Boundary Precipitation Research Articles

Microstructure and mechanical properties of wire arc additive manufactured ZL205A alloy

The evolution of grain structure, precipitates and mechanical properties of the ZL205A alloy prepared by wire arc additive manufacturing were investigated. The results show that the as-deposited alloy consists of equiaxed and columnar grains in the bottom region. As the deposition height increases, the alloy mainly consists of equiaxed grains. The evolution of columnar to equiaxed grains is explained by the solidification theory. The needle-like θ′-Al2Cu precipitates in Cu-rich regions surrounding the α-Al+θ-Al2Cu eutectics. As the part deposited first is subjected to multiple cycles of heating by the subsequently deposited layers, the volume fraction and average size of θ′ precipitates gradually decrease with the increase of deposition heights. The microhardness of the as-deposited alloy fluctuates in the range of 74.6–84.5 HV. The mechanical properties of the horizontal specimens at different heights are affected by the combination of grain structure, precipitates and pores, and that of the specimens in the middle region are optimal. The mechanical properties of the horizontal and vertical specimens show isotropic. Attributed to the grain boundary strengthening and precipitation strengthening, the yield strength (132.9 MPa) and elongation (15.9%) of the as-deposited alloy are approximately 31.3% and 211.8% higher than that of the as-cast alloy, respectively.

The microstructure and mechanical performance optimization of a new Fe-Ni-based superalloy for Gen IV nuclear reactor: The critical role of Nb alloying strategy

The effects of Nb alloying on microstructural evolution and mechanical performance of a newly developed solid-solution strengthened Fe-Ni-based superalloy, considered as the candidate material for high-temperature components manufacturing in fourth Generation (Gen IV) advanced nuclear reactor, were systematically investigated. The results show that the increase in the Nb content obviously increased the volume fraction and average size of MC carbides, and the MC carbides more preferentially distributed at grain boundaries. The average grain size and low Σ coincident site lattice (CSL) boundary exhibited a gradually decreased trend with increasing Nb content. Meanwhile, the thermal stability (grain size and grain boundary precipitation) of the alloy can be improved notably via Nb alloying. The tensile strength of Nb-alloyed alloys at 750 °C was enhanced significantly compared to that of Nb-free alloy, mainly due to the synergistic effect of solid-solution strengthening and fine grain strengthening. It was impressive that the variation of Nb content had no obvious influence on the yield strength, but gradually decreased the ultimate tensile strength. The detrimental effects of the decrease in Σ3 boundaries and intergranular precipitates were assumed to restrict the strengthening factors. The ductility of Nb-alloyed alloys decreased remarkably due to the changed dominant deformation mechanism from planar slip to wavy slip; however, it showed an increasing trend in high Nb-content alloys due to the occurrence of dynamic recrystallization and grain rotation. The present work establishes Nb alloying as an effective method to optimize the microstructure and mechanical properties of solid-solution strengthened Fe-Ni-based superalloys, and the appropriate Nb content is recommended to be controlled below 0.5 wt% for this alloy.

Microstructures and mechanical properties of a carbon-doped CuFeMnNi multiple-principal-element alloy

Carbon-doped CuFeMnNi multiple-principal-element alloys (MPEAs) were prepared via hot-forging and subsequent annealing at 900 °C for different durations varied from 15 min to 360 min. The annealed alloys exhibit dual face-centered-cubic (FCC) structures due to the immiscibility between Cu and Fe, and the microstructures remain thermodynamically stable after long annealing at such a high temperature. A large amount of FeMn-carbides inside and at the edge of Fe-rich dendrite arm were still undissolved after annealing for 15 min and 120 min, which inhibited the growth of recrystallized grains in the region. In addition, most carbides dissolved during annealing at 900 °C for 360 min while the size of Cu precipitates increased with the increase of annealing time. Compared with the forged state, the as-annealed alloys exhibited good softening-resistant capability with the increase of annealing durations. The yield strength and ultimate tensile strength kept constant at 400 ± 10 MPa and 800 ± 30 MPa while ductility distinctly increased from 54% to 65%. Grain boundary strengthening, solid-solution strengthening and precipitation strengthening jointly contributed to the strength of the prepared alloys. The strengthening mechanism related to Cu precipitates gradually changed from shear mode to Orowan mode with increasing annealing time.

Achieving exceptional ultrahigh-strength in low-alloy Mg-1Al-1Ca-0.4Mn (wt%) alloy by regulating bimodal microstructure

A Mg-1Al-1Ca-0.4Mn (AXM1104, wt%) alloy extruded at 250 ℃ with a ram speed of 0.1 mm/s obtains a unique bimodal microstructure. As-extruded alloy exhibits a high tensile yield strength (YS) of 474 MPa but a low ductility of 2.1%, in which the YS exceeds conventionally extruded rare-earth-free Mg alloys reported so far. After annealing at 300 ℃ for 10 min, the annealed alloy achieves a good strength-ductility balance, with a YS of 426 MPa and an elongation of 5.4%. The ultrahigh YS of the as-extruded alloy is attributed to the combined strengthening efect of grain boundary, dislocation and texture, while the as-annealed alloy is mainly strengthened by both grain boundary strengthening and precipitation strengthening from planar Al2Ca nano phase.

Structure evolution and degradation mechanism of compressive strength for (Zr0.25Hf0.25Ti0.25Ta0.25)C high-entropy ceramics with ultra-strength of 3.62 GPa

(Zr0.25Hf0.25Ti0.25Ta0.25)C high-entropy ceramics (ZHTTC) with exceptional compressive strength were prepared by spark plasma sintering at different temperatures (1900–2200 ℃). The effect of average grain size on compressive strength were investigated. The results indicate that the compressive strength degrades with the increase of the average grain size. The grain growth from 3.309 ± 0.603–9.25 ± 1.98 µm as the temperature elevated from 1900 to 2200 ℃. The ZHTTC-1900 achieves the maximum compressive strength of 3620 ± 180 MPa, while the compressive strength of ZHTTC-2200 is 1912 ± 203 MPa, which is 47 % lower than that of ZHTTC-1900. The increase of sintering temperature not only causes the increased average grain size, but also induces the decrease of grain boundary precipitation sites and adhesion of grain boundaries. The above reasons result in the degradation of compression properties.

Precipitation behavior and mechanical properties of a novel Cu–2Fe-0.5Ti alloy produced by thermos-mechanical treatment

The effect of the multi-stage thermo-mechanical treatment on the microstructure and properties of a Cu–2Fe-0.5Ti alloy (wt.%) was investigated. After repeated thermos-mechanical treatment, the alloy has the yield strength, tensile strength, elongation, electrical conductivity and softening temperature of 521 MPa, 554 MPa, 5.9%, 70.4 %IACS and 550 °C, respectively. The nano-scale Fe2Ti and γ-Fe precipitates, accompanying with some submicron-scale primary Fe2Ti phase, are found in the aged alloy. The orientation relationship between the nano-scale Fe2Ti precipitate and the Cu matrix is Fe2Ti [1‾ 21‾ 6]//Cu [01‾ 1] (with a small deviation) and Fe2Ti (02 2‾1‾)//Cu (200), while that between the nano-scale γ-Fe precipitate and the Cu matrix is γ-Fe [110]//Cu [110] and γ-Fe (111)//Cu (111), respectively. The precipitation of Fe2Ti and γ-Fe phase contributes to the improvement of the electrical conductivity and mechanical properties of the alloy. The strengthening mechanisms of the designed alloy is mainly grain boundary strengthening, dislocation strengthening and precipitation strengthening.

Effect of Zr content on corrosion behavior and chemically-milled surface roughness of Al-Cu-Mg alloy

Al-Cu-Mg alloys, as important aircraft skin material in the aerospace field, should have excellent corrosion resistance and lower chemically-milled surface roughness. Fortunately, we found that microalloying with Zr elements can solve the challenges. In this work, the effects of adding the Zr to Al-3.8Cu-1.2Mg alloys the microstructural evolution, intergranular corrosion (IGC), electrochemical corrosion and chemically-milled surface roughness are investigated by using OM, SEM, and TEM studies. The electrochemical measurements tests are performed using a CHI760 electrochemical testing instrument and the chemically-milled surface roughness is measured by JB-4 C precision roughness tester. The experimental results show that the Zr additions to Al-3.8Cu-1.2Mg alloy could refine grain size, improve solidification segregation, increase the spacing of grain boundary precipitation phases, enhance corrosion resistance, and optimize the chemically-milled surface roughness. It is found that with the increase of Zr content, the precipitate-free zones (PFZs) of the alloy gradually become narrower and the grain boundary precipitation phases (GBPs) are discontinuously distributed. In addition, a narrower PFZ and discontinuous GBP are beneficial to enhance the IGC resistance of the alloy. The alloy with 0.2 wt% Zr exhibits the best corrosion resistance and the lowest chemically-milled surface roughness. The finding provides a good strategy for the high corrosion resistance and superior chemically-milled surface roughness properties for applications as aircraft skin material in the aerospace field.

Enhanced strength in Co-free Ni47.5-xFe25Cr25AlxTi2.5 high entropy alloys via introducing dual precipitates

Synergistic strengthening mechanisms (e.g. solid solution strengthening, grain/twin boundary strengthening, dislocation strengthening, and precipitation strengthening) have been shown to effectively achieve a balance between strength and ductility of mechanical properties of metal materials. In this study, Co-free AlxNi47.5-xFe25Cr25Ti2.5 high entropy alloys (HEAs) mainly strengthened by grain boundary/twin-precipitation were successfully prepared by combining rolling and annealing. The phase-field method was used to demonstrate that twins enhance the Hall-Petch effect by introducing additional twin boundaries. Only a small amount of lamellar L12 phase is precipitated in the aged Al2.5 HEA. The tensile strength is 1086 MPa and the elongation is 30.8%. With the increase of Al content, the volume fraction of L12 phase in the alloy increases, and the bulk B2 phase is precipitated. When the Al content increases to 7.5 at. %, the strength of the alloy increases to 1757 MPa, but the plasticity is catastrophically reduced to only 3.4%. The excellent strength of aged Al7.5 HEA is mainly due to the good combination of grain/twin boundary strengthening and precipitation strengthening.

Thermal stability and deformation mechanisms in Ni-Co-Fe-Cr-Al-Ti-Nb-type nanoparticle-strengthened high-entropy alloys

The precipitate morphologies, coarsening kinetics, elemental partitioning behaviors, grain structures, and tensile properties were explored in detail for L12-strengthened Ni39.9Co20Fe15Cr15Al6Ti4–xNbxB0.1 (x = 0 at.%, 2 at.%, and 4 at.%) high-entropy alloys (HEAs). By substituting Ti with Nb, the spheroidal-to-cuboidal precipitate morphological transition, increase in the coarsening kinetics, and phase decomposition upon aging at 800 °C occurred. The excessive addition of Nb brings about the grain boundary precipitation of an Nb-rich phase along with the phase decomposition from the L12 to lamellar-structured D019 phase upon the long-term aging duration. By partially substituting Ti with Nb, the chemically complex and thermally stable L12 phase with a composition of (Ni58.8Co9.8Fe2.7)(Al12.7Ti5.8Nb7.5Cr2.3) ensures the stable phase structure and clean grain boundaries, which guarantees the superb high-temperature mechanical properties (791 ± 7 MPa for yielding and 1013 ± 11 MPa for failure) at 700 °C. Stacking faults (SFs) were observed to prevail during the plastic deformation, offering a high work-hardening capability at 700 °C. An anomalous rise in the yield strength at 800 °C was found, which could be ascribed to the multi-layered super-partial dislocations with a cross-slip configuration within the L12 particles.

Complete Desensitization of Aluminum–Magnesium Alloys via Boron Addition

We address here an important issue related to sensitization effects in Al5083 by mitigating the grain boundary precipitation of the beta phase and demonstrate that the addition of a small amount of boron to Al5083 impedes the precipitation of the beta phase, Al3Mg2, also known as the Samson phase. In Al–Mg alloys, the precipitation of Al3Mg2 usually occurs at grain boundaries in the temperature range of 50 to 200 °C from a supersaturated solid solution of Al–Mg and makes these alloys susceptible to intergranular corrosion and stress corrosion cracking. Upon boron addition, we show, using transmission electron microscopy, that a diboride phase, AlMgB2, forms at grain boundaries instead of the beta phase upon extended annealing at 150 °C. This diboride phase does not dissolve in saltwater, suggesting it is less anodic relative to the matrix. To quantify and compare the dissolution characteristics, we carried out nitric acid mass loss test for Al5083 samples containing 3 wt.% boron treated at 190 h at 150 °C, and fully sensitized Al5083 samples containing 0.0 wt.% boron. We estimate the mass loss to be 4 mg/cm2 for boron containing samples as compared to the mass loss of 45 mg/cm2 for samples without boron, indicating that the addition of boron is highly effective in suppressing the susceptibility to intergranular corrosion in Al5000 series alloys. This provides a potential route to minimize the longstanding problem of ship structure sensitization.

Characterization of boundary precipitation in a heavy ion irradiated tungsten heavy alloy under the simulated fusion environment

In the concerted effort to identify materials capable of surviving the adverse environment of a fusion reactor interior, tungsten heavy alloys have been put forth as candidates. Experimental trials and behavioral studies have yielded positive results for their adoption by taking advantage of the alloy's unique balance of high fracture toughness and high tungsten content; yet due to their relative novelty in the fusion community, there remains a lack of understanding on the response of these materials to the extended high temperature irradiation environment of the reactor interior. To alleviate this issue and provide the necessary data on the behavior of tungsten heavy alloys to the simulated fusion environment, a 90W-7Ni-3Fe alloy has been subjected to elevated temperature sequential Ni+ and He+ ion irradiations to mimic the expected displacement damage and He gas production expected after five years of service as materials for plasma facing components. Atomic-scale structural analyses and nanoscale chemical mapping have identified the formation of two distinct precipitation structures, a surface localized η-carbide and a hexagonal W2C type tungsten carbide, both of which appear to originate at the bi-phase interface between W and the ductile phase. This irradiation enhanced and induced precipitate formation respectively is anticipated to adversely affect these materials by selective embrittlement at bi-phase interfaces leading to a reduction in the material's overall fracture toughness during prolonged high temperature irradiation. It is asserted that any potential exposure to C during fusion reactor operational service should be minimized as to prevent the formation of these phases.

On precipitation hardening behaviour in a triaxial forged Mg-2Zn-2Gd alloy and relationship to mechanical properties

ABSTRACT Currently, there is a significant interest in magnesium (Mg) alloys for structural and functional applications. Mg alloys are lightweight materials (1.7–2.0 g/cm3), whose density is significantly lesser than that of titanium alloys and stainless steels. Furthermore, from the perspective of biomedical devices, they are biodegradable, bioresorbable and biocompatible. The objective of this work is to investigate the transition from grain boundary precipitation behaviour in the as-cast alloy to nanoscale precipitation during triaxial forging, with particular emphasis on exploring the application of small-angle X-ray scattering technique to investigate nanoscale precipitation and complement with the conventional approach of X-ray diffraction and electron microscopy.

Influence of 1:7 H phase stability on the evolution of cellular microstructure in Fe-rich Sm-Co-Fe-Cu-Zr magnets

Fe-rich Sm2Co17-type magnets have many essential applications prospects in high-temperature operation environments. However, the formation of complete cellular microstructure becomes increasingly difficult when the Fe content of Sm2Co17-type magnets exceeds 20 wt%. Herein, the phase stability of 1:7 H phase and its effect on the precipitation of the 1:5 H precipitates in Fe-rich Sm2Co17-type magnets are investigated. The results confirm that the stability of the 1:7 H phase decreases with the increasing of Fe content for the solution-treated magnets. Not only 2:17 R phase but also a lot of 2:17 R′ phase is found in solution-treated magnets when the Fe content exceeds 22 wt%. As the Fe content increases, the density of initial defects decreases for the growth of 2:17 R nanotwins in the solution-treated magnets. Furthermore, the segregation of Cu in the defects-aggregated cell boundaries weakened gradually. Consequently, the nucleation and precipitation of cell boundary precipitates is difficult, and the cellular microstructure becomes incomplete and coarse when the Fe content is above 22 wt%. The present work may provide a further insight for understanding the evolution of cellular microstructure depending on the phase decomposition behavior in Fe-rich Sm2Co17-type magnets.

Corrosion behavior and mechanism of 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet in sea water acidified accelerated test

The microstructure and corrosion properties of a novel 4004/Al-Mn-Cu-Mg-Si/4004 aluminum brazing sheet were investigated in comparison with the traditional 4004/3003/4004. It revealed that, the microstructure after brazing of two aluminum brazing sheets from outer to center were both residual Al-Si eutectic, precipitate-free zone (PFZ), band of dense precipitates (BDP) and core material, respectively. However, the alloying element distribution and the types and amounts of second phases were quite different. The sea water acidified accelerated test showed that 4004/Al-Mn-Cu-Mg-Si/4004 exhibited typical exfoliation corrosion, while the corrosion quickly spread deep into the core material of 4004/3003/4004. The formation of Cu concentration gradient and the strip distribution of Al-Mg-Cu-Si precipitates in the BDP region contributed to the exfoliation corrosion of 4004/Al-Mn-Cu-Mg-Si/4004. The corrosion behaviors of Al-Mn-Cu-Mg-Si and AA3003 core materials are both in form of intergranular corrosion (IGC)，and the later was more severe than the former. For Al-Mn-Cu-Mg-Si core material, high-density fine dispersed cathodic α-Al(Mn, Cu)Si precipitates promoted the formation of a profusion of micro-galvanic cells in the Al matrix, which would retard any effects between the grain boundary precipitations and the adjacent relatively PFZs. Hence, the IGC susceptibility of Al-Mn-Cu-Mg-Si core material was decreased. Besides, less large-sized α-AlFeMn(Cu)Si phases in core material also reduced the localized corrosion attack. At last, the corrosion in depth direction of 4004/Al-Mn-Cu-Mg-Si/4004 was retarded, compared to its counterpart.

Atomic-scale understanding of solute interaction effects on grain boundary segregation, precipitation, and fracture of ultrahigh-strength maraging steels

Understanding the fundamental mechanisms of embrittlement and de-embrittlement is crucial for the development of strategies toward advanced steels with improved performance. In this study, the solute interaction effects on grain boundary (GB) segregation, precipitation, and fracture of Fe–Ni–Ti–(Mo) maraging steels were systematically investigated through a combination of experimental and theoretical techniques, including scanning transmission electron microscopy, atom probe tomography, and first-principles calculations. Our results reveal that the Fe–Ni–Ti maraging steel suffers from serious intergranular embrittlement and the mechanism is related to the formation of coarse Ni3Ti precipitates and associated precipitate-free zones (PFZs) at GBs, which facilitate the crack initiation and growth along the GBs. Interestingly, adding Mo to the maraging steel effectively suppresses the intergranular embrittlement, thereby substantially improving the ductility. Atomistic analyses reveal that Mo de-embrittles the GBs by reducing the segregation of Ni and Ti, which substantially inhibits the formation of coarse Ni3Ti precipitates and PFZs at the GBs, thereby alleviating their harmful impact on the GB cracking. In addition, the Mo segregation enhances the GB cohesion, which may play a minor role in suppressing the GB fracture.

Refractory high-entropy nanoalloys with exceptional high-temperature stability and enhanced sinterability

Nanocrystalline alloys (nanoalloys) are prone to grain growth. It is known that grain boundary segregation and precipitation can stabilize nanoalloys, but the stabilization becomes less effective at high temperatures and adding grain growth inhibitors often reduces sinterability. Herein, we have simultaneously achieved exceptional high-temperature stability and improved sinterability for a class of TiNbMoTaW-based refractory high-entropy nanoalloys (RHENs). Bulk pellets of RHENs were fabricated through ball milling and spark plasma sintering, achieving 93–96% relative densities with 50–100 nm grain sizes for three compositions. For example, Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 sintered at 1300 °C attained ~ 96% relative density with ~ 55 nm mean grain size. Moreover, these RHENs exhibited exceptional stability at 1300 °C. Both Ti17.8Nb17.8Mo17.8Ta17.8W17.8Ni6Zr5 and Ti18.8Nb18.8Mo18.8Ta18.8W18.8Ni6 retained < 150 nm grain sizes after five hours annealing at 1300 °C. Notably, the addition of Ni, a well-known sintering aid for activated sintering of refractory metals such as W and Mo, in high-entropy TiNbMoTaW can promote sintering while maintaining high-temperature stability against rapid grain growth. This may be explained by hypothesized high-entropy grain boundary (HEGB) effects, while we recognize the possible (additional) effects of compositional inhomogeneity and secondary phase (Zener) pinning. These RHENs possess some of the highest temperature stability achieved for nanoalloys and ultrafine-grained metals.

Austenite grain boundary segregation and precipitation of boron in low-C steels and their role on the heterogeneous nucleation of ferrite

The addition of boron (B) to steels suppresses the austenite to ferrite phase transformation dramatically, thus increasing their hardenability. It achieves this through grain boundary (GB) segregation at the austenitic GBs that delays the ferrite nucleation. Though the effects of B segregation on hardenability have long been known, the mechanisms of B segregation and how exactly B suppresses the ferrite nucleation remain elusive. We designed a B-containing low-C steel to study the B segregation and precipitation behavior. We conducted heat treatments with different austenitization temperatures and cooling rates. Site-specific atom probe tomography, complemented by high-resolution secondary ion mass spectrometry reveals B segregation at prior austenite grain boundaries (PAGBs) along with carbo-boride precipitation. The B segregation mechanisms are discussed in detail based on these observations considering their dependence on austenitization temperature and cooling rate. Furthermore, we analyzed the impact of GB energy reduction through nucleation kinetics calculations. From our analysis, we conclude that the reduction in GB energy affects the grain corner and grain edge nucleation substantially. However, the retarding effects of carbo-boride precipitation on ferrite nucleation cannot be completely excluded. We provide a detailed account of the possibilities of how B-containing precipitates may be suppressing ferrite nucleation.

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.

Accelerating matrix/boundary precipitations to explore high-strength and high-ductile Co34Cr32Ni27Al3.5Ti3.5 multicomponent alloys through hot extrusion and annealing

• Hot extrusion reduces grain sizes and simultaneously accelerates precipitation. • Pre-existing dislocation slips and hierarchical precipitates cause high strength. • Interaction among dislocations, stacking faults, and twins ensures acceptable ductility. Annealing-regulated precipitation strengthening combined with cold-working is one of the most efficient strategies for resolving the conflict between strength and ductility in metals and alloys. However, precipitation control and grain refinement are mutually contradictory due to the excellent phase stability of multicomponent alloys. This work utilizes the high-temperature extrusion and annealing to optimize the microstructures and mechanical properties of the Co 34 Cr 32 Ni 27 Al 3.5 Ti 3.5 multicomponent alloy. Hot extrusion effectively reduces grain sizes and simultaneously accelerates the precipitation of coherent L1 2 nanoparticles inside the face-centered cubic (FCC) matrix and grain boundary precipitations (i.e., submicron Cr-rich particles and L1 2 -Ni 3 (Ti, Al) precipitates), resulting in strongly reciprocal interaction between dislocation slip and hierarchical-scale precipitates. Subsequent annealing regulates grain sizes, dislocations, twins, and precipitates, further allowing to tailor mechanical properties. The high yield strength is attributed to the coupled precipitation strengthening effects from nanoscale coherent L1 2 particles inside grains and submicron grain boundary precipitates under the support of pre-existing dislocations. The excellent ductility results from the synergistic activation of dislocations, stacking faults, and twins during plastic deformation. The present study provides a promising approach for regulating microstructures, especially defects, and enhancing the mechanical properties of multicomponent alloys.

Microstructure Characterization and Strengthening Mechanism Analysis of X100 Pipeline Steel

The strengthening mechanism of X100 high steel grade pipeline steel, including grain boundary strengthening, solution strengthening, precipitation strengthening, dislocation strengthening, and texture strengthening, was analyzed by the technics of scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction (XRD), physicochemical phase analysis, and so on. The results showed that the tested steel had a mixed structure of granular bainite and lath bainite, the average effective grain size was refined to about 1 μm by severe hot plastic deformation, the dislocation density was as high as 1.74 × 1015/m2, and the second phase precipitation was mainly (Ti, Nb)(C, N) in submicron scale and nanoscale NbC. Through orientation distribution function (ODF) orientation analysis, it was found that the tested steel had obvious anisotropy and had a strong rolling direction (RD) texture along the rolling direction compared with the 30° direction: {115}&lt;110&gt; and {113}&lt;110&gt;. After the quantitative analysis of strengthening mechanisms, it was found that the highest strengthening increment was caused by high-density dislocations in bainite of about 268 MPa, while the lowest strengthening increment was induced by precipitation particles of about 31 MPa.