Formation Of Ultrafine Grains Research Articles

Achieving balanced mechanical properties in Fe–0.16C plain low-carbon steel by trimodal microstructure and fine cementite

In the present study, for the first time, a trimodal microstructure (including coarse, fine, and ultrafine ferrite grains) was created in plain low-carbon steel by simple thermomechanical processing including intercritical annealing (at 770 °C, 800 °C, and 830 °C), 75% cold rolling, and finally heat treatment at 550 °C. The results showed that martensite with two different morphologies consisting of lath (low-carbon) and plate (high-carbon) were created in the microstructures of 800-75% and 830-75% sheets due to the non-homogeneous distribution of carbon atoms as a result of not using the homogenization treatment. After the final heat treatment, the samples exhibited trimodal grain size distribution of ferrite including coarse grains (larger than 5 micrometers), fine grains (between 1 and 5 micrometers), and ultrafine grains (UFGs) (smaller than 1 micrometer). The formation of ultrafine grains was ascribed to the presence of martensite with lath morphology in the 800-75% and 830-75% samples. The size of cementite particles in the 770-75%-550 sample was larger than the other two samples (800-75%-550 and 830-75%-550) owing to the presence of martensite phase only with plate morphology in 770-75%-550 steel. Unlike the 800-75%-550 sheet, the entire microstructure of the 770-75%-550 and 830-75%-550 samples had not undergone complete recrystallization and several deformed ferrite grains were still visible in the microstructures. The strength and hardness of the heat-treated sheets was larger than that of the as-received steel due to the presence of fine spherical cementite particles and fine ferrite grains in the microstructure of the heat-treated samples. Based on the obtained results, the 800-75%-550 sample revealed the best combination of strength-ductility-toughness due to trimodal microstructure along with the formation of fine spherical cementite particles. The failure mode of all heat-treated samples was ductile and there were both fine and coarse dimples in the fracture surfaces.

Comparison of the corrosion behavior of four Fe-Cr-Ni austenitic alloys in supercritical water

The corrosion behavior of four Fe-Cr-Ni austenitic alloys were investigated after exposure to deaerated supercritical water (SCW) at 650°C. Results show that the corrosion resistance follows the trend:Fe-29Cr-61Ni > Fe-16Cr-75Ni > Fe-25Cr-20Ni > Fe-21Cr-31Ni. The increase in Cr and Ni contents promote a transition from low-protective multilayer oxide scales to protective Cr2O3 scales. After surface grinding, the weight gains of all specimens dropped by one order of magnitude. Surface grinding induces the formation of ultrafine grains and a high density of dislocations, which enhances the corrosion resistance among all studied austenitic alloys. However, the beneficial effects of grinding diminish with increased Cr and Ni contents due to the pre-existing portion of Cr2O3 scales. The lower critical Cr content required to form Cr2O3 scales in high Ni austenitic alloys is primarily attributed to their lower solubility of O and the slower diffusion rates of Ni, which inhibit corrosion behavior.

Interfacial reaction characteristics and mechanisms during dissimilar friction stir lap welding of pure copper and Al0.1CoCrFeNi alloy

In order to explore the weldability and widen the applicability of high entropy alloys (HEAs), dissimilar friction stir lap welding of pure copper and Al0.1CoCrFeNi alloy was investigated in this research. The results indicate that a sound dissimilar weld without tunnels and voids is produced, where a ∼20 μm thick interfacial layer is evident at the Cu-HEA interface. The lap joint is fractured at the upper Cu sheet rather than the lap surface during tensile shear test, suggesting an effective bonding of the dissimilar materials. The lap weld roughly consists of three layers along the plate thickness direction, i.e. welds of Cu, HEA and Cu-HEA interface, respectively. The interface of weld, which has experienced the most severe Cu-HEA interaction during welding, is characterized by uniformly distributed and highly recrystallized ultra-fine grains (∼0.79 μm). Meanwhile, intermetallic compounds particles (Al13Cr2 and Al13Co4) ranging in size of 150∼400 nm are generated at the fine grain boundaries. Fast diffusion of Al, Cr and Co elements and easy segregation of these elements during welding result in the formation of the particles. The sufficient DDRX and the pinning effect of particles are responsible for the formation of ultra-fine grains in the weld interface. The present study lays a theoretical foundation for the joining of conventional Cu and the multi-component HEA, which contributes to the improvement of HEA availability in the field of manufacturing industry.

Intergranular Shielding for Ultrafine-Grained Mo-Doped Ni-Rich Li[Ni0.96 Co0.04 ]O2 Cathode for Li-Ion Batteries with High Energy Density and Long Life.

Deploying Ni-enriched (Ni≥95 %) layered cathodes for high energy-density lithium-ion batteries (LIBs) requires resolving a series of technical challenges. Among them, the structural weaknesses of the cathode, vigorous reactivity of the labile Ni4+ ion species, gas evolution and associated cell swelling, and thermal instability issues are critical obstacles that must be solved. Herein, we propose an intuitive strategy that can effectively ameliorate the degradation of an extremely high-Ni-layered cathode, the construction of ultrafine-scale microstructure and subsequent intergranular shielding of grains. The formation of ultrafine grains in the Ni-enriched Li[Ni0.96 Co0.04 ]O2 (NC96) cathode, achieved by impeding particle coarsening during cathode calcination, noticeably improved the mechanical durability and electrochemical performance of the cathode. However, the buildup of the strain-resistant microstructure in Mo-doped NC96 concurrently increased the cathode-electrolyte contact area at the secondary particle surface, which adversely accelerated parasitic reactions with the electrolyte. The intergranular protection of the refined microstructure resolved the remaining chemical instability of the Mo-doped NC96 cathode by forming an F-induced coating layer, effectively alleviating structural degradation and gas generation, thereby extending the battery's lifespan. The proposed strategies synergistically improved the structural and chemical durability of the NC96 cathode, satisfying the energy density, life cycle performance, and safety requirements for next-generation LIBs.

Intergranular Shielding for Ultrafine‐Grained Mo‐Doped Ni‐Rich Li[Ni0.96Co0.04]O2 Cathode for Li‐Ion Batteries with High Energy Density and Long Life

AbstractDeploying Ni‐enriched (Ni≥95 %) layered cathodes for high energy‐density lithium‐ion batteries (LIBs) requires resolving a series of technical challenges. Among them, the structural weaknesses of the cathode, vigorous reactivity of the labile Ni4+ ion species, gas evolution and associated cell swelling, and thermal instability issues are critical obstacles that must be solved. Herein, we propose an intuitive strategy that can effectively ameliorate the degradation of an extremely high‐Ni‐layered cathode, the construction of ultrafine‐scale microstructure and subsequent intergranular shielding of grains. The formation of ultrafine grains in the Ni‐enriched Li[Ni0.96Co0.04]O2 (NC96) cathode, achieved by impeding particle coarsening during cathode calcination, noticeably improved the mechanical durability and electrochemical performance of the cathode. However, the buildup of the strain‐resistant microstructure in Mo‐doped NC96 concurrently increased the cathode‐electrolyte contact area at the secondary particle surface, which adversely accelerated parasitic reactions with the electrolyte. The intergranular protection of the refined microstructure resolved the remaining chemical instability of the Mo‐doped NC96 cathode by forming an F‐induced coating layer, effectively alleviating structural degradation and gas generation, thereby extending the battery's lifespan. The proposed strategies synergistically improved the structural and chemical durability of the NC96 cathode, satisfying the energy density, life cycle performance, and safety requirements for next‐generation LIBs.

Excellent tensile yield strength with ultrafine grain and tailored microstructure in plastically deformed Ti–Re alloys

The pursuit of strong and ductile titanium alloys for critical structural applications remains a constant focus in metallurgical engineering research. To evaluate the microstructure evolution and strengthening effect of Rhenium (Re) addition, binary alloys of Ti–xRe (x = 0, 2, 4, 6 wt%) were fabricated via spark plasma sintering and hot extrusion techniques. Microstructure characterization by electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) revealed the presence of equiaxed α and β phases in the extruded materials, with an increased fraction of β phase corresponding to higher Re concentrations. X-ray diffraction (XRD) analysis indicated lattice expansion in the β phase due to Re diffusion, suggesting solid solution formation and preferential diffusion of Re into the β phase. Moreover, Re increments of 4 and 6 wt% resulted in significant grain refinement towards the formation of ultrafine grains (0.8 and 0.6 µm, respectively). The extruded Ti–Re alloys exhibited a linear strengthening trend, with an increase of 250–300 MPa per 2 wt% Re added. The yield strength of Ti–6Re exceeded three times higher than that of commercially pure (CP) Ti produced via a similar process, while maintaining over 13 % elongation. Re increment was also found to influence the deformation behavior of the extruded Ti–Re alloys during tensile test. This comprehensive investigation provides valuable insights into the microstructural changes and strengthening mechanisms associated with the addition of Re in titanium alloys, contributing to the ongoing efforts in developing high-performance materials for critical structural applications.

The Microstructure Evolution and Formation Mechanism of Gradient Nanostructure Prepared on CrCoNi Medium-Entropy Alloy.

An equiatomic CrCoNi medium-entropy alloy was subjected to high-energy shot peening (HESP) to fabricate a gradient nanostructure (GNS) in this work. The microstructures of the GNS samples at different depths within the deformed layer were thoroughly investigated. The microstructure exhibited a transformation from unstressed coarse grains to deformed coarse grains, followed by the formation of ultrafine grains, and ultimately reaching a final nanocrystalline structure with a uniform size of approximately 50 nm. Detailed structural analysis indicated that the deformation process was primarily influenced by the interaction between dislocations and deformation twins, which was attributed to the low stacking fault energy (SFE) of the alloy. The nanocrystalline mechanism was divided into three stages. In the coarse-grained deformation stage, the dislocation band divided twin/matrix lamellae into sub-segments, and the cross twin divided coarse grains into ultrafine grains simultaneously. In the ultrafine grain deformation stage, dislocations were arranged around the deformation twins in order to break the twins to form incoherent boundaries, destroying the coherent relationship between the twin and matrix. Finally, in the nanocrystalline deformation stage, the nanocrystalline structure was further divided into smaller segments to accommodate additional strains through the interaction between dislocations and twins.

Effect of deformation methods on microstructure, texture, and properties of a Cu–Mg alloy

The effect of cold working by drawing, rolling, or equal channel angular pressing (ECAP) on the microstructure, the texture, the mechanical properties, and the electrical conductivity of a Cu-0.25%Mg alloy was investigated. The plastic deformation led to the development of continuous dynamic recrystallization that was accompanied by strain-induced low-angle boundary formation and their transformation into high-angle boundaries with straining. The elongated grains evolved after drawing. Sheet rolling provided pancaked ultrafine grain formation assisted by the development of microshear bands. After ECAP almost equiaxed ultrafine grains formed readily inside the deformation microbands. The rapid kinetic of grain refinement assisted by geometric dynamic recrystallization during cold rolling and drawing led to the formation of ultrafine grains with transverse sizes of about 0.20 μm and 0.43 μm, respectively, while the grain size in the ECAP samples was 0.88 μm after total strain of 4. The deformation microstructures led to the significant strengthening resulting in the ultimate tensile strength of about 670 MPa after drawing or rolling to total strain of 4 and 625 MPa after ECAP to total strain of 9.2. Elongation slightly decreased from 21% to 19% after ECAP and dramatically degraded to 6–8% after drawing or rolling. A decrease in elongation was accompanied by a linear increase in the fraction of specific texture components. Electrical conductivity gradually decreased by about 6–10% IACS with straining regardless of deformation method. The strengthening mechanisms and the conductivity variation during deformation are discussed.

Formation of ultrafine grain and mechanical properties in commercial pure titanium subjected to heavy-reduction thermomechanical processing around β transus temperature

The purpose of this study was to investigate the flow stress behavior and formation of ultrafine grains (UFGs) by focusing on the effects of strain-induced dynamic transformation (SIDT) and dynamic recrystallization (DRX) under hot forming near the β transus temperature. Heavy-reduction thermomechanical controlled processing (TMCP) of commercial pure (CP) titanium was performed at deformation temperatures ranging from 700 to 1000 °C, a strain rate of 1 s−1, and a 70% height reduction, followed by water cooling immediately after compression. The flow stress at 0.2 strain decreased significantly from 96 to 25 MPa when the deformation temperature increased from 800 to 900 °C. Equiaxed UFGs with a grain size of 2–4 μm formed in this temperature range. The primary mechanisms of grain refinement in the CP titanium under heavy-reduction TMCP were DRX occurring at 800 °C and a combination of DRX and SIDT (β → α) at 900 °C. Of the 700, 900, and 1000 °C TMCP-treated specimens subjected to a 90° V-bending test, the yield force of the 700 °C TMCP-treated specimen was the highest. When the TMCP temperature was increased from 900 to 1000 °C, the yield force decreased significantly, whereas the springback amount increased. The α phase grain refinement achieved by the TMCP at 700 °C increased the force, and the retained β phases within the 1000 °C TMCP-treated specimen increased the springback amount.

Overcoming the strength-ductility trade-off in additive manufactured AlSi10Mg plates by friction stir welding at a low rotational speed

The main problem in post-treatment of laser powder bed fusion (L-PBF) AlSi10Mg is the sharp reduction in strength when the ductility enhances, i.e. the problem of strength-ductility trade-off. Friction stir welding (FSW) at a low rotational speed of 200 rpm was employed to join AlSi10Mg plate produced by laser-powder bed fusion in conjunction with overcoming the strength-ductility trade-off. The microstructure, texture, and mechanical properties of the as-built and friction stir welded samples were examined. FSW resulted in the elimination of porosities, formation of ultrafine-grains with an average grain size of 0.7 μm, increment in dislocation density and Taylor factor, and fragmentation of Si particles. In addition, after FSW, B/B¯ and C shear texture components were mainly formed in the center of the stir zone, while at the shoulder-dominated zone the A1∗(111)[1¯1¯2] and A2∗(111)[112¯] components might be formed. The main grain structure formation mechanism was continuous dynamic recrystallization occurring by a gradual increase in misorientation of low angle grain boundaries and geometrically dynamic recrystallization. Consequently, tensile elongation was enhanced by ≈173%, while a negligible decrease in strength (≈9%) was observed. During tensile loading, the joint was fractured from the stir zone by the initiation of cracks from the surface of the sample, resulting in a bimodal dimple structure.

A machine learning enabled ultra-fine grain design strategy of Mg–Mn-based alloys

Grain size is the critical characteristic of ultra-fine grain Magnesium (Mg), which is a concrete representation of the whole heat deformation procedure. In this paper, a design strategy was proposed to quantitatively investigate the composition and process conditions for the preparation of ultrafine grains. Herein, a dataset of Mg–Mn-based wrought alloys was constructed, and the average grain size was set as the design target. Based on this dataset, five machine learning (ML) algorithms, including the k-nearest neighbor (kNN), support vector machine (SVM), decision tree (DT), random forest (RF), and artificial neural network (ANN), were integrated to conduct in-depth analysis and make predictions. Among these models, the computational accuracy of both the DT classifier and ANN predictor is around 90%. The main factors affecting the formation of ultrafine grains were found by ML, and the interrelationships between the features were quantitatively analyzed as well. Then, four suggested routes with conditions were extracted from the tree models for preparing ultrafine grain Mg alloys. And four new Mg alloys were designed through these routes and taken as experimental validation. After testing, the actual grain sizes are close to the predictions, and the accuracy of the experimental verification exceeds 80%. Compared with conventional “trial and error” design methods, the grain design strategy proposed in this paper brings new thought and good prior guidance for developing high-performance commercial Mg alloys.

Microstructure evolution of adiabatic shear band in AZ31 alloy under dynamic compression

The formation of adiabatic shear band (ASB) and its damage behaviour in AZ31 alloy under high strain rate compression (2000 s−1) were investigated in this study by using a split Hopkinson pressure bar. Microstructure of the ASB was characterised by electron back-scattering diffraction and transmission electron microscopy, and the adiabatic shear damage behaviour was analysed through finite element simulation by LS-DYNA. The results show that the ASB and the surrounding microstructure form a gradient microstructure distribution, and the formation of ultra-fine grains in the ASB is due to rotational dynamic recrystallisation. The combination of work hardening and grain refinement in ASB leads to a high microhardness. Micro-voids grow in the ASB and eventually form macro-cracks, leading to material failure.

Formation of ultrafine grains in magnesium alloy during dynamic extrusion at room temperature

Magnesium alloy is a class of lightweight structural materials with promise for many applications. Here, the authors report a significant grain refinement, high deformation twin density and obvious nanograined structures in magnesium alloy through a novel high strain rate plastic deformation method—dynamic extrusion. By using finite element simulation and transmission electron microscopy (TEM), the authors find that the stain rate during plastic deformation affects the microstructure evolution process, which results in the dynamic deformation mechanisms transition. The results are of interests not only for understanding the deformation mechanisms under dynamic plastic deformation but also for nanostructures formation of the magnesium alloy.

Formation of ultrafine grains and twins in the β-phase during superplastic deformation of two-phase brasses

During superplastic deformation of microduplex-structured brasses, the strain is primarily localized in the β-phase where deformation occurs by grain boundary sliding (GBS) and dislocation slip/creep mechanisms. Dynamic recrystallization and twinning were observed in the b.c.c. β-phase leading to a transformation of the initial coarse β-phase grains with average sizes of ∼4–5 µm to ultrafine grains with sizes below 1 µm. Alloying with Al refined the β-grains, improved the superplastic behavior and reduced the occurrence of residual cavitation.

Achieving superior combination of yield strength and ductility in Mg–Mn–Al alloys via ultrafine grain structure

Ternary magnesium-based alloys with an aluminum content of 0.5 wt.% and a varying manganese content of x = 1, 2, and 3 wt.% (Mg-xMn-0.5Al) were prepared and characterized. The study focused on the effect of Mn addition on the formation of ultrafine grains (UFGs, average grain size <1 μm) and the mechanical properties of the alloys. After extrusion at a relatively low temperature of 250 °C, the average grain sizes of Mg-2Mn-0.5Al and Mg-3Mn-0.5Al samples were successfully refined to values of 0.9 ± 0.39 μm and 0.8 ± 0.34 μm, respectively, of submicron scale. The Mg-3Mn-0.5Al sample simultaneously demonstrated superior yield stress (YS, 274 MPa) and fracture elongation (FE, 48.9%). The high YS was associated with the UFG structure, residual dislocations, and nanosized precipitates, while the high FE was attributed to the low dislocation storage rate and UFGs with random orientations. The formation of UFGs is mainly caused by a recrystallization process promoted by particle-stimulated nucleation. The grain growth is suppressed by dynamic precipitated α-Mn particles.

Microstructural, mechanical and tribological characterization of Cu-Co-Ni-Be alloy processed via equal channel angular pressing

Effects of different processes including various combinations of equal channel angular pressing (ECAP) and aging on the microstructure, mechanical and tribological properties of the quadruple Cu-Co-Ni-Be alloy were investigated. ECAP process results in a significant grain refinement and brings about an ultrafine-grained (UFG) microstructure. Grain boundary strengthening and strain hardening mechanisms increase the strength and hardness of the ECAPed alloy significantly. Subsequent aging of the ECAPed sample results in a further increment in the hardness and strength due to the formation of precipitates besides the UFG structure. Wear resistance and operative wear mechanisms of the alloy depend on the applied processes strongly. ECAPed + aged sample with the highest hardness exhibit the highest wear resistance among all samples. Detailed investigations show that various wear mechanisms are operative in the Cu-Co-Ni-Be alloy samples. Adhesive wear is the mean wear mechanism in the initial and aged samples, and it is more prominent in the initial sample than the aged one due to the its quite softer and more ductile nature. Limited abrasive wear is also operative in these samples. delamination wear, on the other hand, is observed to be the main wear mechanism in the ECAPed and ECAPed + aged samples. Wear particles generated due to the delamination process cause three-body abrasive wear as an another wear mechanism in the UFG samples. Besides, oxidative wear is also operative in these samples and it is more prominent than that of the initial and aged ones. The achieved results show that UFG formation by any manner and subsequent aging processes are effective to improve wear resistance of the Cu-Co-Ni-Be alloys.

Large and Severe Plastic Deformation of Metals: Similarities and Differences in Flow Mechanics and Structure Formation

Large and severe plastic deformations (SPD) are two widely known approaches for the processing of metallic billets that have a different effect on the microstructure and physico−mechanical properties of materials. After a large deformation is implemented, for example, by longitudinal rolling or drawing, dislocation substructure cells and microbands are formed in billets. As a result of SPD, in particular, after equal‐channel angular pressing (ECAP), ultrafine nanoscale grains are formed in metals,[1,2] which provides their high mechanical and functional properties. Herein, in both cases, structure formation is largely conditioned by the patterns in the mechanics of plastic deformation. The difference in the produced microstructures is determined by different scalar and vector characteristics of large and severe strains. In the former case, monotonic strain accumulation takes place in a billet. In the latter case, due to the active nonmonotonic deformation of a billet, heavy strain is accumulated, and rotational modes are realized, which, as a result, produces shear bands with further formation of ultrafine grains with high‐angle grain boundaries. As a conclusion, a more precise definition of SPD is introduced, and recommendations are given for increasing the efficiency of the process of UFG structure formation in metals.

Fabrication of fine-grained, high strength and toughness Mg alloy by extrusion−shearing process

A novel extrusion−shearing (ES) composite process was designed to fabricate fine-grained, high strength and tough magnesium alloy. The structural parameters of an ES die were optimized by conducting an orthogonal simulation experiment using finite element software Deform-3D, and Mg−3Zn−0.6Ca−0.6Zr (ZXK310) alloy was processed using the ES die. The results show that the optimized structural parameters of ES die are extrusion angle ( α ) of 90°, extrusion section height ( h ) of 15 mm and inner fillet radius ( r ) of 10 mm. After ES at an extrusion temperature and a die temperature of 350 °C, ZXK310 alloy exhibited good ES forming ability, and obvious dynamic recrystallization occurred in the forming area. The grain size decreased from 1.42 μm of extrusion area to 0.85 μm of the forming area. Owing to the pinning of second phase and formation of ultrafine grains, the tensile strength, yield strength and elongation of alloy reached 362 MPa, 289 MPa and 21.7%, respectively.

Abrasion wear of austenitic manganese steels via jaw crusher test

The abrasion wear of the austenitic manganese steels with 12% Mn and 20% Mn was evaluated via jaw crusher tests. The abrasive mass (i.e. gravel) used per test was 100 kg and 14.3 mm of mean sized particles. The wear characterization was carried out with the evaluation of mass loss, hardness profile, worn surface roughness, and wear micromechanisms. Additionally, the subsurface's microstructure was analysed by a Focused Ion Beam (FIB). The stationary and movable jaws presented differences in mass loss and wear micromechanisms. The stationary jaw has higher wear than the movable one. These results are related to the micromechanism of microcutting and microploughing to the stationary jaw and the micro-indentation to the movable jaw. For the relative wear of the jaws' materials, there was a significant difference for the values of stationary jaw conditions, resulting in higher wear resistance for the high manganese alloy applied in this condition. FIB results show that the wear promotes the formation of ultrafine grains and deformation twins in the subsurface layer.

Mechanical behaviour of Aluminium Alloy AA6063 processed through ECAP with optimum die design parameters

ABSTRACT Equal Channel Angular Pressing (ECAP) is one of the most effective techniques for the formation of ultra-fine grained bulk material. The material is passed through the channel in the specially designed die in the ECAP process. The die design parameters like corner angle, channel angle and other parameters play an important role in the formation of ultra-fine grains. The die with optimum design parameters, with channel angle 90° and corner angle 20°, was designed and fabricated in the laboratory and commercial Aluminium AA6063 was used as sample material to investigate the mechanical behaviour/properties before and after the ECAP process. In the present work, the mechanical behaviour of commercially pure Aluminium processed by ECAP die with an optimum die design parameter is studied and examined. The path adopted was BC and pressing was done from 0 to 3 numbers of times. The results revealed that the grain size reduced from 530 to 220 nm and tensile strength increases from 275.8 to 368.4 MPa after three numbers of ECAP passes.