Segregation Band Research Articles

Microstructure and mechanical properties of non-heat-treated Al–8Si-0.2Mg-0.5Mn alloy inoculated by Al–Nb–B refiner

Al–Nb–B refiner has been considered as an alternative to Al–Ti–B refiner for hypoeutectic Al–Si alloys due to Si-poisoning effect. In this paper, the effects of Al–Nb–B and Al–Ti–B refiner on the microstructure and mechanical properties in indirect squeeze casting Al–8Si-0.2Mg-0.5Mn alloy were compared. Results show that the grain sizes in the edge chilling layer, segregation band, and the center region of the Al–Nb–B refined alloys were refined by 13.9%, 12.5%, and 17.6% compared to Al–Ti–B refined alloys, respectively. The yield strength, tensile strength, and elongation of Al–Nb–B inoculated alloys were superior to those of the unrefined alloys and Al–Ti–B inoculated alloys. The coexistence of NbAl3 and NbB2 particles in the Al–Nb–B refiner exhibited excellent anti Si-poisoning capability. They significantly decreased the grain size and uniformly distributed the eutectic Si particles and Fe-rich phases, resulting in an increase in the yield strength of the alloy by 16.1% compared to the Al–Ti–B inoculated alloy. The distribution of iron-rich phases in the segregation band was affected by the inoculation. The size of α-Al15(Fe, Mn)3Si2 phase in Al–Ti–B inoculated alloys was refined, and the number significantly increased, resulting in a decrease in tensile strength of 12.82 MPa. In the Al–Nb–B inoculated alloys, the size and number of α-Al15(Fe, Mn)3Si2 phase decreased, and the δ-Al4FeSi2 phase increased, which effectively improved the fracture toughness and ductility. In addition, with the increase in grain refinement (from unrefined to Al–Ti–B to Al–Nb–B inoculated alloy), the fracture behavior changed from trans-granular fracture to intergranular fracture, improving the fracture toughness.

Revealing the failure mechanism of industrial Fe-25Cr-20Ni stainless steel plate: Fine-grained segregation band and brittle phase induced delamination cracking

The use of heat-resistant stainless steel thick plates in industrial production often presents challenges due to the presence of uneven microstructure and segregation, which can pose a persistent challenge for the prolonged safe operation of critical components made from such materials. This study aimed to investigate the fracture mechanism of an industrial Fe-25Cr-20Ni stainless steel plate at room temperature through uniaxial tensile experiments and fully reversed strain-controlled tension-compression fatigue experiments. Delamination cracks were observed on the fracture surfaces of both experiments. The specimens with fracture delamination were systematically studied by detailed microstructural evolution, and the failure mechanism and main reasons of fracture delamination were analyzed. The results show that the fracture delamination is related to the fine-grained segregation band (FGSB) consisted of fine grains and sigma phase located at the center of the thickness of the stainless-steel plate. The FGSB demonstrates low transgranular and intergranular fracture modes and exhibits bamboo knobs crack during plastic deformation, leading to a mixed fracture mode of ductile and brittle fracture. Furthermore, FGSB, as an anisotropic microstructure, which has a high stress concentration with nearby grains during plastic deformation. The sigma phase fracture and the weakening of the interface strength of the microstructure in FGSB provide a potential initiation site for fatigue crack initiation, thereby contributing to the possibility of aggravating fatigue damage and deteriorating fatigue performance. It is concluded that FGSB, as a metallurgical defect in stainless steel plates, is responsible for delamination failure.

Effect of Segregation Band on the Microstructure and Properties of a Wind Power Steel before and after Simulated Welding

This article uses scanning electron microscopy (SEM) and electron back-scattering diffraction (EBSD) to study the effect of C and Mn segregation on the microstructure and mechanical properties of high-strength steel with 20 mm thickness used for wind power before and after simulated welding. A Gleeble-3500 (GTC, Dynamic Systems Inc., Poestenkill, NY, USA) was used to study the microstructure evolution of the simulated coarse-grained heat-affected zone (CGHAZ) of experimental steel under different welding heat inputs (10, 14, 20, 30 and 50 kJ/cm) and its relationship with low-temperature impact toughness (−60 °C). The results indicate that alloy element segregation, especially Mn segregation, significantly affects the impact toughness scatter of the steel matrix, as it induces the formation of low-temperature martensite or hard phase, such as M/A (martensite/austenite) constituent. In addition, segregation also reduces the low-temperature impact toughness of the simulated welding samples and increases the fluctuation range. For high-strength steel with yield strength higher than 460 MPa used for wind power generation, there is an optimal welding heat input (~20 kJ/cm), which enables the simulated coarse-grained heat-affected zone (CGHAZ) to obtain the highest impact toughness due to the formation of lath bainite (LB) and the finest crystallographic block units. Excessive or insufficient heat input can induce the formation of coarse granular bainite (GB) or lath martensite (LM), leading to a larger size of crystallographic block units, reducing the hindering effect of brittle crack propagation and deteriorating low-temperature impact toughness.

Influence of TiN Inclusions and Segregation Bands on the Mechanical Properties and Delayed Crack in Thick NM550 Wear-Resistant Steel.

The formation mechanism of the delayed crack after flame cutting and mechanical properties in thick NM550 wear-resistant steel are studied by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and an electron backscattered diffractometer. The delayed crack is formed at the segregation zone (SZ) located in the center of the 65 mm thick steel plate. The strength of the non-segregation zone (NSZ) with a martensite microstructure is slightly higher than that of SZ with a mixture microstructure of martensite plus bainite, and the plasticity of NSZ is significantly better than that of SZ. There exists a more severe segregation in the SZ, and only a slight segregation in the NSZ. The average grain sizes of the segregation bands in the NSZ and SZ are 15.72 µm and 6.76 µm, respectively. The number density of TiN larger than 5 µm in the NSZ and SZ is 0.031 and 1.156 number/mm2, respectively. Therefore, a high hardness segregation band with fine grains and a high dislocation density, along with the large number of coarse TiN inclusions within it, results in delayed cracking. For TiN inclusions close to the crack, microvoids or microcracks around the TiN are formed, and the delayed crack will propagate along the edge of the TiN or through the TiN inclusions.

Effect of the segregation band on the hydrogen embrittlement susceptibility of quenched and partitioned steel

In this study, the effect of segregation bands on the hydrogen embrittlement susceptibility of QP1180 was examined by slow strain rate tensile tests, and it was shown that almost all hydrogen-induced cracks originated at the segregation bands, which eventually led to the fracture. In addition, the hydrogen distribution characteristic of QP1180 was also visualized by hydrogen microprinting tests, showing that the local hydrogen concentration of QP1180 is as follows: Segregation band > Retained austenite > Prior austenite grain boundaries > Block boundaries > Martensite > Ferrite.

Axial segregation characteristics and size-induced flow behavior of particles in a novel rotary drum with curved sidewalls

Particle mixing and segregation are common phenomena in rotary drums, which are challenging to be controlled and driven artificially in powder technology. In this work, the discrete element method (DEM) was applied to construct the novel rotary drum composed of different shaped curved sidewalls. By varying the operation parameters of particle and sidewall shapes as well as the length-to-diameter (L/D) ratio of drums, the axial mixing and segregation processes of binary size-induced particles were investigated. The results show that the axial flow velocity of the particle mixtures is noticeably weakened once the particle angularity increases, making the non-spherical particles to mix better in rotary drums compared to the spherical particles. Besides, in the short drums with size-induced spherical particles, the axial segregation characteristics are significantly enhanced by the convex sidewalls while suppressed by the concave sidewalls. However, for size-induced non-spherical particles, the axial segregation structure can be present in rotary drums with plane and concave sidewalls while not in drums with convex sidewalls. Moreover, the axial segregation band structure of spherical particles eventually increases proportionally with the increased drum L/D ratios. In contrast, the non-spherical particles cannot form obvious multi-proportional segregation bands.

Structural Anisotropy Parameters’ Effect on the Low-Temperature Impact Strength of Alloy Steels in Rolled Products

The present work is devoted to the study of the influence of the parameters of the structural anisotropy of rolled products on the low-temperature impact strength of alloyed steels. A quantitative metallographic analysis of the microstructure of rolled steel samples obtained after testing for low-temperature toughness was carried out. It was established that the main reason for the decrease in the low-temperature impact strength of rolled steel samples is a highly developed segregation band enriched with carbon films formed at the stage of steelmaking conversion in violation of the technology of continuous casting of steel. The microstructural analysis of rolled stock samples was used in the work, and studies of the fracture surface of rolled stock samples were carried out with a scanning electron microscope using X-ray microanalysis methods. The studies carried out showed that the metallurgical quality of sheets of one heat, as well as individual samples within one sheet, varied over a wide range, from satisfactory to unacceptably low. It was established that the main reason for the decreasing low-temperature impact strength of rolled products was a highly developed segregation band enriched with carbon films, formed at the stage of steelmaking in case of violation of the continuous casting of steel technology. The multivariate statistical analysis carried out showed that only the size of the segregation band has an effect on the low-temperature impact strength of 10 mm thick rolled coil samples.

Effect of Mg and Si contents and TiC nanoparticles on the center segregation susceptibility of twin-roll cast Al–Mg–Si alloys

Center segregation during the twin-roll casting (TRC) has always been a barrier to the fabrication of high-performance Al–Mg–Si strips. In this work, Al–Mg–Si alloys with different Mg and Si contents were fabricated by vertical-type twin-roll caster to investigate the effect of Mg and Si contents on central segregation susceptibility. Alloys that undergo a eutectic reaction to form Si would lead to the enrichment of eutectic Si in the central region due to the low reaction temperature of eutectic Si, resulting the severe central segregation. For alloys with the same eutectic reaction during the solidification, the alloy with higher |ⅆT/ⅆ(fs1/2)| showed agglomeration of the eutectic phases due to its lower solidification rate. For the alloys with the same Mg/Si ratio, the width of central segregation increased with the increase in Mg and Si contents due to the increase in the proportion of near-equiaxed grains in the central region. The introduction of TiC nanoparticles could mitigate the central segregation band due to the control of solute diffusion by the fine near-equiaxed grain structure.

Interpretation of microstructure evolution and mechanical properties under aging treatments of a Fe–24Mn-0.6C–2Al-0.6V austenitic steel for cryogenic application

Aging treatment is an effective way of strengthening high-Mn strip steels based on automotive industry. However, the role of this method in enhancing yield strength and cryogenic toughness (−196 °C) of high-Mn steels has still unclear so far. Besides, solute segregation has always been a vital factor affecting the mechanical properties of high-Mn steels, whereas it is still lacking in adequate attention. In the present work, the microstructural evolution, including solute macrosegregation, grain boundary segregation and precipitation, and the resultant mechanical properties under different aging treatment were elaborated. It was found that the banded segregation of C, Mn and V elements formed during aging treatment strongly affects the cryogenic toughness. Tridimensional EPMA analyses demonstrate that there exists dense banded segregation of C, V and Mn elements in longitudinal section and cross section for both aged steels (aging at 500 °C and 800 °C). However, more obvious banded segregation of C and V elements developed in the steel aged at 800 °C (800A), and more micron-scale VC carbides tend to precipitate at segregation band, which leads to the easy initiation and propagation of microcracks along the segregation band. Moreover, numerous nano-scale VC precipitates significantly increase the yield strength of 800A steel by 100 MPa, but there exist severer grain boundary segregation and precipitation after aging treatment, which enhances the twining stress by increasing local SFE of grain boundary regions. The twinning formation hence became harder, further deteriorating the cryogenic toughness of the 800A steel.

Effect of Zn Addition on the Microstructural Heterogeneity and Mechanical Properties of Vacuum‐Assisted High‐Pressure Die Casting Al–Si–Mg–Cu alloys

Herein, Al–8Si–0.5Mn–0.5 Mg–0.2Cu–xZn alloys (x = 0, 1, 2, and 3 wt%) are produced by a vacuum‐assisted high‐pressure die casting process. The effects of Zn addition on the surface layer thickness and segregation band, the number density of externally solidified crystals (ESCs), and mechanical properties of the alloys are systemically investigated. As the Zn content increases, the size and number density of the ESCs increase. The morphology and size of the eutectic Si phase could be modified by the addition of Zn. In addition, the microstructural heterogeneity indicates that the thickness of the surface layer and segregation band zone increase with increasing Zn content. Proper Zn addition can effectively promote the improvement of strength and elongation in the as‐cast state. When the Zn content is 2 wt%, the yield strength, ultimate tensile strength, and elongation of the alloy are 167, 311 MPa, and 10.8%, respectively.

Effect of two-stage solution on microstructure and mechanical property of IN718 alloy fabricated by selective laser melting

The substructure characteristics and mechanical properties of the IN718 alloy prepared via selective laser melting in the as-deposited state and different solid solution states were studied. The effects of the substructure characteristics, which include precipitates and grain boundary features, on the microhardness of the IN718 alloy were investigated. It was found that the main substructure of the as-deposited alloy is a cellular structure, which is formed owing to the 50-nm-thick segregation band of the Nb and Ti elements. The two-stage and single-stage solution processes can both prevent the microsegregation of the as-deposited state, but the two-stage solution process can make the composition more uniform; at the same time, it leads to a large number of twin boundaries. The δ phase precipitates mostly in the single-stage solution process at 980 °C, but the two-stage solution process can effectively inhibit the precipitation of the δ phase. The hardness of the alloy after the two-stage solution process is lower than that of the alloy after the single-stage solution process. The main strengthening mechanism of the as-deposited alloy consists of strain strengthening. The strengthening effect of the δ precipitate is lower than the strain strengthening caused by the grain boundaries of the alloy.

Effect of segregation on the microstructure and properties of a quenching and partitioning steel

• Mn segregation decreases ferrite and increases secondary martensite fraction in the Q&P steel. • Mn segregation causes a lower fraction and stability of retained austenite in the Q&P steel. • Mn segregation degrades the strength and ductility of the Q&P steel due to insufficient TRIP effect. Effect of alloying element segregation on the microstructure and mechanical properties of an intercritically annealed quenching and partitioning steel is experimentally studied. Phase constitution of the segregation band and matrix was compared to elucidate the effect of Mn enrichment on the relevant phase transformation processes and the stability of retained austenite. Tensile specimens including and excluding the segregation band were carefully prepared to reveal the effect of segregation on the mechanical properties. The present study provides direct evidence of the influence of segregation on the microstructure and demonstrates the harmful effect of Mn segregation on both the strength and ductility in the studied quenching and partitioning steel.

Segregation behavior, microstructure and properties of TA2/AA7005 composite plates manufactured by horizontal twin-roll casting under the effect of an electromagnetic oscillation field

Three TA2/AA7005 composite plates were manufactured by horizontal twin-roll casting (HTRC) under various external fields, and the formation mechanism of segregation in the cast-rolling clad process as well as the effect of external fields on the segregation behavior, microstructure and properties of the composite plates were studied. In the non-field (NF) plate, due to the high cooling rate and asymmetric heat transfer in the cast-rolling clad process, a central macro-segregation band and an interface segregation band were formed in the aluminum matrix. After applying the electromagnetic oscillation field (EMOF), the electromagnetic oscillation stirring effect was induced in the melt of the cast-rolling zone. The dendrites grown at the solidification front were broken by a shearing force and re-entered the melt. In addition, the temperature gradient at the solidification front decreased, which inhibited the growth of dendrites and avoided blocking the equilibrium distribution of solute elements due to the “bridging” of dendrite arms. After applying the EMOF, the central and interface segregation bands of the composite plate were nearly eliminated and the microstructure became more homogeneous. As a results, under the action of the EMOF, the tensile strength, yield strength, effective elongation, and interfacial bonding strength of the TA2/AA7005 cast-rolling composite plate were 303.8 MPa, 204.7 MPa, 21.6%, and 25.3 N/mm respectively, which were significantly improved in comparation with the NF plate.

Genetic relationship of microstructure and mechanical properties between ferrite/pearlite steel plate and billet

This paper proposed the double-pass compression thermal/force simulation experiment to in situ analyze the phase transformation of continuous casting billet during the controlled rolling and controlled cooling process. The genetic relationship between the central segregation of the billet and solute element distribution of the steel plate, and also the genetic relationship of microstructure and mechanical properties between them were experimentally studied. The results show that the microstructure and mechanical properties between the billet and corresponding batch of steel plate have the genetic relationship, and the link of the relationship is central segregation. The phase transformation of supercooling austenite will be affected by the central segregation of Mn and C elements, and the granular bainite abnormal segregation band is the transformation product of Mn and C element segregation region.

Particle mixing and segregation behaviors in the rotating drums with adjacent axial segmentations in different speed directions

The axial mixing and segregation behavior of particles with different properties is a complicated movement process, whose mechanism is still indistinct. Consequently, it is very important to explore dynamic repose angle and flow characteristics of the particles along the axial direction during the mixing and segregation process. Focusing on the mixing and segregation phenomenon in the rotary drum, this paper studies the particle mixing and segregation behavior by changing the speed direction of adjacent axial segmentations. The results show that alternating axial segregation band characteristics can be formed in adjacent axial segmentations with same velocity direction, while the overall mixing degree is better under the condition of the opposite velocity direction. Although the different rotational speed directions of adjacent axial segmentations will cause opposite axial mixing and segregation phenomenon, the general rules of the dynamic angle of repose and axial flow velocity are similar in the rotary drum. In addition, the axial flow structure of the binary particles varies with rotational speed direction of the adjacent axial segments, but not with the drum length to diameter (L / D) ratio and particle filling rate.

Investigation on microstructure heterogeneity of the HPDC AlSiMgMnCu alloy through 3D electron microscopy

The microstructural characteristics of the high pressure die casting (HPDC) AlSiMgMnCu alloys in the skin layer, segregation band and the central zone were investigated in three dimensions by serial block-face scanning electron microscopy (SBFSEM). The results present that the volume fractions of α-Al15(Fe, Mn)3Si (α-Fe phase), β-Mg2Si (β phase) and Q-Al5Cu2Mg8Si6 (Q phase) in the skin layer and the central zone are higher than those in the segregation band zone. The volume fractions and sizes of the θ-Al2Cu phase (θ phase) in the skin layer and the central zone are lower than those in the segregation band zone. In the skin layer, the preferential growth orientation of the α-Fe phase with plate-like morphology is parallel to the heat transfer direction, while the other phases exhibit a complex seaweed-like morphology. In the central zone and the segregation band zone, the shearing stress and turbulent flow during the filling process lead to the morphology of α-Fe and β-Mg2Si relatively regular. However, the morphology of Q-Al5Cu2Mg8Si6 and θ-Al2Cu phases are irregular owing to their lower solidification temperatures. The distribution and morphology of β-Mg2Si, Q-Al5Cu2Mg8Si6 and θ-Al2Cu in the segregation band zone mainly determined the distribution and morphology of the α-Fe phase.

Effect of Cu on the Microstructure of Al-8Zn-2.5Mg-xCu Alloys Fabricated by Twin roll casting

The effect of Cu content on the microstructure and mechanical properties of Al-8.0Zn-2.5Mg-xCu (x: 0, 1, 2, 3) aluminum alloys manufactured by the twin-roll casting process was investigated. The Al-8.0Zn-2.5Mg-xCu alloy showed an increase in surface defects with increasing Cu content. This is because the amount of residual liquid in the final solidification region increased from 9.6 wt.% to 18.3 wt.% as the Cu content increased from 0Cu to 3 Cu alloy. For the 3Cu alloy, as the amount of residual liquid in the final solidification region exceeded the critical point, a large number of surface defects and internal shrinkage defect were observed. The main secondary phases of the four alloys were the T(Mg32(Al, Zn)49) and η(MgZn2) phases, and their fraction increased with Cu content. These secondary phases mainly existed in the center segregation band, and a fine η(MgZn2) phase was additionally observed. In terms of mechanical properties, as the Cu content increased, the hardness of the center matrix, secondary phase, and overall hardness increased respectively. Although the yield strength increased, the tensile strength and elongation decreased because the center segregation band was widened from 684 μm to 790 μm with increasing Cu content.

Effect of Cr segregation on creep strength in grade 91 steels

ABSTRACT Several heats of T91 steel and P91 steel were evaluated, focusing on the distribution of segregation and its effect on creep strength. The difference between the maximum and minimum Cr content, ΔCr, at a constant distance (x) was estimated in the range of 1 mm along the wall thickness direction for T91 steels. The standard deviation of ΔCr was employed as an indicator of the degree of segregation. The standard deviation correlated with time to rupture at 650°C when the Cr content is relatively high. However, no clear correlation between them was observed when the Cr content is very low in the range of specification. In addition to Cr segregation, the Cr content itself should be considered as a cause of reduced creep strength when the Cr content is low. For P91 steel, the distribution of segregation was inhomogeneous as compared with T91 steel. For example, a strongly segregated area was confirmed at the position of 3/4 t from the outer surface of the pipe. On the other hand, no segregation band was observed in other areas of the same pipe.

Effect of non-uniform microstructure on rolling contact fatigue performance of bainitic rail steel

Rolling contact fatigue (RCF) is the main failure mode of wheel-rail system. In this work, the influence of non-uniform microstructure on the rolling contact fatigue performance in bainitic rail steel was investigated. According to the results, an increase in degree of segregation enabled material to improve the wear resistance through the improvement of the local hardness of the rail, but seriously reduced the rolling contact fatigue performance of the rail. The crack propagated along the segregation band, thereby leading to the failure of the specimen. The main cause of fatigue crack initiation was the non-uniform strain distribution under cyclic loading due to different hardness values between the matrix and the segregation region. A large number of dislocations produced by plastic deformation on the surface of the specimen segmented grains into nanocrystals. Meanwhile, it can be seen from the in-situ heating transmission electron microscope (TEM) that the friction heat generated during RCF test was enough to support the occurrence of dynamic recovery.

Analysis of the mechanical properties and microstructure of titanium surfaces designed by electromagnetic induction nitriding

Nitride has high hardness and excellent wear resistance. It is frequently prepared on a material surface to improve material performance. The nitriding layer can be prepared in different ways, so the bonding strength and microstructure between the nitriding layer and the matrix differ, which will directly affect the surface mechanical properties of the material. In this study, pure titanium (TA1) was nitrided using electromagnetic induction nitriding, and the microstructure of nitriding layer was analysed using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy-electron backscatter diffraction (SEM-EBSD) and scanning electron microscopy-energy-dispersive x-ray spectroscopy (SEM-EDS). In addition, the mechanical properties of the nitriding layer were studied using a nanoindentation and scratch tester. The experimental result shows a 20 μm induction nitriding layer composed of TiN, Ti2N and α(N)-Ti. The compound layer (Ti2N and TiN) was approximately 3 μm. The surface was contaminated with C and O elements, and evident segregation bands were found between the induction nitriding layer and matrix. The induction nitriding layer can considerably improve the wear resistance of titanium alloy, but the bonding force between the induction nitriding layer and matrix decreases owing to the segregation band.