Search Results (597)

To mitigate the poor corrosion resistance of 45 steel components, ZrC-reinforced Ni-based composite coatings were fabricated on 45 steel substrates via laser cladding. The phase composition and microstructure were systematically characterized by XRD, FE-SEM, TEM, and EBSD, and the corrosion performance was evaluated by electrochemical measurements. The results show that the composite coatings consist of a dendritic γ-(Fe, Ni) matrix, interdendritic eutectics, and ZrC second-phase blocks or short dendrites. The volume fraction of ZrC increases with its addition content, while the ZrC grain size is first refined and then coarsened as the scanning velocity increases. IPF orientation analysis reveals that the γ-(Fe, Ni) phase exhibits a preferred <001> crystallographic orientation along the coating thickness direction. The coating with 30 wt% ZrC achieves the optimal corrosion resistance, with a corrosion current density of only 7.18% that of the substrate and a polarization resistance 13.34 times higher. Full article

As the automotive industry accelerates its transformation toward lightweighting and high-performance design, the demand for ultra-high-strength welded tube steels in body and chassis structural components has become increasingly urgent. However, these materials face severe challenges in cold bending formability, damage resistance, and cracking resistance. In this work, a Nb-containing high-strength steel for automotive welded tubes was developed, and the effects of the composite microalloying design on the microstructure, bending formability, and strengthening–toughening mechanisms of the experimental steel were systematically investigated. Using characterisation techniques including scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and X-ray diffraction (XRD), the synergistic action and quantitative contributions of grain-refinement strengthening, dislocation strengthening, and precipitation strengthening were thoroughly elucidated. The results show that after austenitisation and tempering, the microstructure of the experimental steel is predominantly lath martensite, with martensite lath packets separated by high-angle grain boundaries, which account for 66.2% of the boundary population. Nb significantly refines the prior austenite grains and martensite blocks through grain-boundary pinning, contributing 216 MPa via grain-refinement strengthening; the high dislocation density of the martensitic structure (3.05 ± 0.0018) × 10¹⁵ m⁻² provides a dislocation-strengthening contribution of 778 MPa; and nanoscale (Nb, Ti)C precipitates contribute an additional precipitation strengthening of 154 MPa. The synergistic effect of these three mechanisms enables the 1.2 mm thick welded tube steel to achieve a tensile strength exceeding 1400 MPa and an elongation above 9%. In summary, through reasonable microalloying design and an appropriate processing route, a Nb-containing high-strength steel for automotive welded tubes can be obtained that combines high strength, good ductility, and excellent formability. The elucidation of the synergistic enhancement from multiple strengthening mechanisms and the bending failure mechanism provides a valuable reference for the microalloying design of ultra-high-strength welded tube steels. Full article

High-speed directed energy deposition (HS-DED) was employed to fabricate a high-quality tungsten carbide (WC)-reinforced Inconel 625 metal matrix composite (MMC) coating on 316L stainless steel. The exceptionally high scanning speed (~30,000 mm/min) significantly reduced thermal exposure during processing, promoting uniform WC particle retention, negligible porosity (<0.1%), and strong metallurgical bonding with the substrate. Microstructural characterization using SEM, EBSD, and XRD revealed a refined Inconel 625 matrix with limited WC dissolution and pronounced accumulation of geometrically necessary dislocations (GNDs), indicating strong heterogeneous deformation-induced strengthening. The resulting coating exhibited high hardness, superior shear bond strength (623 MPa), and a defect-free structure, outperforming conventional high-velocity oxy-fuel (HVOF)-sprayed coatings. These results demonstrate that HS-DED enables the fabrication of dense, well-bonded, and mechanically robust MMC coatings, offering a promising alternative to conventional thermal spray technologies for demanding wear and structural applications. Full article

Laser-directed energy deposition (L-DED) is a key technology for fabricating wear-resistant Stellite-6 coatings on power-plant components. This study aims to clarify the surface cracking mechanism of L-DED Stellite-6 coatings deposited on 22Cr15Ni3.5CuNbN (SP2215) boiler tubes during long-term high-temperature service. The coatings were exposed at 650 °C for up to 5000 h, and their microstructural evolution and mechanical degradation were systematically characterized using SEM/EDS, TEM, EBSD, microhardness testing, and impact testing. High-temperature service induces the decomposition of M₂₃C₆ precipitates at dendrite boundaries, releasing Cr, C, and W atoms that subsequently migrate toward the coating surface. Owing to the rapid interstitial diffusion of C, a carbon-enriched surface region preferentially develops, accompanied by progressively increasing coverage of surface Cr₂O₃ and subsurface M₂₃C₆. Consequently, the surface hardness increases from 441.6 HV0.1 at 0 h to 613.1 HV0.1 after 5000 h, whereas the impact-absorbed energy decreases from 82.8 ± 4.2 J to 5.9 ± 2.5 J. An increase of 1 HV0.1 in hardness corresponds to an approximately 0.448 J reduction in impact-absorbed energy. Mechanistically, local stress concentration associated with Cr₂O₃ formation, interfacial sliding promoted by lattice mismatch, and crack nuclei originating from pores between chain-like M₂₃C₆ precipitates collectively promote crack initiation and propagation. These results demonstrate that precipitate decomposition, elemental redistribution, and subsequent oxide/carbide evolution govern the progressive surface embrittlement and cracking of L-DED Stellite-6 coatings during long-term high-temperature service. This study provides mechanistic insight into the coupling between microstructural evolution and surface failure and offers a theoretical basis for microstructural regulation and long-term reliability assessment of wear-resistant Co-based coatings used in power-plant components. Full article

Selective laser melting (SLM) produces elemental segregation and nonequilibrium secondary phases in GH4169 superalloys, but the effects of P and C variations on grain-boundary phase evolution and high-temperature tensile behavior remain unclear. Alloys with primarily varied P (0.034–0.058 wt.%) and C (0.009–0.066 wt.%) contents were fabricated and heat treated identically. Microstructures were characterized by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM), and tensile properties were evaluated at 650 °C. The as-built alloys exhibited columnar grains, cellular substructures, cell-wall segregation, and Nb-rich Laves-phase particles. After heat treatment, cellular substructures largely disappeared, γ″ precipitates formed in the γ matrix, and fine Ti/Al-rich particles were tentatively attributed to γ′; δ phase, residual Laves phase, and MC-type carbides were present at grain boundaries. Increasing P reduced the δ-phase area fraction from 2.20% to 1.38% and changed its distribution from continuous chains to semi-continuous and discrete arrangements. Increasing C raised the MC-type carbide area fraction from 0.34% to 1.48% while decreasing the δ-phase area fraction from 1.68% to 0.98%, accompanied by carbide coarsening. The 0.050P alloy exhibited the highest yield and ultimate tensile strengths, whereas the 0.009C alloy showed the best strength–ductility balance. These results reveal distinct P- and C-related changes in grain-boundary phase evolution and tensile behavior at 650 °C. Full article

SACM645 steel is a representative nitriding steel containing aluminum, chromium, and molybdenum, and is widely applied to mechanical components where high surface hardness, wear resistance, and fatigue strength are required. Because its strengthening response is strongly influenced by nitrogen absorption and nitride formation, SACM645 provides a suitable model material for evaluating rapid nitriding processes. In this study, induction heating (IH) nitriding was applied to SACM645 steel in order to clarify the feasibility and characteristics of rapid nitriding under ultrashort processing times. Cylindrical specimens were nitrided at 570 °C various times using high-frequency IH, and the resulting microstructure, nitrogen content profiles, and hardness distributions were systematically analyzed by optical microscopy, SEM/EBSD, EPMA, and microhardness testing. A compound layer accompanied by an increase in surface hardness was formed within only 8 min, with the near-surface hardness reaching 819 HV0.1. The surface nitrogen content rapidly increased to about 7–8 mass%. After the formation of a compound layer, its growth followed diffusion-controlled kinetics, consistent with classical nitriding models. The compound layer was predominantly composed of the ε phase. In the diffusion layer, a strong linear correlation was observed between local nitrogen content and hardness increment, irrespective of nitriding time. These results indicate that IH nitriding enables rapid surface hardening and controlled compound-layer formation within short processing times, providing a rational approach for rapid nitriding of SACM645 steel. Full article

Centrifugally cast 35Cr45NiNb alloy has been widely employed in ethylene-cracking furnace tubes owing to its excellent carburization and creep resistance. However, the influence of microstructural degradation and temperature on its high-temperature tensile behavior remains poorly investigated. In this study, an accelerated aging method at 1200 °C for 230 h (A1) and 430 h (A2) was employed to simulate approximately 4 and 8 years of service at 1050 °C, based on the Larson-Miller parameter. The equivalence was validated by the nearly identical precipitate area fractions of the A1 specimen (16.6%) and an ex-service specimen (14.8%). Combined with SEM and EBSD characterization, tensile tests at 950, 1000, and 1050 °C were conducted to elucidate the relationship between microstructure and high-temperature tensile properties. During aging, the skeletal interdendritic M₇C₃ carbides transformed into blocky M₂₃C₆, NbC evolved into the brittle G-phase (Ni₁₆Nb₆Si₇), fine secondary M₂₃C₆ precipitates formed, and the initially continuous primary-carbide network progressively coarsened. Yield and ultimate tensile strengths decreased monotonically with increasing temperature, whereas aging produced pronounced hardening at the expense of ductility, as secondary-carbide precipitation strengthening outweighed the weakening of the primary carbide network. The fracture mode transitioned from mixed quasi-cleavage fracture at 950 °C, initiated by stress concentration at coarse phase interfaces, to ductile rupture at 1000 and 1050 °C. GND analysis further revealed an aging-dependent transition in the dominant deformation mechanism, from dislocation pile-up at the carbide network, to recrystallization after prolonged aging. Full article

Controlling carbon removal while limiting surface oxidation is essential for constructing composition gradients in high-carbon medium-Mn steel. In this study, solid-state decarburization of Fe-12 wt%Mn-2.7 wt%C alloy sheets was investigated in H₂O-H₂ and CO₂-CO atmospheres by combining thermodynamic calculations with XRD, SEM, OM, EBSD, and GDOES characterization. Thermodynamic analysis showed that, above 1190 K, the critical gas partial-pressure ratio for Fe oxidation is lower in CO₂-CO than in H₂O-H₂, while the competitive-oxidation analysis further indicated a wider selective-oxidation window in the H₂O-H₂ atmosphere. Experimentally, H₂O-H₂ produced a relatively uniform oxide layer with a clear interface, whereas P_CO2/P_CO ≥ 0.29 promoted finger-like MnO growth along grain boundaries in CO₂-CO. In both atmospheres, increasing temperature accelerated carbon removal. At 1363 K and 50 min, increasing P_H2O/P_H2 from 0.47 to 0.51 and further to 0.56 progressively reduced the carbon concentration at a depth of approximately 450 μm from approximately 0.50 to 0.45 and finally to 0.30 at%, demonstrating effective regulation of the through-thickness carbon gradient. EBSD of the specimen treated at 1323 K for 50 min with P_H2O/P_H2 = 0.47 revealed a near-surface α + γ microstructure and a γ-dominated near-center region, with the number-weighted mean grain size increasing from approximately 14.5 to 59.5 μm. These results establish a processing-microstructure relationship among atmosphere-dependent selective oxidation, carbon removal, and carbon-gradient microstructure formation in medium-Mn steel. Full article

This study comparatively investigates the coarse-grained heat-affected zone (CGHAZ) responses of two industrial X65 seamless pipeline steels with distinct composition–microstructure characteristics under simulated girth-welding thermal cycles. One steel exhibits a predominantly bainitic initial microstructure, whereas the other consists of a ferrite–bainite dual-phase microstructure. Low-temperature Charpy impact testing, microhardness measurements, scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), prior-austenite grain reconstruction, and JMatPro 13.0-based continuous cooling transformation (CCT) calculations were employed to evaluate their heat-input sensitivity and microstructural evolution. At heat inputs of 7–10 kJ/cm, both steels maintained high impact toughness at −20 °C, with average absorbed energies of approximately 250 J. A pronounced difference emerged at 15 kJ/cm the bainite-dominated steel retained relatively high impact toughness and higher crack-initiation and -propagation energies, whereas the ferrite–bainite steel exhibited a marked toughness reduction. At higher heat inputs of 20–30 kJ/cm, both steels showed substantial toughness deterioration associated with severe prior-austenite grain growth and coarsening of the bainitic transformation products. Microstructural and crystallographic analyses showed that the bainite-dominated steel generally retained finer prior-austenite grains and more refined crystallographic features under the investigated thermal cycles. Detailed characterization at 15 kJ/cm further revealed finer prior-austenite grain, packet, and block structures, together with more tortuous crack-propagation paths. JMatPro calculations predicted a lower bainitic transformation temperature for this steel, which is consistent with the experimentally observed tendency toward finer bainitic transformation products. The superior CGHAZ toughness retained by the bainite-dominated steel is therefore associated with the combined effects of alloy composition, initial metallurgical state, transformation behavior, and hierarchical crystallographic refinement rather than the initial microstructure alone. The results highlight the importance of coupled composition–transformation–microstructure effects in determining the welding heat-input tolerance of industrial X65 seamless pipeline steels. Full article

D406A steel serves as a critical structural material for load-bearing components in aerospace solid rocket motors. To achieve an excellent strength-plasticity balance of D406A steel in this work, the quenched specimens austenitized at 890 °C were subjected to tempering treatments at 320 °C, 350 °C, 380 °C and 410 °C, respectively. SEM and EBSD characterization were adopted to systematically investigate the effects of tempering temperature on the microstructure, grain boundary characteristics, local strain, Schmid factor and mechanical properties. The results reveal that the lath martensite gradually undergoes recovery and disintegration with increasing tempering temperature, while the fraction of low-angle grain boundaries rises first and then falls, reaching the maximum value of 48.2% for the specimen tempered at 350 °C. At this tempering temperature, the KAM distribution is uniform, the Schmid factors shift toward the medium-to-high range, the proportion of grains with soft orientation increases, and the deformation coordination capacity is optimal. The specimen tempered at 350 °C exhibits an ultimate tensile strength of 1633.9 MPa, a yield strength of 1262.5 MPa and a Vickers hardness of 485.4 HV, achieving the optimal synergy between strength and plasticity. Full article

Metal parts produced by directed energy deposition go through a complex thermal history during the deposition stage, which can result in heterogeneous microstructures and the accumulation of residual stress. While individual post-processing heat treatments are widely used to address these issues, the industrial logistics of manufacturing large components often demand specific sequences of combined treatments (e.g., applying stress relief prior to substrate detachment to prevent distortion, followed by high-temperature solubilization, or vice versa). The microstructural and mechanical consequences of altering this sequence remain underexplored. Therefore, this study aimed to investigate the effects of different post-processing heat treatment sequences on the mechanical properties of AISI 316L deposited through laser-directed energy deposition. Tensile and Charpy impact tests were carried out on the specimens under five conditions: (i) as-built; (ii) stress relief; (iii) solubilization; (iv) stress relief and solubilization; and (v) solubilization and stress relief. A statistical analysis of variance supported a comparison between each treatment’s influence on the mechanical properties under each condition. Furthermore, the typical microstructures were assessed by optical microscopy, scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD), and X-ray diffraction (XRD). The solubilization treatment reduced the ultimate tensile strength (from ~618 MPa to ~576 MPa) and the yield stress (from ~424 MPa to ~299 MPa), while no significant change was observed in elongation (ranging from 27% to 38%) due to high data dispersion. The stress relief, however, did not significantly change these mechanical properties. Considering the heat treatment combinations, the solubilization had a stronger impact on tensile stress than the stress relief, regardless of the treatment order. Impact resistance was not significantly affected by any of the heat treatments, maintaining an average of ~114 J. The solubilization treatment fully recrystallized the microstructure, while the stress relief did not promote any significant changes at an optical microscopy level. Ultimately, this study demonstrates that the microstructural transformations induced by the solubilization step dominate the final mechanical baseline, indicating that the sequence order is not a determining factor. This finding grants critical flexibility for industrial manufacturing logistics, allowing stress relief to be strategically applied when most convenient for dimensional stability without compromising final part performance. Full article

A solid solution treatment (SST) followed by single-stage aging (0–30 h, 140 °C) was performed on 7075 aluminum alloy specimens with longitudinal axes oriented at 0°, 45° and 90° to the rolling direction. The mechanical properties and microstructure were analyzed by tensile testing, optical microscope (OM), electron backscatter diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM). The results show that the average tensile strengths of the as-received 7075 aluminum alloy in the three directions were 304 MPa (0°), 295 MPa (45°) and 297 MPa (90°), respectively, with an anisotropy index (AI) of 0.97, indicating that the as-received samples exhibited negligible anisotropic mechanical properties. After SST, elongated grains with coarse size were formed, which is primarily attributed to the inheritance of the deformed fiber texture introduced by hot rolling. EBSD analysis of the 30 h aged specimens revealed that, within the same analyzed area, the total grain-boundary length in the 45° direction (16.4 cm) was much larger than that in the 0° (10.4 cm) and 90° (13.5 cm) directions. As the grain morphology showed no significant change between the SST and aged conditions, this grain-boundary distribution was representative of the microstructural state established during SST and persisted throughout the artificial aging process, contributing to the anisotropic mechanical properties. During artificial aging, prolonged aging time significantly facilitated the precipitation, with the 30 h aged sample exhibiting a significantly higher density of precipitates compared to the 6 h aged sample, leading to enhanced mechanical properties. The tensile strengths of the 30 h aged samples increased to 165 MPa, 236 MPa and 196 MPa in the three directions, respectively. Meanwhile, due to the fixed crystallographic orientation relationship between the precipitates and the Al matrix, the precipitates tended to form on specific planes, which enhanced the anisotropic mechanical properties. Consequently, the AI value increased from 0.97 (as-received) to 1.43 (30 h aged) with prolonged aging time. Full article

At present, the detection methods for the microstructure and mechanical properties of aluminum alloys are mainly based on SEM, EBSD, TEM, tensile tests, and microhardness tests, which are time-consuming and destructive. In this paper, laser ultrasonic non-destructive detection is employed to obtain ultrasonic signals from Al-Cu-Mg aluminum alloy subjected to various heat treatment processes. The results reveal empirical correlations between the characteristic values of the ultrasonic signals and the material’s state. Specifically, the characteristic values exhibit an inverse correlation with the precipitated phase content. When both the precipitated phase content and the average grain size vary significantly, distinct deviations in the characteristic values are observed, which can serve as indicators of microstructural changes. The extracted ultrasonic eigenvalues also show promising, empirically derived correlations with mechanical properties, with frequency-domain attenuation coefficients demonstrating relatively higher sensitivity based on fitting analyses within the current dataset. These observed variations are tentatively discussed as plausible consequences of grain boundary scattering and changes in matrix solid solution strengthening associated with precipitate dissolution. Overall, the findings suggest the potential of laser ultrasonics as a rapid non-destructive evaluation tool, providing a preliminary scientific basis for further development of methods to assess the microstructure and mechanical properties of Al-Cu-Mg alloys within the tested parameter space. Full article

To explore the effect of grain size on the tensile deformation mechanism of GH4169 nickel-based superalloy, specimens with average grain sizes of 10.7 μm and 70.6 μm were fabricated via different heat treatment processes, and in situ tensile experiments were carried out at room temperature using combined in situ SEM-EBSD technology. The results show that the fine-grained specimen exhibits a yield strength of 794 MPa and an ultimate tensile strength of 1372 MPa, while the coarse-grained specimen presents a yield strength of 317 MPa and an ultimate tensile strength of 782 MPa. During tensile loading, the fine-grained specimen undergoes uniform deformation with strong grain boundary coordination, accompanied by homogeneous grain orientation rotation; its average kernel average misorientation (KAM) value increases moderately from 0.82 to 0.89. In contrast, the coarse-grained specimen suffers severe inhomogeneous deformation and readily generates continuous slip bands, featuring abrupt local changes in grain orientation, with the KAM value rising from 0.8 to 1.04. Slip systems with high Schmid factors induce localized intragranular deformation. Benefiting from a high grain boundary density, the fine-grained specimen effectively restrains strain localization and achieves more uniform bulk deformation of the alloy. Full article

This paper investigates the stress corrosion cracking (SCC) behavior under constant loading conditions and the early-stage grain-scale deformation mechanisms of a friction-stir-welded (FSW) joint of 7A52 aluminum alloy. The results show that when a constant load equivalent to 1.0 times the yield strength (439.43 MPa) of the base metal is applied, the joint fractures after 72 days of immersion in a 3.5 wt% NaCl solution, with the fracture located in the heat-affected zone on the advancing side (AS-HAZ). The fracture surface exhibits a mixed-mode morphology characterized by both brittle and ductile features. Observations suggest that cracks mainly initiate at the bottom of corrosion pits, at grain boundaries, and at the interfaces between precipitates (such as Mg-Si-rich, Al-Fe-rich, or Al-(FeMn)-rich) and the Al matrix. It is suggested that the initiation mechanisms are closely related to galvanic corrosion, interfacial weakening, and mechanical property mismatch. In situ tensile and EBSD results indicate that the AS-HAZ is the first region to undergo deformation. As the load increases from 400 N to 1500 N, the degree of strain localization intensifies, with high-strain regions preferentially concentrated at grain boundaries. Grain boundary damage is likely a key mechanism responsible for the initial failure on the advancing side of the FSW joint. Further in situ SEM observations reveal that during the early stage of tensile deformation, as the load increases from 300 N to 455 N, the grain surface in the AS-HAZ evolves from a flat morphology to a typical orange peel appearance. Meanwhile, grain boundaries change from clearly visible to blurred, slip traces increase, and multiple slip systems are activated within the grains. The continuous pile-up of dislocations at grain boundaries leads to a sharp increase in local stress concentration, ultimately inducing grain boundary instability and crack nucleation. Full article