Formation Of Fine-grained Structure Research Articles

The Influence of Structure and Phase Composition of Titanium Alloy on Superplastic Deformation

The paper presents research into the behavior of high-temperature plastic deformation of titanium alloy Ti–4.74 wt.% Al–5.57 wt.% Mo–5.04 wt.% V alloy (VT22) depending on its structure and phase composition. It is shown that the formation of fine-grain structure in this alloy is not a sufficient condition for realizing superplastic deformation. It is found that the formation of the ultra-fine grain structure in VT22 alloy leads to the temperature decrease down to 823 K at the beginning of plastic deformation. This allows achieving the percent elongation of the alloy specimens over 1300% within the temperature range of 973–1073 K and at a 6.9·10–3 s–1 initial tensile rate. The grain boundary sliding is considered to be the main deformation mechanism.

On structural and phase transitions in aluminum alloys

Mathematical modelling of behavior of group of the industrial aluminum alloys with initial varying grain size structure showing superplastic properties in certain temperature and strain rate ranges is presented. It is known that the structural superplasticity is connected with facilitated by fine-grained structure formation (1…10 microns) at the preliminary stage. However, for realization of superplasticity of “dynamic type” there has to be a replacement of an initial varying grain size structure state of material with another, ready for superplasticity. Therefore the used definition “the dynamic superplasticity” reflects consecutive change of states which happens in material with initial varying grain size structure under the changing temperature-rate conditions: initial varying grain size → equiaxed fine-grained structure (4…7 microns) formed under the temperature-rate conditions of superplasticity → coarse-grained at further increase in strain rate. These changes are caused by simultaneous action of deformation rates and structural (phase) transitions of evolutionary type in open nonequilibrium systems. In particular, for the considered commercial aluminum alloys with initial varying grain size structure such irreversible transition is dynamic recrystallization. The association of deformation process with metal flow that has irreversible structural and phase transition of indistinct type at one of the stages, allows using synergetic approach. The mathematical model formulated from positions of solid mechanics use allow to research nonequilibrium system reaction to behavior of thermodynamic response functions – the specific heat and entropy – and to establish implementations features of the irreversible indistinct phase transitions observed in the conditions of dynamic superplasticity for aluminum alloys.

Effect of temperature of isothermal multidirectional forging on microstructure development in the Al-Mg alloy with nano-size aluminides of Sc and Zr

The effect of deformation temperature on structural changes in the homogenized ingot of a complex-alloyed aluminum alloy 1570C (Al-5% Mg-0.2% Sc-0.1% Zr) during hot isothermal multidirectional forging (IMF) was investigated. The alloy with equiaxial grains of 25 μm and coherent precipitates of Al3(Sc, Zr) 5–10 nm in diameter was subjected to IMF at the temperatures of 325° and 450 °C (∼0.6 and 0.8 Tm, respectively) at a strain rate of 10−2 s−1 to a total strain of ε = 8.4 with a pass strain of 0.7. It has been shown that grain refinement takes place during IMF at both temperatures. The formation of new fine (sub)grains surrounded by low- and high-angle boundaries started in the vicinity of the original grain boundaries in the earlier stages of deformation. With increasing strain, the volume fraction and boundary misorientations of such crystallites increased; that results in almost complete replacement of the original structure with a new fine-grained one via continuous dynamic recrystallization. The alloy grain refinement was controlled by the interaction of lattice dislocations and/or grain boundaries with precipitates, effectively inhibiting migration of the boundaries as well as dynamic recovery, leading to rearrangement of lattice dislocations, and their annihilation. Herewith, the kinetics of the fine-grained structure formation was practically independent on the processing temperature, resulting in roughly similar microstructures after the same strains, which characterized by the closed mean misorientation angles and fractions of high-angle boundaries. However, the size of (sub)grains processed at 325 °C was significantly lower than at 450 °C, i.e., 1.2 and 2.5 μm, respectively. This indicated that parameters of the developed structure were thermally activated and controlled by precipitates. The features of microstructure formation and the nature of grain refinement of a complex-alloyed aluminum alloy under high-temperature severe plastic deformation conditions are discussed.

Physical-Mechanical Properties of γ-Irradiated SiC Ceramics for Radioactive Wastes Immobilization

The interest in silicon carbide (SiC-based) ceramics and composites as matrix material for nuclear waste immobilization is grown up. Long-term chemical durability and radiation resistance of SiC are important factors for radionuclides immobilization. Advantages of SiC-based ceramics as structural materials in nuclear applications are the high-temperature properties, high density and reduced neutron activation. The use of radiation resistant materials is a strong requirement for safe and environmentally beneficial energy system. The SiC ceramics stability under irradiation for temperatures up to 1273 K is also very important for nuclear power applications. The SiC matrices doped by additives of Cr, Si were fabricated using High Speed Hot Pressing Method. Additives content was in the range from 0.5 to 3 wt %. Microstructural characteristics of silicon carbide ceramics were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and infra-red spectroscopy (IR) methods. The results of microcracking under indentation conditions were revealed the lack of cracks in the SiC ceramics with Cr additives before and after irradiation process. In addition, it was demonstrated that samples of SiC with alloying additives Cr and Si possess high mechanical parameters under γ-irradiation process. The strength of ceramics increases with the uniform and fine-grained structure formation. The modification of phase composition and mechanical properties of the SiC ceramics with Cr and Si additives under γ-irradiation were analyzed for further development of radiation resistant and matrix materials for radioactive wastes immobilization.

Creating Weldable High-Strength Structural Steel with Nanostructuring

Exploitation of the Arctic calls for the creation of economical high-strength steel capable of operating at low temperatures. Research shows that, to that end, means of controlling the steel structure must be identified, so as to create quasi-isotropic fragmented nanostructure within the metal. The formation of finegrain structure is possible by a combination of intense plastic deformation with recrystallization and phase transformations. To confirm the theoretical and experimental preconditions for this process, experiments are conducted on the Gleeble-3800 instrument and a Kvarto-800 four-high rolling mill. Experimental steel melts with a carbon equivalent Cequ = 0.44–0.87% are investigated. In simulation on the Gleeble-3800 instrument, compressive deformation is applied in two stages: roughing at 1080–1100°C; and finishing at 950 and 820°C. That simulates the deformation cycle in industrial Kvarto-5000 four-high mills. The grain size in the steel is decreased from 6.5 to 2.2 μm after deformation at 950°C and 1.1 μm after deformation at 810°C. Fragments smaller than 500 nm constitute 20–37% of the steel structure. In the steel with Cequ = 0.44–0.65%, the yield point is 500–700 MPa, which is 40% greater than the standard values. In the steel with Cequ = 0.65–0.87%, the yield point is 700–1150 MPa. These values are obtained with increase in nickel content in the steel to 3%. At higher Ni concentrations, no improvement in yield point is seen. After rolling on the Kvarto-800 mill, with Cequ = 0.60–0.87% in one pass (with 70% reduction) at 1100°C and direct quenching with subsequent tempering at 600°C, the yield point is 1060 MPa. In this case, variation in the Ni content and Cequ has little influence on the yield point. The steel consists of bainite (mean grain size 6.9–8.3 μm), with a large dislocation density (1–2) × 1015 m–2 and considerable fragmentation within the grain. On the basis of the new technology, a group of low-temperature steels with yield points of 270–690 MPa and Cequ = 0.32–0.65% is created. The thickness of the steel sheet is up to 130 mm; it may operate at temperatures as low as –60°C. Such steel may be used for atomic icebreakers, other Arctic vessels, and fixed and floating drilling platforms for oil and gas extraction from Russia’s Arctic shelf. This research demonstrates the possibility of creating structural steels with relatively little alloying (up to 20–30%) and with standardized chemical composition.

Formation of Fine-Grained Structure and Superplasticity in Commercial Aluminum Alloy 1565ch

A process for fabricating 5-mm-thick sheets from alloy 1565ch with recrystallized grains less than 10 μm in size is developed, which involves superplastic forming and provides elevated mechanical properties at room temperature. The parameters of superplasticity and the mechanical properties of alloy 1565ch are determined. Test forming of a dome-shape model part is performed.

FUNDAMENTAL APPROACHES IN THE DEVELOPMENT OF HIGH-STRENGTH STRUCTURAL EASY-TO-WELD STEELS WITH NANOSTRUCTURING

The Russian Arctic region development considered as one of the most important national tasks to be realized makes it necessary the creation of economic high-strength cold-resistant steels. Investigations have shown that for this purpose to be achieved it is necessary to develop the mechanism of structure control aimed at forming a quasiisotropic fragmented structure in the metal volume to be refined till the nanolevel. The fine-grained structure formation is most affected by an intensive plastic strain combined with recrystallization and phase transformations. A number of investigations were conducted using a plastometer Gleeble-3800 and a rolling mill Quarto-800 for supporting theoretical and experimental assumptions. The experiments were carried out on the experimental heats of steels with the carbon equivalent CE = 0.44 – 0.87 %. In modeling of the technological processes on the plastometer Gleeble-3800, compression deformation was conducted in 2 stages – roughing at 1100 – 1080 °С and finishing at 950 °С and 820 °С thatsimulated a strain cycle on the industrial mills Quarto-5000. It has been found that the steel grain was refined from 6.5 to 2.2 µm after deformation at 950 °С and to 1.1 µm – at 810 °С. 20 – 37 % of fragments of less than 500 nm were recorded in the steel structure. For steel with CE = 0.44 – 0.65 % the yield strength changed from 500 MPa to 700 MPa that was 40 % higher than the level of standard values. For steel with CE = 0.65 – 0.87 %, the yield strength increased from 700 MPa to 1150 MPa. This strength value was achieved with increasing the nickel content of steel up to 3 %, with further increase in its concentration the yield strength remained the same. After rolling the steel with CE = 0.60 – 0.87 % on the mill Quarto-800 in one pass with a 70 % deformation at temperature of 1100 °С and direct quenching followed by tempering at 600 °С, the yield strength of 1060 MPa, thus, the effect of the nickel content and the change of CE within the stated limits on the steel hardening were insignificant. The steel structure is bainite with the average grain size of 8.3 – 6.9 µm with a high dislocation density of (1 – 2)·1015 m–2 and a great extent of intragranular fragmentation. Based on the new technology, a group of cold-resistant steels with yield strength of 270 – 690 MPa and CE = 0.32 – 0.65 % up to 130 mm in thickness and operating temperatures down to –60 °С has been developed. These steels were used in the building of nuclearpowered ice-breakers, ice-going ships, stationary and floating drilling platforms for the exploration and extraction of hydrocarbons in the Arctic Russian offshore zones. The completed work shows the possibility of developing structural steels with a significantly lower level of alloying (by 20 – 30 %) as compared to the steel analogues as well as that of unifying the chemical composition of steels.

Структура ультрамелкозернистого сплава А2024 при комбинированной термодеформационной обработке

The present work studies the effect of preliminary heat treatment on the features of ultra fine-grained (UFG) structure formation in Al-Cu-Mg alloy by means of high pressure torsion in Bridgman anvils. Heat treatment was carried out following two regimes: regime 1 included heating to a temperature of 500°C, 16 hours and quenching in water, and regime 2 additionally included two-stage T6I6 aging: 120°C, 1.5 hours and 160°C, 6 hours, after quenching with water quenching after each stage. The number of revolutions of the anvil was n = 5, 10, which, according to the calculation, corresponded to the true logarithmic strain e ≈ 5.5, 6.5. It is shown that after the preliminary two-stage aging T6I6 of coarse-grained alloy fragmented grain / subgrain structure with high dislocation density was formed at e = 5.5. The relaxation processes were suppressed by blocking of the grain boundaries by dispersed particles of the strengthening S phase. It was shown that at e = 6.5 alloy structure is refined to the nanolevel, and the process of dynamic dissolution of S phase precipitates started parallel. It is accompanied by the appearance of defect-free grains and a mixed structure was formed. In the case of the torsion of hardened coarse-grained alloy, the main mechanism of grain refinement to 450 nm is dynamic recrystallization. At e = 6.5 the dynamic aging of the supersaturated Al solid solution becomes the additional relaxation channel of the elastic energy and the hardness of the UFG alloy increases up to 3000 MPa.

Improvement of mechanical properties of the Ti–45Al–5Nb–1Mo–0.2B (аt %) intermetallic alloy by means of microstructure controlling

The effect of heat and thermomechanical treatments conditions on the microstructure and main mechanical characteristics (obtained by tensile, high-temperature long-term strength, fracture toughness, and high-cycle fatigue tests) of the Ti–45Al–5Nb–1Mo–0.2B (аt %) alloy was studied. Before the treatments, the sequence of phase transformations in the alloy after its solidification was determined by testquenching method. The obtained data were used to develop conditions for the heat and thermomechanical treatments. It was found that a small but stable increase in the plasticity and strength of the cast alloy is observed after three-stage annealing at temperatures that correspond to the (α + γ)- and (α2 + β(В2) + γ)-phase region. The thermomechanical treatment at temperatures corresponding to the (α(α2) + β(В2) + γ)-phase region and subsequent two-stage annealing at temperatures that correspond to the (α + β(В2) + γ)- and (α2 + β(В2) + γ)-phase region lead to the formation of fine-grained duplex structure. This determined the substantial improvement of the low-temperature plasticity and strength (δ = 3.1% and σu = 860 MPa at 20°C, respectively) and retained high creep resistance to 700°C.

Recrystallization-based formation of uniform fine-grained austenite structure before polymorphic transition in high-strength steels for Arctic applications

Growing demand for materials suitable for Arctic marine constructions necessitates the development of lowcost steels with excellent low-temperature properties. One way of addressing this issue is to obtain a finegrained structure by careful control of the recrystallization process. The research methods used in this paper includes conducting experiments and making observations. The study investigates mechanisms of grain structure formation in low-carbon cold-resistant F620 steels additionally alloyed with Nb or V during hot plastic deformation. The obtained results evidence that static recrystallization is the most effective means to refine the austenite structure under conditions of industrial rolling. On this basis, a temperature-strain scheme of fractional hot rolling is proposed that provides the most refined uniform structures of bainite and/or martensite after quenching. Additionally, the results show that substitution of the alloying element V by Nb makes the structure of the hot-rolled austenite more finely dispersed. New cold-resistant F620 steels for Arctic applications alloyed with Nb of 50-mm thickness were produced utilizing the research results. These grades of steels have reduced alloying content and exhibit improved properties, in particular, cold resistance down to −60 °С and high fracture toughness at −50 °С.

Effect of Process Parameters on the Microstructural Characteristics and Mechanical Properties of AZ80A Mg Alloy During Friction Stir Welding

In this paper, the influence of process parameters on microstructural characteristics & mechanical properties of AZ80A Mg alloy during friction stir welding (FSW) are investigated in a detailed manner. The tensile fracture surfaces obtained from successfully fabricated joints are subjected to tensile tests and microstructural investigations were done using scanning electron microscope. From the experimental results, the joints produced under a 5 kN axial force value at 1000 rpm and at a feed rate of 1.5 mm/min were found to exhibit superior mechanical properties and metallurgically defect free weldments when compared with other joints. The chemical compositions of these defect free joints were analyzed using energy dispersive spectrometry. Moreover, ideal level of heat generation, uniform flow of the plasticized material and formation of fine grain structure with uniform distribution in the FSW zone were found to be the main reasons for these superior mechanical properties and flawless joints.

Effect of deformation with Bridgman anvils on the structure, hardness, and critical current of a massive MgB2-based sample

The structure and properties of synthesized massive MgB2-based samples subjected to deformation with Bridgman anvils have been studied. Deformation results in the formation of fine-grained structure of the MgB2 phase, enhancement of interconnection of grains, complete disappearance of friable MgB2-phase areas, and abrupt increase in the microhardness.

INFLUENCE OF NITROGEN ALLOYING AT STRENGTHENING AND STABILITY OF AUSTENITE STEEL TYPE Cr18Ni10

The hardening and austenite stability as a result of nitrogen alloying steel type Cr18Ni10 in the temperature range, which is usual for the application of such steels as corrosion-resistant structural heat-resistant and/or cryogenic ones was studied. It is shown that the nitrogen alloying is perspective for strengthening and increasing of stability of austenitic stainless steels. Additional strengthening due to the preliminary cold or warm deformation hardening increases a tendency to the martensite formation under load, which limits the operating temperature of these steels. High-strength non-magnetic nitrogen-alloyed steels on the base of Cr18Ni10 steels containing up to 0.22 % of nitro gen are suitable for cryogenic application of non-deformed articles only. Otherwise, a strain-induced martensite will always be formed in them at temperatures below –70 °С. High strength, ductility and toughness of these steels can be achieved simultaneously only as a result of the trip-effect or fine-grained structure formation. Nitrogen effectively strengthens solid solution in a high-temperature condition. The combination of hot and warm deformation can be used for additional hardening of steels during at thermomechanical treatment, including strain-aging process, which is effective for applications of such steels as heat resistant ones.

STRUCTURE AND PROPERTIES OF COMPOSITE COATINGS OF NICKEL – CHROMIUM BORIDE NANO

The structure and properties of the composite coatings of nickel - chromium boride nano are investigated. It was set that the chromium diboride state of nano provides when used in the technology of composite nickel formation of fine-grained structure, reducing internal stress and improve their physical and mechanical properties, is to increase the hardness, resistance to wear and corrosion in liquid and gaseous media. Isothermal annealing of composite coatings improves adhesion to the substrate.

The influence of degree of deformation under isothermal abc pressing on evolution of structure and temperature of phase transformations of alloy based on titanium nickelide

The article discusses investigations into evolution of microstructure and variation of temperatures of martensitic transformations of the alloy Ti49.8Ni50.2 (at %) with increase in the degree of deformation under isothermal abc pressing (T = 723 K). It is discovered that, at the initial stages of the abc pressing in the vicinity of forge cross, the grain size sharply decreases; in some cases, this decrease exceeds the initial size by about an order of magnitude. Possible mechanisms of formation of such structure are analyzed. It has been demonstrated that, at the degrees of true deformation e > 2, the grain structure in total specimen bulk is refined through the mechanism of continuous dynamic recrystallization, which leads to formation of a homogeneous fine grain structure with a high volumetric portion of submicrocrystalline and nanostructured fractions. It is established that, at all considered degrees of deformation, the temperatures of martensitic transformations remain actually constant, which can be attributed to intense behavior of dynamic and postdynamic recovery, as well as formation of the martensite phase upon cooling from the pressing temperature.

Influence of Physical Properties of B<sub>4</sub>C Powder on the Strength Properties of the Ceramics Manufactured by SPS Sintering

Ceramics samples in the form of a parallelepiped with high strength characteristics have been made. For the manufacture of the ceramics samples a powder mixture from submicron В4С powder with additives (1 wt%, 5 wt%, 10 wt%) of boron carbide nanopowder was used. The physical properties of the powder mixtures and strength properties of sintered ceramics have been studied. It was shown that the use of submicron fractions of the boron carbide powder together with nanoadditives is a determining factor in the formation of dense fine-grained structure providing improved physical and mechanical properties of the ceramics.

Формирование мелкозернистой структуры в процессе высокотемпературной интенсивной деформации высоко¬прочного алюминиевого сплава (обзор)

Формирование мелкозернистой структуры в процессе высокотемпературной интенсивной деформации высоко¬прочного алюминиевого сплава (обзор)

Low temperature sintering properties of LiF-doped BaTiO3-based dielectric ceramics for AC MLCCs

The effects of LiF addition on phase evolution, microstructure and dielectric properties of BaTiO3 based ceramics were investigated. The LiF addition lowered the sintering temperature of BaTiO3 based ceramics effectively from 1,380 to 1,100 °C. Orthorhombic-to-pseudocubic phase transition appeared as increasing sintering temperature or LiF content. Li+ dissolved in the lattice of BaTiO3, replaced Ti4+ site, produced oxygen vacancies and further inhibited the grain growth. F− entered into O2− site, might cause weakening of oxygen bond strength, together with the low-melting-point liquid phase sintering of LiF, facilitated the formation of uniform dense and fine-grained structure and maintained good dielectric properties. Samples with 2 wt% LiF sintered at 1,100 °C showed excellent dielectric properties: er = 1,956, tanδ = 0.8 % (at 1 kHz), ΔC/C25 4.50 kV/mm.

Nonequilibrium dynamics of a two-defect system under severe load.

In the framework of a two-defect model, based on a variation of the Landau technique, the kinetics of structural defect generation in solids under severe external load is investigated. The approach is based on a special form of kinetic equation in terms of internal energy, which is applied here to the description of an important practical problem of fine-grained structure formation in metals under severe plastic deformation. It unifies strengthening curves over the entire range of deformation, including the Hall-Petch and linear strengthening sections.

Effect of Electromagnetic Induction and Heat Treatment on the Mechanical and Wear Properties of LM25 Alloy

Mainly for aluminum castings the properties depend on the grain structure, it is desirable to achieve a fine equiaxed grain structure. The formation of fine equiaxed grain structure depends on the rate of solidification, addition of grain refiners. Al-Ti-B master alloys are used in Al-Si alloys casting industries as a grain refiner to improve the casting properties. This research work investigates the combined effect of the addition of 2% Al-5Ti-1B and electromagnetic induction with T6 heat treatment on the mechanical properties and wear properties of LM25 alloy casting. The mechanical properties assessed are hardness and ultimate tensile strength (UTS), microstructure examination, fracture analysis of the castings are studied. It is found that mechanical properties and wear properties of LM25 alloy have been considerably improved by subjecting the alloy to combined effect of the addition of 2% Al-5Ti-1B and electromagnetic induction with T6 heat treatment.