•Pre-formed martensite eliminates the incubation time for bainite transformation.•Below-Ms austempering delays the finishing time for bainite transformation.•Samples with pre-formed martensite show ...relatively uniform strain partitioning.•Optimal toughness is obtained when austempering is performed at 30–60 °C below Ms.
The roles of pre-formed martensite (PM) in below-Ms bainite formation, microstructure, crystallography, strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction, transmission electron microscopy, micro digital image correlation technique and mechanical tests. It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms, indicative of its acceleration effect at the early stage of transformation. This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite, with initial bainite tending to form around the PM. However, the finishing time for below-Ms bainite transformation, especially at even lower temperatures, is retarded, owing to the dividing effect of PM on parent austenite grains, the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite. PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block. The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite. The specimens with PM show relatively uniform strain partitioning among various phases, contrasting with the specimens without PM, for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite (M/A) blocks or between adjacent M/A blocks. The impact absorption energies of the specimens with PM, when austempered at 30–60 °C below Ms, are more than twice higher than those of the specimens without PM, at no expense of tensile properties.
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The research aims to effectively enhance the low-temperature toughness of 9 wt% Ni steel, which is used in storage containers for alternative energy sources such as liquefied natural gas. To achieve ...this, a lamellarizing heat treatment was applied to 9 wt% Ni steel, initially manufactured using the conventional quenching-tempering (QT) process, thereby introducing a quenching-lamellarizing-tempering (QLT) process. The Charpy impact toughness of the 9 wt% Ni steel was then evaluated under two extreme cryogenic conditions: 196 °C and −253 °C. The 9 wt% Ni steels subjected to the QLT treatment demonstrated exceptional Charpy impact toughness values of 238 J at −196 °C and 217 J at −253 °C. This study presents an examination of the microstructural and micro-hardness evolution throughout the various stages of the QLT heat treatment process. A comparative investigation was undertaken by contrasting these results with samples treated using quenching (Q), quenching-tempering (QT), and quenching-lamellarizing (QL) treatments. The QLT treatment induces a substantial volume fraction of retained austenite, combined with a strategically distributed micro-hardness profile. In this profile, the soft phase interspersed among hard phases can alter the crack propagation path and absorb energy, thereby enhancing toughness. This synergy contributes to the exceptional Charpy impact toughness observed at −253 °C. These results suggest that the introduction of the QLT heat treatment is an effective method for enhancing the cryogenic impact toughness of 9 wt% Ni steel at temperatures below −196 °C, compared to the conventional QT process.
The poor impact toughness and flame retardant performance have greatly restricted the engineering application of epoxy thermoset. To obtain the high-performance epoxy composites, the renewable ...vanillin-based flame retardant toughening agent (PVSi) was synthesized and incorporated into epoxy. The use of PVSi macromolecules can significantly enhance the impact toughness of epoxy. With 5 wt% of PVSi, the impact strength of the epoxy was maximally raised by 189.69%, from 12.42 kJ/m2 of the neat EP to 35.98 kJ/m2 of EP/PVSi5 composites. The toughening effect of PVSi macromolecules on epoxy was closely linked to its structural features, such as the flexible phenylsiloxane, active imine and polar phosphaphenanthrene groups. Simultaneously, the EP/PVSi5 composites reached up to the V-0 rating in vertical burning test (UL-94) and 29.5% in limiting oxygen index (LOI), with only 0.27 wt% ultra-low phosphorus loading. Additionally, the suppressed heat release, the evidently reduced toxic pyrolytic volatiles, and the promoted charring capability of EP/PVSi composites can be obtained, with phosphaphenanthrene, phenylsiloxane and diaminodiphenylsulfone groups in PVSi macromolecules jointly playing a role. These results indicated the improved fire safety of epoxy. Furthermore, the free radical scavenging effect of P· and PO·, the fuel dilution effect of nonflammable NH3 and SO2, the catalytic charring effect of the pyrophosphoric acid and metaphosphoric acid, the charring-stability effect of phenylsiloxane group and the suppression effect of high-quality carbon layers were analyzed and summarized. It was expected that PVSi would pave the way for the development of more highly efficient flame retardant toughening agents and high-performance epoxy thermoset.
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•A novel renewable polymeric flame retardant toughening agent PVSi was synthesized.•Multiple flame retardant elements are contained in PVSi.•PVSi obviously improves the fire safety of EP with ultra-low phosphorus loading.•PVSi significantly improves the impact toughness of EP.•The mechanisms of flame retardancy.
The volume fraction and morphology of the microstructural constituents are extremely important when discussing the impact toughness of nanostructured bainitic steels. On the other hand, ...microstructural features can be influenced by plastic deformation of austenite prior to isothermal bainitic transformation. This paper aims to investigate the effect of 10% ausforming process before austempering heat treatment on impact toughness of nanostructured low temperature bainitic steels austempered at 300°C. Results indicate that the volume fraction and morphology of high carbon retained austenite within the microstructure are important factors affecting the impact toughness energy. However results showed that the role of bainitic sheaves cannot be ignored due to their valuable effect of arresting/deflecting the cracks. It has been found that bainitic sheaves grow at more limited crystallographic variants in ausformed samples which can influence the value of the impact energy absorbed during straining the samples.
The hierarchical martensitic features in ultra-high strength stainless steel (UHSSS), including the prior austenite grains, martensite packets, blocks and laths with the descending size, were refined ...to various extents by employing different thermomechanical processes and then carefully characterized. Their relation to yield strength and impact toughness was analyzed. We conclude that the refinement of martensitic structures could lead to the significant increase of yield strength, which follows the Hall-Petch relation with the effect grain size defined by high angle boundaries (HABs). Impact toughness of UHSSS depends on the frequency and capability for retained austenite (RA) grains at both HABs and martensite lath boundaries to trap the propagating cracks via strain-induced transformation, in which the film-like RA grains at lath boundaries appear to make the greater contribution.
The effects of varying the Nb content (1.04 wt%-1.55 wt%) in NiCrMo weld metals on the microstructure, tensile strength and cryogenic impact toughness were investigated. The microstructure of the ...three different weld metals was observed to be composed of austenite matrix and precipitates by characterization techniques such as SEM, EDS, EBSD, and TEM. The precipitates within the grains were NbC and Ni3Nb phases, and the precipitates on the grain boundaries were NbC phases. With the increase of Nb content, the average grain size of austenite decreased from 205.3 μm to 161.4 μm, and the area fraction of precipitates increased from 1.41% to 3.97%. The cryogenic impact value of the welded joints decreased from 97 J to 76.3 J, while the tensile strength increased from 682.5 MPa to 710 MPa, indicating that the increase in the area fraction of the Nb-rich phase had a deleterious effect on the cryogenic impact toughness and was conducive to the elevation of the tensile strength. When the Nb content in the weld metal was 1.25 wt%, the comprehensive mechanical properties of NiCrMo welded joints were the best, with a tensile strength of 695.5 MPa and a cryogenic impact value of 86.7 J.
•Investigate the effects of variations in Nb content in NiCrMo welded joints.•Study the effects of Nb Content variations on precipitates.•Use characterization techniques such as SEM, EDS, EBSD, and TEM to research the microstructure of different weld metals.•Establish a connection between the mechanical properties of welded joints and the microstructure of the weld metal.
Gradient-structured (GS) materials are capable of displaying high strength without compromising ductility, which can result in damage-tolerant structures. However, due to the difficulties in ...fabricating bulk GS materials, there has been only limited studies on the fracture behavior in GS metals. In the present work, the impact toughness of the macroscale GS pure Ni plates was investigated using instrumented Charpy impact testing. The gradient orientation was found to have a significant influence on the impact toughness of GS Ni. For gradient structures that transition from coarse grains (CG) to nano-grains (NG), termed CG→NG gradients (in the present study from ∼8 μm to ∼30 nm), the absorbed energy and the tensile strength were increased, respectively, by 1.6 and 2.3 times from those exhibited by uniform coarse-grained structures, demonstrating a simultaneous enhancement in strength and impact toughness. Analysis of load-displacement curves revealed that the resistance to both crack initiation and propagation were significantly enhanced as the crack penetrated through the CG→NG gradient structure, leading to markedly rising dynamic R-curve behavior estimated from nonlinear-elastic fracture mechanics J-based measurements. The superior fracture resistance in the CG→NG gradient structure was found to originate from sustained ductile fracture by microvoid coalescence, taking place not only in the initial CG zone, but also within the latter NG regions where adiabatic shear bands form during impact; in these latter regions, plasticity becomes enhanced due to grain coarsening induced by recrystallization under the dynamic loading. The present work not only reveals how the dynamic fracture resistance can be significantly enhanced in GS metals, but also provides structure-design strategies for developing superior metallic materials for impact engineering applications.
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The laser cladding additive manufacturing technology and high-entropy alloys can serve as ideal techniques and materials for the surface repair of high-speed train axles. Coatings of FeCoCrNiMox ...(x = 0.2, 0.5) were prepared on EA4T axle steel using laser cladding technology, and their phase, structure, and mechanical properties were analyzed using X-ray diffraction (XRD), electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) characterization techniques. Additionally, the mechanical properties of the coatings were tested, and first-principles calculations were used to verify and calculate the material's mechanical performance. The research findings indicate that an increase in Mo content leads to a greater degree of lattice distortion, causing a leftward shift in diffraction peak positions. Significant differences in the microstructure from the substrate to the coating surface were observed, with columnar grain structure at the bottom of the cladding layer and equiaxed dendrites dominating the coating surface. The increase in Mo content promotes the formation of σ phase while also refining the grain size. ECCI results show that both types of coatings consist of a high-density dislocation cell structure and Mo-rich particles. With an increase in Mo content, the peak hardness of the coating increased from 456.5 HV to 469.4 HV, while the impact energy decreased from 56 J to 16 J, attributed to the increase in dislocation density and the greater quantity of σ phase. First principles calculations verified that the comprehensive mechanical properties of FeCoCrNiMo0.2 are superior, providing theoretical guidance for the optimization and design of the coatings.
Experimental method to explore the Mo element of first principles calculation of laser cladding high entropy alloys FeCoCrNiMox (x = 0.2, 0.5) organization and mechanical properties of impact. Display omitted
•High entropy alloy coatings with different Mo content were prepared by laser cladding.•Mo element promotes the formation of σ phase and has the effect of refining the grain.•Increasing the Mo content significantly raises the hardness and reduces the toughness.•First principle calculation confirms that FeCoCrNiMo0.2 has better mechanical properties than FeCoCrNiMo0.5.
In this study, the impact toughness and deformation mechanism of base metal (BM), heat affected zone (HAZ) and weld zone (WZ) of Ti6321 alloy joint at different temperatures (20 °C, 0 °C, −20 °C, ...−40 °C, −60 °C and −196 °C) are studied. The microstructure of BM consists of primary α (αp), acicular α and β phase. The HAZ is composed of αp, acicular α, acicular martensite (α′) and β phase. The WZ is composed of acicular α′, lath α′ and β phase. All the BM, HAZ and WZ obtain the highest impact toughness at 20 °C, which are 51 J, 75 J and 72 J, respectively. Under −196 °C, brittle fractures occurs in the BM, HAZ and WZ, and their impact toughness are 18 J, 22 J and 30 J, respectively. Compared with the impact test at −196 °C, the fracture surface of 20 °C by impact test has higher kernel average misorientation and geometrically necessary dislocation. The fracture impact resistance at 20 °C is greater. Grain deformation at 20 °C consumes more energy and the crack propagation path is more tortuous.
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