lower impact resistance, the W10 samples were heat-treated. W10 after QT550 (quench and temper heat treatment at 550 °C) sample developed a large number of M6C-type carbides and martensite in the matrix, which improved the properties of the matrix by diffusion strengthening, leading to a hardness of 70.0 HRC and a reduction in weight loss to 0.035 g. Compared to the Cr20 reference iron, W10 after QT550 was 22.8% higher in hardness and 76.8% lower in abrasion. Concurrently, an experiment was conducted on the production of roller sleeves. With the use of W10 after QT550, the lifetime of the roller sleeve was increased by 1.6 times.

Effect of aging treatment on the microstructure, cracking type and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam

This study aimed to provide a comprehensive understanding of how aging treatments (namely, HT1 and HT2) affect the microstructure, cracking behavior, and crystallographic texture of IN939 fabricated by powder bed fusion-laser beam (PBF-LB) method. Although both aged samples demonstrated similar grain structure and recrystallization behavior according to the electron backscatter diffraction (EBSD) analysis, as well as the precipitation of bimodal γ′ phase and MC- and M23C6-type carbides, notable differences were observed in the size and morphology, particularly the γ′ phase. The HT1 sample displayed coarsened primary γ′ phase, with sizes reaching up to 2 μm and exhibiting varied morphologies, including irregular and cuboidal shapes. Additionally, this treatment led to the formation of some γ′-γ eutectic regions and plate-like η phase, along with the decomposition of MC-type carbides into M23C6-type carbides. In contrast, the HT2 sample displayed uniformly distributed spherical primary γ′ phase with sizes ranging from 70 to 120 nm, accompanied by very fine secondary γ′ phase. Furthermore, it was found that changes in both aged sample microstructures could result in the formation of strain-age cracks due to the γ′ phase formation and liquation cracks due to the partial remelting of lower melting point phases. The findings also revealed that with the application of aging treatments, the hardness of the as-fabricated sample (339.8 ± 3.4 HV) increased to 440.2 ± 5.6 HV and 508.1 ± 4.8 HV for the heat treatment of HT1 and HT2, respectively.

The effect of direct aging treatment on microstructure and mechanical properties of the GH4099 superalloys produced by selective laser melting

The effect of direct aging (DA) treatment on microstructure and mechanical properties of the GH4099 superalloys produced by selective laser melting (SLM) was investigated by the microhardness tests, room- (RT) and high-temperature (HT) tensile tests, field emission scanning electron microscopy (FE-SEM), and electron backscattered diffraction (EBSD) in this study. Both microhardness and RT tensile results show that DA treatment improves mechanical properties more than solution-aging (SA) treatment. By analyzing the effect of the microstructural features on the yield strength, we found that the dominant strengthening mechanisms responsible for higher yield strength of the DA specimens are the grain size hardening of fine γ matrix, high-density dislocation strengthening, Orowan dislocation bypass strengthening of smaller but dense M23C6-type carbides, and chemical strengthening of smaller but dense γ΄ nanoprecipitates. Furthermore, a transgranular fracture is a dominant fracture type for RT tensile specimens produced by DA treatment. Conversely, the failure type of SA tensile specimens is a typical intergranular fracture mechanism. The underlying mechanism for the above phenomenon is attributed to the effects of transgranular and intergranular M23C6-type carbides.

Microstructure and Mechanical Properties of GH3535 Superalloy Joints Using Electron Beam Welding

To achieve low‐stress and small‐deformation welding of thin‐walled structures for the fourth‐generation nuclear power plants, GH3535 superalloy is welded to itself using electron beam welding (EBW). The microstructure and mechanical properties of the GH3535 superalloy joints are characterized by scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), hardness, and tensile tests. A macro‐defect‐free GH3535 superalloy joint could be obtained using 22 mA welding beam current under 65 kV welding voltage with 300 mm min−1 welding speed. Different sub‐grain solidification morphologies are observed at the weld zone because of the influence of composition undercooling on the solidification process of the weld pool. The M6C type carbides formed in the weld zone and heat‐affected zone (HAZ) take place eutectic transformation under the action of thermal cycles. The eutectic transformation of M6C has no significant effect on the strength of HAZ, whereas this phenomenon is beneficial to hinder the dislocation movement in the weld zone, thereby promoting the tensile strength of the joint. The Vickers hardness of weld zone and HAZ are 250.6 and 252 HV0.5, remarkably lower than that of the base metal (261.2 HV0.5), due to the grain coarsening in those zones. The tensile strength of the GH3535 superalloy joint reached 791 MPa, ≈93.5% that of the base metal, with the fracture occurred in the weld zone.

Investigating the influence of post-deposition heat treatment (PDHT) on the characteristic changes in wire arc additively manufactured 410 martensitic stainless steel thin-wall structure

Obtaining the desired material characteristics of an as-deposited martensitic stainless steel (MSS) structure fabricated by wire arc additive manufacturing (WAAM) is challenging due to several factors like inhomogeneity, precipitation, phase transition, etc. This investigation elucidates the impact of post-deposition heat treatment (PDHT) on the microstructural and mechanical properties of the WAAM-processed 410 MSS thin-wall structure and compares it with the as-deposited 410 thin-wall structure. The MSS thin-wall structure consists of retained austenite (γ), delta (δ) ferrite, martensite laths and Cr-rich M23C6 type carbide. The M23C6 volume fraction increases by up to 18 %, and the γ-phase fraction reduces by up to 40 % by applying PDHT. The PDHT process significantly changed the texture from {110}<011> to {001}<110>. The PDHT process increases the dislocation density (1.974 × 10−15 m−2) than the as-deposited condition (0.924 × 10−15 m−2). The total low energy boundary (LAGB+CSL) contribution of the as-deposited sample is 19.5 %, whereas the PDHT sample demonstrated a higher fraction of low energy boundaries of 32.4 %. The PDHT sample showed a much smaller distribution of grains with an average grain size of 11 μm compared to the as-deposited condition. The overall hardness increases from 468 HV to 487 HV after PDHT compared to the as-deposited condition with lower inhomogeneity across the build direction. The mean 0.2 % proof stress, ultimate tensile stress, and elongation of the PDHT sample increased by 17 %, 5 %, and 60 %, respectively, compared to the as-deposited sample.

Effect of rare earth element on H13 microscopic structure and mechanical properties

In view of the phenomenon that irregular inclusions are easy to use as crack sources, rare earth H13 steel with different gradients is designed. The results showed that ellipsoidal rare earth inclusions (Ce2O3, CeAlO3 and Ce2O2S) were formed from irregular Al2O3 inclusions with the assistance of rare earths. The carbides that precipitated from liquid phases were arranged in a network along the grain boundary, forming composite carbides that are rich in Mo and V. Along the grain boundary, the morphology of the composite carbides got finer, and MC-type carbides enclosed the modified inclusions into nuclei, facilitating their precipitation. After annealing, the microstructure revealed that the carbides were dispersed in black segregated patches along the grain boundary and were discontinuous. The average diameter of carbides decreased from 109.7 to 79.2 nm in the same region according to a statistical comparison of the number of carbides before and after the addition of rare earth elements. The carbides were distributed alongside primary carbides as tiny particles of M7C3 and M23C6 secondary carbides. Especially when the rare earth content was 0.032 wt-%, the impact toughness increased to 34%. This work offers a reference value for the practical application of rare earth in hot work die steel in addition to revealing the strengthening mechanism of ultra-high strength RE-13 steel.

Effect of post-weld heat treatment on microstructure and mechanical properties of twinning-induced plasticity (TWIP) steel joints

The effect of post-weld heat treatment on microstructure and mechanical properties of twinning-induced plasticity (TWIP) steel joints obtained by melting electrode inert gas-shielded welding (MIG welding) was systematic studied. A total of two post-weld heat treatment processes are proposed, i.e., post-weld annealing treatment (PWAT) with temperature/time of 1100 ℃/1 h and subsequent furnace cooling and post-weld solution treatment (PWST) with temperature/time of 1100 ℃/1 h and subsequent water quenching, which were compared with the case of post-weld non-treatment (PWNT). Following annealing treatment at 1100°C, the joint tensile strength dropped by 6.6 % (from 845.46 MPa to 789.85 MPa), while its elongation at break increased by 40.87 % (from 29.80 % to 41.98 %). The significant increase in plasticity was due to the significant elimination of weld stresses in the joints after annealing. Moreover, the stresses in the weld metal of the PWST joints were instead significantly elevated and the carbides were solidified, which resulted in the lower strength. When post-weld heat treatment was applied, the impact toughness of the joints was lower than when untreated. This was primarily due to the greater percentage of the low-angle grain boundaries and the precipitation of MC-type carbides. Consequently, post-weld annealing treatment was an effective method to further enhance the strong plasticity of new twinning-induced plastic steel joints.

Effect of Inconel 718 Filler on the Microstructure and Mechanical Properties of Inconel 690 Joint by Ultrasonic Frequency Pulse Assisted TIG Welding.

Ultrasonic frequency pulse assisted TIG welding (UFP-TIG) experiments were conducted to join Inconel 690 alloy (IN690) by adding Inconel 718 alloy (IN718) as the filler. The effect of the filler on the microstructure, mechanical properties, and ductility dip cracking (DDC) susceptibility of IN690 joints were investigated. The results show that a variety of precipitates, including MC-type carbide and Laves phases, are formed in the weld zone (WZ), which are uniformly dispersed in the interdendritic region and grain boundaries (GBs). The increase in the thickness of the IN718 filler facilitates the precipitation and growth of Laves phases and MC carbides. However, the formation of Laves phases in the WZ exhibits a lower bonding force with the matrix and deteriorates the tensile strength of IN690 joints. Due to the moderate content of Laves phases in the WZ, the IN690 joint with 1.0 mm filler reaches the maximum tensile strength (627 MPa), which is about 96.5% of that of the base metal (BM). The joint with 1.0 mm filler also achieves the highest elongation (35.4%). In addition, the strain-to-fracture tests indicate that the total length of cracks in the joint with the IN718 filler decreases by 66.49% under a 3.8% strain. As a result, the addition of the IN718 filler significantly improves the mechanical properties and DDC resistance of IN690 joints.

Understanding the metal-mold interfacial reactions during directional solidification of superalloys using Al2O3-rich ceramic mold

This study investigates the reactions occurring at the metal-mold interface during directional solidification of Ni-based superalloys, exerting a significant influence on both the surface quality and the desired composition of the alloy. The origin and nature of these reactions are systematically investigated by integrating thermodynamics and microstructural evolution of the alloy. It was observed that Hf from the melt reacts with the Al2O3-based ceramic mold, leading to the formation of HfO2 at the interface. This compound adheres to the surface of the ingot and also infiltrates into the melt near the interface. In order to validate these findings, an exploration of four alternative Ni-based superalloys, both with and without Hf content, was conducted, revealing an absence of comparable reactions at the interface in Hf-free alloys. Additionally, it was explored that the interface formed due to the metal-mold reaction may serve as a preferential nucleation site for MC-type carbides in C-containing Ni-based superalloys. This study concludes that the Hf content is the primary determinant influencing reactions at the metal-mold interface.

Effects of Hf and C on microstructure and mechanical properties of Re0.1HfxTa1.6W0.4(TaC)y refractory medium-entropy alloy

The trade-off between high-temperature strength and room-temperature ductility represents a critical concern for practical application of refractory high-entropy alloys (RHEAs). Here, a set of five Re0.1HfxTa1.6W0.4(TaC)y refractory medium-entropy alloys (RMEAs) were fabricated using the vacuum arc melting technique, and the effect of Hf and C on the microstructures and mechanical properties of these alloys was investigated. These RMEAs exhibit excellent high-temperature strength and favorable plasticity at room-temperature. The microstructures of the Re0.1HfxTa1.6W0.4(TaC)y alloys are hypoeutectic or eutectic structures, consisting of BCC solid solution and primary and secondary carbides. The carbides can be classified into two types: M2C and MC. Hf and C have an important influence on the grain size, carbide type and spatial configuration of the alloy, and in turn, influence the mechanical properties of the alloy. For instance, the content of MC-type carbides is proportional to the addition of Hf element, whereby a higher proportion of Hf results in reduced plasticity at room-temperature and enhanced softening resistance at elevated temperatures in the alloy. The carbides and BCC matrix exhibit synergistic strengthening behavior. The ultimate compressive strength of the five Re0.1HfxTa1.6W0.4(TaC)y alloys at 1450 °C is >950 MPa, while maintaining a fracture strain exceeding 11% at room-temperature. Notably, the Re0.1Hf0.1Ta1.6W0.4(TaC)0.25 alloy demonstrates a compressive strength of 1110 MPa at 1450 °C, which is 2.3 times greater than that of the NbMoTaW alloy at 1400 °C. Additionally, the fracture strain of the Re0.1Hf0.1Ta1.6W0.4(TaC)0.25 alloy at room-temperature is 16.9%, which is 8 times higher than that of the NbMoTaW alloy. The Re0.1HfxTa1.6W0.4(TaC)y alloy exhibit excellent mechanical properties at both room and high temperatures, making it a potential candidate for applications in fields that require high strength at ultra-high temperatures.

Study on hot compressive deformation behavior and microstructure evolution of G13Cr4Mo4Ni4V steel

To solve the problem of poor uniformity of microstructure of high alloy bearing steel G13Cr4Mo4Ni4V after rolling, the deformation behavior of the steel and its effect on the microstructure evolution was studied systematically. Gleeble-3800 thermo-mechanical simulator was used for precise control of deformation temperature ranging from 1000 to 1200 °C and deformation rate 0.01–10 s−1. Based on modified stress and strain curves by temperature and friction, the processing map of tested steel was drawn. Electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) were used to analyze the typical microstructure and carbide properties of different hot working domains respectively. The results showed the processing map could be divided into four regions: domain A 1000–1150 °C, 0.1–10 s−1; B 1000–1100 °C, 0.01–0.1 s−1; C 1100–1150 °C, 0.01–0.1 s−1 and D 1150–1200 °C, 1–10 s−1. Domain A was characterized by incomplete dynamic recrystallization (DRX) which included fine recrystallized grains and elongated deformed grains, whose softening mechanism was mainly discontinuous DRX (dDRX) and limited DRV and continuous DRX (cDRV) related to the ferrites and carbides. For domain B, the deformed grains gradually disappear with temperature rising, instead serrated grains appear with dDRX as dominating softening mechanism. A large number of carbides hinder DRX at 1000 °C with low temperature. With the increase of temperature, carbides dissolve and DRX degree increases. Grain coarsening is obvious because of high deformation temperature and low strain rate in domain C, although the nanoscale MC type carbides prevent recrystallization grain coarsening to some extent. Completely DRX resulted in uniform and fine with defect-free deformation microstructure in domain D, whose volume fraction of recrystallization reached up to 0.96 according to EBSD results. By considering the uniformity of microstructure and the distribution of carbide comprehensively, domain D is an ideal region for hot working.

Morphology and Distribution of Primary Carbides in Forged Cr-Ni-Mo-V/Nb Steel.

This study aims to investigate in situ the three-dimensional (3D) morphology and distribution of primary carbides (PCs) in electro-slag remelting (ESR) forged 30Cr3Ni3Mo2V steel. A facile non-aqueous electrolytic etching method was applied to prepare 3D PCs on the matrix. The morphology, composition, and element concentrations of PCs were characterized using a combination of optical microscopy (OM), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and electron back-scattered diffusion (EBSD). The precipitation, type, and composition of PCs in the same steel were also simulated using Thermo-Calc software Version 2015a. The results indicate that PC is rich in Nb, which is a potential heterogeneous nucleating agent. Both the size and number of PCs increase from the edge to the center of the ingot. The large-sized PCs present three dominant types of morphology, which vary in different regions, i.e., a bulky type dominates in the edge region, a lamellar type dominates in the middle region, and a stripy type dominates in the core region. The results of EBSD analysis show that the orientation of PCs with different morphologies is different and that more nanosized V-rich type carbides are precipitated on the matrix. The thermodynamic calculations show that MC precipitates from the liquid phase when the solid phase fraction is greater than 0.985 and that the MC-type carbides are rich in Nb, which agrees well with the experimental results.

Microstructural study of microwave welded austenitic stainless steel

The joining of austenitic stainless steels through microwave hybrid heating has gained much attention due to its advantages, like uniform and volumetric heating. However, to establish a microwave joining process for promising applications, a microstructural study of the welded joint is still an area to explore. In the present study, microstructural analysis of austenitic stainless steel (SS304) joints is explained through optical microscopy, scanning electron microscopy (SEM), elemental mapping, X-ray diffraction (XRD) and Vickers' micro-hardness tester. Further, detailed microstructural analysis of interlayer and heat-affected zone (HAZ) is studied through electron back-scattered diffraction (EBSD). XRD results reveal the development of various Ni, Cr and the M23C6 type carbides at the joint interlayer region. SEM results point toward non-epitaxial grain evolution along the fusion zone. The mean microhardness of 350 ± 20 HV was determined at the joint interlayer region. A relatively fine grain structure was observed along the fusion boundary. However, moving away from the joint, two different forms of coarse grain heat-affected zone (CGHAZ) were observed. The presence of two different forms of CGHAZ is confirmed through inverse pole figure maps and grain size plots. The presence of high-angle boundaries (HABs) at 60° rotation describes the twin boundaries characterized along the &lt;111&gt; axis. The existence of improved fractions of HABs in the microwave-joined specimen of SS will favor the microstructure to retard bulk deformation. The joint specimen endures a maximum force of 6.98 kN, and the ultimate tensile strength of 613 MPa with 4.75% of elongation. It is observed that microhardness observations and the microstructural studies in the research help establish a solid microstructure-property correlation for the joined material.

Coupling of alloy chemistry, diffusion and structure by grain boundary engineering in Ni–Cr–Fe

The diffusion–microstructure correlations for grain boundaries (GBs) in the technologically-relevant Ni-based 602CA alloy are investigated. Prolonged annealing treatments up to 744 h create distinct GB complexions with specific segregation–precipitation–structure states. Globular M23C6-type carbides at straight GBs and plate-like carbides together with NiAl-enriched (γ′-type) particles at hackly GBs are found to co-exist. Moreover, an atomic-scale GB spinodal-like decomposition, especially at straight GBs, is observed. The co-existence of the two distinct states of general high-angle GBs, indicated by tracer diffusion experiments and verified by a detailed structure examination, is explained via state-of-the-art measurements of local elastic strains. In a course of annealing at 873 K, the relatively “fast” diffusivities are found to increase by a factor of 10 or more as a result of a coupled evolution of the GB plate-like precipitates and the irregular GB structures, whereas the relatively ”slow” diffusivites remained practically unchanged representing the contributions of straight interfaces with spherical precipitates. Thus, the diffusion properties of high-angle GBs evolve together with characteristic changes of GB complexions distinguished by a growth of carbide- and γ′-type precipitates and a concomitant generation of GB dislocation networks. The obtained results provide novel insights into grain boundary tailoring by utilizing structure – kinetics correlations involving segregation, precipitation and the evolution of interface defects.