Formation Of Ultrafine-grained Structure Research Articles

Multiple semi-coherent particles strengthened ultra-fine-grained Al composites for neutron shielding materials

Neutron shielding materials face imbalanced behaviors among shielding, strength, and ductility properties. Based on the requirement of the high property shielding particles, a superior semi-coherent τ(Al4MgGd) phase was designed and predicted by cluster expansion (CE) method using density functional theory calculations. To realize its shielding property, the Powder Metallurgy-based routines (i.e., powder fabrication, spark plasma sintering, and hot extrusion techniques) are used to fabricate 6TiB2/Al-6Mg-5Gd (wt.%) composite with dispersed refined τ phases and homogenized TiB2 distribution. The atomic structure of ternary phase τ is examined by aberration-corrected high-angle annual dark-field (HAADF) scanning transmission electron microscope (STEM) and energy dispersive X-ray spectroscopy (EDXS) STEM experiments, which is well complied with the calculated compound (Al4MgGd). In detail, the τ(Al4MgGd) phase has a semi-coherent interface both with α-Al and TiB2, which is consistent with the prediction of interface relationships. With the optimized interfaces, the TiB2 and τ phases can effectively promote recrystallization and suppress grain growth, leading to the formation of ultra-fine grain structure. Then, the composite exhibits advanced shielding properties (Macroscopic transmission cross section ∼24.1 /cm, higher than 30 %B4C/Al) and optimized synergic mechanical properties (Ultimate tensile strength ∼506 MPa, elongation ∼12.9 %), which are far higher than available Al-based neutron shielding materials. Finally, the underlying strength-ductility mechanisms are discussed. Critically, the design and optimization of shielding particle interfaces are reliable strategies for developing novel structural-functional integrated materials.

Rational Methods of Plastic Deformation Providing Formation of Ultrafine-Grained Structure in Large-Sized Products

Based on the peculiarities of plastic deformation mechanics and physical mesomechanics, the processes are considered and technological schemes for their realization are proposed to ensure the formation of ultrafine-grained structure in axisymmetric products of large sizes. Deformation of coarse-grained materials in temperature-velocity conditions of superplastic deformation and methods of severe plastic deformation are taken as methods of preparation of such a structure. All of these deformations include large strain, and they are carried out at low hydraulic press speeds, but at different temperatures: in the mode of superplasticity at hot temperatures, and in the mode of severe plastic deformation at warm or cold temperature deformation. The first mode allows grains to be refined to microcrystalline sizes of 1–10 microns, and such a material acquires the ability to deform in a state of superplasticity, i.e., in tension with low resistance to deformation and high elongation. The second mode (severe plastic deformation) refines grains to submicro- (1÷0.1 µm) and nanosizes (less than 0.1 µm), thus giving metal materials record structural strength, as well as the possibility to use processing in extended temperature-velocity conditions of superplastic deformation in comparison with microcrystalline structure.

SUPRAPLASTIC DEFORMATION OF ALUMINUM-LITHITHIUM ALLOYS 1450 AND 1460, ALLOYED WITH ZIRCONIUM AND SCANDIUM

The article presents the results of research directed to the determination of mechanisms of an ultrafine-grained structure formation in aluminum-lithium alloys 1450 and 1460, which include dispersed particles of zirconium and scandium aluminides, which are effective stabilizers of the microstructure, as well as for the determination of the features of structural changes during the course supraplastic deformation.

EFFECT OF PRELIMINARY DEFORMATION ON THE FORMATION OF ULTRAFINE-GRAINED STRUCTURE DURING EQUAL CHANNEL ANGULAR PRESSING OF MAGNESIUM ALLOYS

The formation of ultrafine-grained structure is very desirable in the microstructural design of magnesium alloys, in particular Mg-Zn-Ca medical alloy, for a substantial increase in their strength and corrosion resistance. However, conventional processing of these alloys by equal channel angular pressing is not easily applicable due to their low deformability, which often leads to rapid fracture of billets. In this paper, computer simulation data and principles of physical mesomechanics are used to demonstrate that preliminary deformation of Mg alloy billets by reduction at high temperatures and low strain rates significantly increases their deformation capacity and enables equal channel angular pressing at lower temperatures, resulting in billets with ultrafine-grained structure. Consideration is given to the physical nature of the established effect.

Influence of ultrafine surface grain size on the passive film chemistry and critical pitting temperature (CPT) in AISI 316L austenitic stainless steel

ABSTRACT Severe shot peening (SSP) was applied to induce a profound surface deformation characterised by the formation of ultrafine-grained structures with an average grain size of 99 ± 5 nm on AISI 316L. The impact of these ultrafine-grains on the critical pitting temperature (CPT) was investigated in 3.5 wt.% NaCl solution. Additionally, this study comprehensively explored pit chemistry using electrochemical techniques and mechanistic analyses. Moreover, a comparative investigation of the chemical composition of passive films, developed on severe shot peened (SSPed) samples, was undertaken in contrast to their solution-annealed (SAed) counterparts, employing X-ray photoelectron spectroscopy (XPS). Potentiostatic measurements showed that the mean CPT value was 8.8 ± 0.3 °C for the SAed samples, whereas the SSPed counterparts demonstrated a notably higher CPT, averaging at 13.6 ± 0.5 °C. Insights from pencil electrode measurements revealed shallower pit depths and lower critical current density requirements for passivation in the SSPed sample. Furthermore, the passive films formed on the SSPed samples showed a higher concentration of Cr and Mo. These observations were comprehensively elucidated through effective diffusion equations, leading to a discussion regarding the improved resistance to pitting corrosion exhibited by the SSPed samples, thus shedding light on the pivotal role of surface grain refinement in corrosion behaviour improvements.

Enhanced in-vitro biodegradation, bioactivity, and mechanical properties of Mg-based biocomposite via addition of calcium-silicate-based bioceramic through friction stir processing as resorbable temporary bone implant

Bioabsorbable Mg and alloys have been used as metallic biomaterials for orthopedic implant applications due to their similar mechanical properties to human natural bone. High biodegradation rate is the main reason for limiting clinical applications of Mg and alloys in biological media. In order to achieve long-lasting bio applications of bioabsorbable WE43 Mg alloy, it is necessary to improve strength and bioactivity and control biodegradation rate of this material. In this study, Mg-based biocomposites are produced using friction stir processing (FSP) by addition of wollastonite (CaSiO3) calcium-silicate-based bioactive reinforcing particle into Mg matrix. The results showed that applied severe plastic deformation during FSP caused formation of ultra-fine grain structure with homogenous distribution of secondary phase particles, which resulted in a remarkable improvement of ductility, strength and load bearing capacity of FSPed Mg matrix. Also, biodegradation behavior of resulting biocomposite was evaluated using electrochemical methods and in-vitro immersion test in simulated body fluid (SBF) and phosphate-buffered saline (PBS) at 37.5 °C. The results revealed that processed bioabsorbable Mg alloy showed a lower biodegradation rate compared to monolithic Mg alloy. Furthermore, the presence of wollastonite bioceramic particles significantly enhanced strength and reduced biodegradation of the Mg-based biocomposite. Moreover, the addition of wollastonite bioceramic particles positively affected the bioactivity of Mg matrix by forming a cauliflower shaped bone-like hydroxyapatite layer on the surface after 28 days of immersion in SBF and stimulate bone tissue regeneration.

Fracture Kinetics and Mechanisms of Ultrafine-Grained Materials during Fatigue Tests in the Low-Cycle Fatigue Region

In this paper, the fracture kinetics and mechanisms in the low-cycle fatigue region were analyzed for different ultrafine-grained (UFG) materials with body-centered cubic (bcc), hexagonal close-packed (hcp) and face-centered cubic (fcc) lattices. Three-point bending principle fatigue tests were performed. The tests show that the UFG structure formation in the investigated materials has an ambiguous effect on the total number of cycles to failure (life) of the samples. The number of cycles to fatigue crack initiation (Nin) is about 20% of the total life of the samples, irrespective of the material state and the crystal lattice type. At the same value of the stress intensity coefficient range (∆K), for the majority of the investigated UFG materials, the fatigue crack propagation rate (dl/dN) is close to or lower than that of the initial coarse-grained (CG) materials. For the UFG materials, the coefficient n in the Paris equation is, in most cases, lower than that for the CG materials, which indicates that the UFG materials are less sensitive to cyclic overload. The fatigue fracture mechanisms of the investigated CG and UFG materials are rather similar, although the fracture of the UFG materials is accompanied by the formation of many secondary cracks, irrespective of the crystal lattice type.

Effect of Rotary Swaging on the Structure, Mechanical Characteristics and Aging Behavior of Cu-0.5%Cr-0.08%Zr Alloy

A study of the effect of rotary swaging (RS) on the microstructure and properties of the pre-extruded and pre-quenched Cu-0.5%Cr-0.08%Zr alloy was performed. RS leads to the formation of an ultrafine-grained (UFG) microstructure. UFG structure formation caused by RS increases the ultimate tensile strength (UTS) up to 443 ± 5 MPa and 597 ± 9 MPa for pre-quenched and pre-extruded alloys, respectively. Additionally, the reduction in ductility occurs after RS. It should be noted that UTS is increased for a pre-quenched alloy, while the strength of a pre-extruded alloy is dropped. The growth of UTS for the pre-quenched alloy is associated with the precipitation of fine Cr particles, whereas the recovery processes in the pre-extruded alloy induce the reduction in its UTS. An additional advantage of RS is an increase in the fatigue limit of the pre-quenched alloy up to 265 MPa, and of the pre-extruded alloy up to 345 MPa. The combination of extrusion and RS allows for the increase of the UTS of the Cu-0.5%Cr-0.08%Zr alloy up to 597 ± 9 MPa, while the levels of ductility and electrical conductivity are 10.9 ± 0.9% and 82.0 ± 1.7% IACS, respectively.

True fracture stress of UFG samples of Al 6101 alloy

This paper presents the results of studies on the true fracture stresses of the Al alloy 6101 with an ultrafine-grained (UFG) structure processed by equal channel angular pressing — conform (ECAP-C) method and the same alloy after artificial aging (AA state). The grain size in the UFG state was about 500 nm. Aging particles and Al3Fe particles were present in the structure of AA and UFG state. Strength and yield stress increase as a result of the formation of UFG structure. The true strain to failure in AA and UFG states of 6101 alloy, taking into account the measurement error, is the same. In this case, the true fracture stress of samples with the UFG structure is markedly higher than the true fracture stress of samples with the AA structure. An explanation for the increase in the true fracture stress of specimens with UFG structure based on a generalization of the Hall-Petch relation and the Zener-Strauss model, a criterion of pore formation on the particle, is proposed.

Effect of Multidirectional Isothermal Forging on the Formation of Ultrafine-Grained Structure in Alloy 1570C

Effect of Multidirectional Isothermal Forging on the Formation of Ultrafine-Grained Structure in Alloy 1570C

Formation of grain structure and its change during the superplastic deformation of samples of alloys Al-4,1 wt.% Cu-0,5 wt.% Zr, 1450 and 1460, doped with zirconium and scandium

The article presents the results of research aimed at establishing the mechanisms of formation of ultrafine-grained structure in samples of Supral Al-4.1 wt.% Cu-0.5 wt.% Zr, 1450 and 1460 alloys, containing dispersed particles of zirconium and scandium aluminides which are effective stabilizers of the microstructure as well as at establishing the peculiarities of its change during superplastic deformation. It is established that static recrystallization plays an important role in the formation of ultrafine-grained grain structure with an average grain size d =5 μm in samples of the alloy Al-4.1 wt.% Cu-0.5 wt.% Zr. It takes place during their heating in air to the test temperature T = 773 K. Finally, the formation of the grain structure in the samples is completed in the initial stages of their superplastic flow due to the passage of continuous dynamic recrystallization. It was found that the grain structure in the working parts of the samples of the alloy Al-4.1 wt.% Cu-0.5 wt.% Zr, superplastically deformed to fracture by hundreds of percent, is ultrafine-grained ( d ≈10 μm) and equiaxed. It is established that recrystallization annealing of 1450 alloy samples does not lead to the formation of a homogeneous fine-grained structure in them. It is shown that an almost homogeneous microstructure with an average grain size d =3÷6 μm is created directly during their superplastic deformation at temperatures of 753, 763, 773, 778K and flow stresses σ = 2,0 ¸ 8,0 MPа to degrees of relative deformation εrel, amounting to 20-50%, due to the implementation of continuous dynamic recrystallization. During the subsequent stages of the superplastic flow, the average grain size increases in the working part of the samples of alloy 1450. It is shown that in the samples of alloy 1450, which were deformed to failure under the optimal conditions, the grains retain an equiaxed shape, and their average size is 10 – 15 μm. It is established that the ultrafine-grained grain structure in the working parts of samples of alloy 1460 with an average grain size d =5 μm increases during their superplastic deformation, which is carried out at temperatures in the range T = 753÷853 K and at flow voltages σ equal to 3.0÷6.0 MPa. It is shown that in samples of alloy 1460, which were deformed to failure by 1000% at Т = 823 К and s = 3,5 MPa, average grain size d ≈15 μm.

THE EFFECT OF PRECIPITATION ON MICROSTRUCTURE AND CORROSION BEHAVIOUR OF ZK60 SUBJECTED TO SEVERE PLASTIC DEFORMATION AS BIODEGRADABLE MATERIAL

Microstructure and corrosion behaviour of ZK60 subjected to severe plastic deformation (SPD) as biodegradable material has been investigated in terms of grain refinement process and precipitation formation. Equal channel angular pressing (ECAP) was one of the well-known SPD techniques due to their advantages to promote dislocation and grain fragmentation. ZK60 billet was cut to a 10 mm diameter cylindrical shape for biodegradable material application. ECAP process was carried out up to two passes by Route Bc at 423 K temperature process. Microhardness test was performed at ECAP processed sample, and the structure observation was carried by optical microscope and transmission electron microscope. The corrosion behaviour of the material was investigated by an anodic polarization curve. The ECAP process promotes dislocation accumulation efficiently and ultrafine-grained structure formation. It may improve the formability characteristic of ZK60. The hardness also showed significant increment during the ECAP process due to the high level of deformation. Corrosion behaviours and microstructure observation during the ECAP process showed a correlation that concluded that grain refinement and precipitation formation influenced the electrochemical properties. The alloying element such as Zn and Zr promoted the protective film for corrosion due to their ability for pitting protection. ECAP improved the precipitation formation for corrosion resistance, microstructure uniformity and material formability.

Theoretical Foundations of the Use of Severe Plastic Deformation for Formation of Ultrafine Grain Structure in Superalloys

Theoretical Foundations of the Use of Severe Plastic Deformation for Formation of Ultrafine Grain Structure in Superalloys

The effects of Y pre-alloying on the in-situ dispersoids of ODS CoCrFeMnNi high-entropy alloy

Oxide dispersion strengthened CoCrFeMnNi high-entropy alloys (ODS-HEAs) were prepared using two different powder preparation methods classified by yttrium addition strategy to investigate the effects of in-situ and ex-situ oxide dispersoid formation on the microstructure and mechanical properties. Systematic microstructural analysis was carried out by X-ray diffraction (XRD), electron backscattered diffraction (EBSD), high-resolution transmission electron microscopy (HRTEM), atom probe tomography (APT), and small-angle neutron scattering (SANS). Cryo-milled powder analysis, grain structure evolution after spark plasma sintering, dispersoid characteristics, and matrix/dispersoid interface structure analysis of the in-situ and ex-situ dispersoids within the high-entropy alloy (HEA) matrix were performed. The in-situ dispersoid formation was dominantly observed in the Y-alloyed ODS-HEA through the construction of a coherent interface relationship with complex chemical composition, leading to an increase in the Zener pinning forces on the grain boundary movement. ODS-HEA with in-situ oxide dispersoids enhanced the formation of ultrafine-grained structures with an average diameter of 330 nm at a sintering temperature of 1173 K. This study shows that the Y pre-alloying method is efficient in achieving fine coherent dispersoids with an ultrafine-grained structure, resulting in an enhancement of the tensile strength of the CoCrFeMnNi HEA.

Effect of Deformation Temperature on Formation of Ultrafine-Grained Structure in the Age-Hardenable Cu–Cr–Zr Alloy

The effect of the temperature of plastic deformation performed by equal channel angular pressing on the structure and physical and mechanical properties of the age-hardenable Cu–Cr–Zr alloy has been studied. Plastic deformation results in the formation of an ultrafine-grained structure in some regions with an average grain size smaller than 1 µm, a supersaturated solid solution decomposition, and the precipitation of disperse particles. The fraction of the supersaturated solid solution which is decomposed is shown to increase with increasing deformation temperature. The density of microshear bands, the dislocation density, and the fraction of high-angle boundaries and the ultrafine-grained structure increase with increasing volume fraction of disperse particles.

Effect of repeated warm rolling cold rolling and annealing on the microstructure and mechanical properties of AISI 301LN grade austenitic stainless steel

Global warming now days is an important issue over the world. To reduce the effect of global warming high strength structural steels are required to develop which will able to make safe body white future vehicle. In this matter advanced thermomechanical treatments of steel play a vital role. In present context, a repeated warm rolling, cold rolling and annealing treatments were applied to AISI 301LN grade austenitic stainless steel. Due to formation of ultra-fine grain structure (Average grain size ≈ 4 µm) at treated materials as compared to large grain (Average grain size ≈ 35 µm) structure of as-received material, the treated steel was showing excellent mechanical properties. It was showing 667 MPa Y.S., 1095 MPa U.T.S. and 40% elongation as compared to 405 MPa Y.S., 841 MPa U.T.S. and 78% elongation of base material. The average misorientation angle of the treated specimen was slight high (≈ 390) in compare with as received hot rolled material (≈ 340). This is the indications of highly recrystallized structure of the treated materials. It indirectly helps to improve the mobility of dislocations around grain boundary towards improvement of the ductility of the material. The fractographic image of the treated material is supporting the high ductile behavior along with high strength of this material.

Evolution of the Microstructure and Mechanical Properties of Copper under ECAP with Intense Cooling

Tough-pitch copper M1 is studied after deformation by the method of equal channel angular pressing (ECAP) in a die with parallel channels with the use of cryogenic cooling in liquid nitrogen. Comparative analysis of the properties of the copper after the ECAP with cooling in liquid nitrogen and at room temperature is performed. Special features of formation of ultrafine-grained structure in quenched copper M1 under the ECAP in the die with parallel channels are determined. The role of cooling with liquid nitrogen after the ECAP is shown to be positive from the standpoint of deceleration of the processes of static and dynamic recrystallization and improvement of the combination of the mechanical properties of the copper.

Orientation-dependent microstructure development during high-rate shear deformation of copper

ABSTRACT The microstructure transformations in copper subjected to high-rate deformation via dynamic channel angular pressing (DCAP) have been examined after one and four DCAP passes using electron backscattering diffraction. The focus was on the interrelation between microstructure and texture evolution and on the role of deformation twinning in these processes. During the first pass, a mesoscopic banding is shown to be the main mechanism of grain fragmentation, while the texture is qualitatively similar to that of face-centered cubic metals subjected to equal channel angular pressing, including the presence of characteristic texture components C, A1 and A2. A correspondence between structural elements and texture components occurs. Specifically, the mesobands have orientations near either an A1 or A2 ideal orientation, whereas the matrix has an orientation near C. Inside the A1-bands, microtwins with orientations near A2 are observed. Detailed analysis of the A2-bands suggests that they may be of twinning origin. Similar orientation dependence of microstructure, though on a much finer scale, is observed after four DCAP passes. Based on the microstructure examination, it was suggested that mesoscopic banding together with deformation twinning continue to be principal mechanisms of grain refinement until the fourth pass, resulting in the formation of ultrafine-grained structure.

Эволюция структурно-фазового состояния титанового сплава системы Ti-Al-V-Mo в процессе формирования ультрамелкозернистой структуры с использованием обратимого легирования водородом

Эволюция структурно-фазового состояния титанового сплава системы Ti-Al-V-Mo в процессе формирования ультрамелкозернистой структуры с использованием обратимого легирования водородом

Nature of dynamic ferrite transformation revealed by in-situ neutron diffraction analysis during thermomechanical processing

Nowadays, a new concept of process utilizing dynamic ferrite transformation, which can achieve ultrafine-grained structure with a mean grain size of approximately 1 μm, has been proposed. This paper reports transformation mode of dynamic ferrite transformation and formation mechanism of ultrafine-grained structure revealed by our novel technique of in-situ neutron diffraction analysis during thermomechanical processing. Dynamic ferrite transformation occurs in a diffusional manner, whose partitioning behavior changes from para- to ortho-equilibrium with the progress of transformation. Moreover, we propose that dynamic recrystallization of dynamically-transformed ferrite is the main mechanism for the formation of ultrafine-grained structure.