Fraction Of High-angle Boundaries Research Articles

Effects of Cooling Media on Microstructure and Mechanical Properties in Friction Stir Welded SA516 Gr.70 Cryogenic Steel Joints.

SA516 Gr.70 steels were welded by friction stir welding (FSW) under various media of air, water, and water + CO2 cooling, and the effect of the cooling media on the microstructure and mechanical properties of joints was systematically analyzed. The nugget zone (NZ) under the air-cooling condition contained coarse bainite + martensite. Martensite was obtained by decreasing the cooling media temperature. Furthermore, tensile fracturing of the joints occurred in the basal metal (BM), and the ultimate tensile strength of the joints under various cooling media was similar to that of the BM. However, with decreasing cooling media temperature, the total elongation of the joints noticeably increased. Good strength (545 MPa) and elongation (16.8%) were obtained in the joints under the water + CO2 cooling condition since the fine martensite microstructure enhanced the plastic deformation capacity of the joints. In addition, in the NZ under water + CO2 cooling condition, good toughness of 110 J/cm2 was obtained due to a high fraction of high-angle boundaries and fine martensite.

Grain refinement and strain delocalization in TRIP high-entropy alloys during hot deformation

Transformation-induced plasticity (TRIP) high-entropy alloy (HEAs) have shown promising mechanical properties, making them potential candidates for various applications. The preparation of structures with fine grains and heterogeneous orientation is important to improve the strength and ductility of TRIP-HEAs. However, the evolution and mechanism of grain refinement with heterogeneously oriented structures of TRIP-HEAs during thermal deformation and its effect on strain delocalization are not clear. In this paper, the microstructure evolution and strain delocalization of a TRIP-HEA, Co35Cr25Ni15Mn15Fe10 (atomic percent, at %) was examined through hot compression tests conducted at temperatures ranging from 900 ℃ to 1000 ℃, and strain rates ranging from 0.001 s−1 to 0.1 s−1. The results revealed that the average grain size of the alloy decreases with increasing deformation temperature and reducing strain rate due to the appearance of dynamic recrystallization (DRX) and annealed twins. After deformation at 1000 °C-0.001 s−1, the average grain size of the alloy decreases from 169.85 μm (initial sample) to 21.5 μm, and the volume fraction of DRX and twin boundaries (TBs) increases to 81.9 % and 37.6 %, respectively. The DRX mechanism of the TRIP-HEA transformed from discontinuous dynamic recrystallization (DDRX) at 900 °C-0.1 s−1/0.01 s−1 to geometric dynamic recrystallization (GDRX) at 900 °C-0.001 s−1 and then to continuous dynamic recrystallization (CDRX) at 950 °C/1000 °C-0.001 s−1. Furthermore, the occurrence of DRX promotes the formation of annealed twins, resulting high volume fraction of high angle boundaries (HAGBs) and twin-matrix pairs, and contributing to grain refinement. Encouragingly, these HAGBs and twin-matrix pairs not only contributes to grain refinement, but also facilitates strain delocalization via dispersing strain and forming strain gradient, thus leading to coordinated deformation. This result is of great significance for understanding and improving the mechanical properties of TRIP-HEAs, and it can help in the design of high-performance TRIP-HEAs for various applications.

Substructure and texture evolution of a novel near-α titanium alloy with bimodal microstructure during hot compression in α+β phase region

Ti65 alloy has extensive potential applications in manufacturing high-temperature components in the aerospace industry. During thermomechanical processing in the α+β phase region, complicated microstructure and texture evolution occurs. In this work, isothermal hot compression experiments were conducted to comprehensively investigate the substructure and texture evolution of Ti65 alloy with bimodal microstructure via the electron backscatter diffraction (EBSD) technique, and the influence of processing parameters on the evolution of substructure and texture was systematically analyzed. The experimental results showed that the distribution of grain boundaries and the transformation of low-angle boundaries (LABs) to high-angle boundaries (HABs) exhibited sensitivity to temperature and strain rate. The fraction of sub-grain boundaries decreased, while the fraction of HABs increased with the increasing of strain rate or temperature due to the spheroidization of the lamellar α phase accompanied by the wedging of the β phase during deformation and the relative misorientation angle distribution between different α variants precipitated from β phase. Temperature rising at a high strain rate resulting from adiabatic heat promoted atomic activity, caused dislocations to be absorbed by sub-grain boundaries or HABs, and finally reduced the dislocation density in both αp and αs phases. Dynamic spheroidization was the main mechanism with increasing temperature, and coarsening of the lamellar α phase was the main mechanism with increasing strain rate. With the increase of temperature or decrease of strain rate, the texture of the αp phase became stronger and the basal and pyramidal slip could be activated. For the αs phase, spheroidization and variant selection together influence the orientation distribution. As a result, the texture evolution showed a complicated tendency.

Effect of Single Crystal Growth and Solidification Grain Boundaries on Weld Solidification Cracking Behavior of CMSX-4 Superalloy

Single-crystal superalloys have been popularly employed in high-temperature parts of gas turbines, such as blades. However, the welds of such alloys are highly susceptible to solidification cracking, which limits their applicability to high-temperature turbine blades. In this study, the effects of characteristics of weld solidification on solidification cracking susceptibilities (solidification brittle temperature range, BTR) were fundamentally investigated for the CMSX-4 single-crystal superalloy. We applied a transverse-Varestraint test procedure for both the linear and oscillated arc welds by changing the weld solidification characteristics, such as the degree of single crystal growth and formation of solidification grain boundaries. The BTR for the CMSX-4 alloy is 336 K for linear welding condition, whereas the values are 434 K and 342 K for 0.6 and 1.5 Hz oscillated welds. Interestingly, the BTR continuously increases with the weld oscillation frequency. By contrast, almost no changes in the weld mushy-zone temperature range are theoretically calculated for each welding condition via the diffusion-controlled Scheil model. The mechanism underlying the increase in BTR under oscillation welding is clarified based on the relationship between the achievement ratio of the weld single crystal growth and fraction of high-angle (>15<sup>o</sup>) solidification boundaries, which affect severe dendrite coalescence undercooling. The lower fraction of the high-angle weld solidification grain boundaries attributed to the superior achievement ratio of weld single crystal growth, which reduces the dendrite coalescence undercooling and BTR. Consequently, it enhances the solidification crack propagation resistance.

Microstructural study of microwave welded austenitic stainless steel

The joining of austenitic stainless steels through microwave hybrid heating has gained much attention due to its advantages, like uniform and volumetric heating. However, to establish a microwave joining process for promising applications, a microstructural study of the welded joint is still an area to explore. In the present study, microstructural analysis of austenitic stainless steel (SS304) joints is explained through optical microscopy, scanning electron microscopy (SEM), elemental mapping, X-ray diffraction (XRD) and Vickers' micro-hardness tester. Further, detailed microstructural analysis of interlayer and heat-affected zone (HAZ) is studied through electron back-scattered diffraction (EBSD). XRD results reveal the development of various Ni, Cr and the M23C6 type carbides at the joint interlayer region. SEM results point toward non-epitaxial grain evolution along the fusion zone. The mean microhardness of 350 ± 20 HV was determined at the joint interlayer region. A relatively fine grain structure was observed along the fusion boundary. However, moving away from the joint, two different forms of coarse grain heat-affected zone (CGHAZ) were observed. The presence of two different forms of CGHAZ is confirmed through inverse pole figure maps and grain size plots. The presence of high-angle boundaries (HABs) at 60° rotation describes the twin boundaries characterized along the <111> axis. The existence of improved fractions of HABs in the microwave-joined specimen of SS will favor the microstructure to retard bulk deformation. The joint specimen endures a maximum force of 6.98 kN, and the ultimate tensile strength of 613 MPa with 4.75% of elongation. It is observed that microhardness observations and the microstructural studies in the research help establish a solid microstructure-property correlation for the joined material.

The Correlation of Kissing Bond, Microstructure and Mechanical Properties of the 2195 Al-Li Alloy Friction Stir Welding Joints at Different Welding Speeds

Reasonable welding speeds are a prerequisite for obtaining high-quality joints by friction stir welding (FSW). In this paper, 2195-T8 Al-Li alloy FSW joints were successfully fabricated at different welding speeds (100–600 mm/min) with a constant rotation speed. The effect of welding speed on the microstructure and mechanical properties was analyzed under different experimental methods. Microstructural characterization was conducted using optical microscopy (OM), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). Mechanical properties were measured by a hardness test and a tensile test. The results showed that the original T1 precipitates disappeared in the nugget zone (NZ), generating many dislocations. With welding speeds increasing, joints obtained at lower welding speeds developed coarser T1 precipitates in the heat-affected zone. Also, the equiaxed grains with a bigger size and a higher fraction of high angle boundaries (HABs) were detected in the NZ of these joints. As the welding speed increased, the area of hardness value changes gradually shrunk, which was consistent with the trend of the cross-section morphology. A kissing bond and macroscopic cracking were observed in the joints that were fabricated at the higher welding speeds. The appearance of those defects significantly reduced the ultimate tensile strength and elongation of the joints at high welding speeds. Fracture morphologies of the different joints were all characterized in quasi-cleavage fractures.

An investigation into the reverse transformation of ferrite to austenite during friction stir processing of a duplex low-density steel

The reverse transformation of ferrite-to-austenite during friction stir processing of a duplex lightweight steel resulted in an abrupt decrease of the ferrite volume fraction within the stirred zone. This was explained in terms of diffusion-governed phenomena from the thermodynamic points of view. However, owing to the recorded thermal history of the process and the low exposure time at the maximum temperature, the probability of shear assisted diffusion was considered. The fraction of high angle and low angle boundaries within the stirred zone was extremely higher than those observed in thermomechanically affected zone or heat affected zone. These high/low angle boundaries acted as preferred diffusion paths and provided a proper condition to increase the lattice diffusion. The results of electron probe micro-analysis indicated that the alloying elements remained relatively un-partitioned in long range order. In addition, the grains were arbitrarily sheared towards the shear surface/ direction and volume fraction of shear texture component was substantially higher than that of recrystallization texture component. The above observations suggest that the abrupt drop in the ferrite content at the vicinity of the rotating tool in the shear layer may be also induced by shear assisted massive transformation.

Microstructure-based cleavage modelling to study grain size refinement and simulated heat affected zones of S690 high strength steel

Study of the cleavage behavior of heat treated S690 steel by a microstructure-based approach combined with finite element analysis is present in this paper. Cleavage simulations of steels subjected to heat treatments that cause grain refinement or simulate heat affected zones are performed, and are compared with experiments. It is found that the experimental improvement of toughness from grain refinement is 80% of what would be expected based on the model. The 20% difference is due to the lower number fraction of high-angle misorientation boundaries. It is also found that the resistance to micro-crack propagation is more effective in heat affected zones, which can be explained by the residual compressive stress in martensite-austenite constituents. This research assesses the balance between microstructural parameters for controlling cleavage toughness.

Gradient microstructure, recrystallization and mechanical properties of copper processed by high pressure surface rolling

• High pressure surface rolling generates a gradient microstructure in copper. • Nano- and ultrafine-scale structures develop near the surface. • Lightly deformed grains are observed at depths of 1–3 mm. • Partial recrystallization improves the combination of strength and ductility. The microstructure, hardness and tensile properties have been studied in copper processed by high pressure surface rolling (HPSR) both in the as-deformed condition and after subsequent annealing at 150 °C. It is found that a gradient structure with significant differences in the scale of microstructural features is formed by HPSR. The deformed microstructure varies from nano- and ultrafine-scale structures with a large fraction of high angle boundaries near the surface to lightly deformed grains at depths of 1–3 mm below the surface. Tensile tests of 1-mm-thick specimens demonstrate that the as-deformed material has a high strength and a low uniform elongation. Annealing at 150 °C results in partial recrystallization, which creates new through-thickness gradients. Except for the topmost layer and several bands in the adjacent layer, recrystallization is more pronounced close to the surface. The fraction recrystallized is at least 80% at depths of 60–300 µm after annealing for 960 min. The fraction recrystallized in the subsurface decreases with increasing depth, and the deformed layer at depths greater than 500 µm remains largely non-recrystallized after annealing. This partially recrystallized condition demonstrates an improved combination of strength and ductility.

Effects of Severe Plastic Deformation and Ultrasonic Treatment on the Structure, Strength, and Corrosion Resistance of Mg-Al-Zn Alloy.

Nowadays, there is a great demand for increasing the strength and corrosion resistance of magnesium alloys for their wider use in machine engineering, oil industry, and medicine. This paper is devoted to a study on the effects of the combined process of reduction and equal channel angular pressing, as well as the subsequent ultrasonic irradiation on the structure, strength, and corrosion properties of the Mg-Al-Zn alloy. Deformation processing results in an increase of the strength up to 280 ± 10 MPa. A fine-grained structure is formed with a grain size of 10–20 µm and small recrystallized grains 1–2 µm in size. The corrosion resistance in the HCl medium falls down significantly. Action of ultrasound on the deformed specimen leads to an increased fraction of high-angle boundaries, in particular, the fractions of special, fully overlapping Σ13a boundaries and twin boundaries of Σ15b and Σ17a systems. Due to the ultrasonic treatment, the strength of the Mg-Al-Zn alloy increases up to 310 ± 5 MPa, while the corrosion resistance in HCl almost doubles.

Investigation on uniaxial ratcheting fatigue behaviors and microstructure evolution of ultrafine-grained 6061 aluminum alloy

The motivation of the present study is to explore ratcheting-fatigue behavior and the corresponding micro-deformation mechanism of ultrafine-grained aluminum alloys 6061 (UFG AA6061) under two stress-controlled low-cycle fatigue (LCF) loading modes, i.e., mean tension stress (MTS) and mean compressive stress (MCS). The uniaxial ratcheting tests demonstrate that the fatigue-ratcheting damage of UFG AA6061 is prone to occur under MCS-LCF loading, rather than MTS-LCF loading. Detailed microstructure characterizations by Electron backscatter diffraction (EBSD) techniques under these two loading modes show that the grain aspect ratios decrease greatly and the fractions of high angle boundaries increase remarkably in the case of MCS-LCF loading. These observations are mainly ascribed to the room temperature continuous dynamic recrystallization (CDRX), in which low angle boundaries evolve into high angle grain boundaries and the formation of equiaxed grains is accelerated. In contrast, there were no obvious change in the grain aspect ratios or the fractions of high angle boundaries at MTS-LCF loading condition, while the Schmid factor of non-octahedral slip system {100} <110> increases and the texture evolve gradually. The deformation mechanism is grain boundary sliding (GBS) and grain rotation would accommodate the process of GBS. An analysis of the microstructure-properties relationship showed the superposition effect of residual shear deformation and uniaxial LCF ratcheting deformation is responsible for the CDRX at room temperature. And the occurrence of GBS is mainly caused by the activation of non-octahedral slip system.

Ultrafine-Grained Microstructures of Al-Cu Alloys with Hypoeutectic and Hypereutectic Composition Produced by Extrusion Combined with Reversible Torsion.

In this study, binary as-cast Al–Cu alloys: Al25Cu (Al–25%Cu) and Al45Cu (Al–45%Cu) (in wt%) were severely plastically deformed by extrusion combined with a reversible torsion (KoBo) method to produce an ultrafine-grained structure (UFG). The binary Al–Cu alloys consist of α-Al and intermetallic Al2Cu phases. The morphology and volume fraction of α-Al and Al2Cu phases depend on the Cu content. The KoBo process was carried out using extrusion ratios of λ = 30 and λ = 98. The effect of phase refinement has been studied by means of scanning electron microscopy with electron backscattering diffraction and scanning transmission electron microscopy. The mechanical properties were assessed using compression tests. Detailed microstructural analysis shows that after the KoBo process, a large number fraction of high-angle boundaries (HABs) and a very fine grain structure (~2–4 μm) in both phases are created. An increase of λ ratio during the KoBo processing leads to a decrease in average grain size of α-Al and Al2Cu phases and an increase in fraction of HABs. UFG microstructure and high fraction of HABs provide the grain boundary sliding mechanism during KoBo deformation. UFG microstructure contributes to the enhanced mechanical properties. Compressive strength (Rc) of Al25Cu alloy increases from 172 to 340 MPa with an increase of λ. Compressive strain (Sc) for Al25Cu alloy increased from 35 to 67% with an increase of λ. High fraction of intermetallic phase in Al45Cu alloy was responsible for room temperature strengthening of alloy and low compressive strain. The deformed Al45Cu alloy with λ = 30 showed that Rc is 194 MPa and Sc is equal to 10%.

High Temperature Mechanical Properties and Microstructure Evolution of Ti-6Al-4V Alloy Linear Friction Welding Joints

The combination of linear friction welding (LFW) and hot forming processes is limited due to a lack of research on the high-temperature flow behavior and microstructure evolution of welded joints. In this paper, an electric-assisted high-temperature uniaxial tensile test platform based on digital image correlation (DIC) is built, and uniaxial tensile tests of Ti-6Al-4V alloy with LFW joints are performed at different temperatures (923–1023 K) and different strain rates (0.001 s−1–0.01 s−1). Then the microstructure of the LFW joints under different hot deformation conditions have been analyzed by scanning electron microscope (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The results indicate that the high-temperature flow behavior of LFW joints shows an obvious correlation between temperature and strain rate: the yield stress decreases from 203 MPa at 923 K to 105 MPa at 1023 K, and increases from 85 MPa to 130 MPa when the strain rate increases from 0.001 s−1 to 0.01 s−1 at 973 K. The hot deformation mechanisms with deformation conditions have been characterized, which changes from the mechanism of dislocation creep to the mechanism of self-diffusion as the deformation temperature increases from 923 to 1023 K. Especially, the fraction of high angle boundaries (HABs) rapidly rise from 49.2% to 64.1% with the increasing temperatures, the discontinuous dynamic recrystallization (DDRX) become the primary mechanism of nucleation during high-temperature deformation of LFW joints.

Effect of multiaxial deformation on structure, mechanical properties, and corrosion resistance of a Mg-Ca alloy

This article provides a report on the effect of multiaxial deformation (MAD) on the structure, texture, mechanical characteristics, and corrosion resistance of the Mg-0.8 (wt.)% Ca alloy. MAD was carried out on the alloy in the as-cast and the annealed states in multiple passes, with a stepwise decrease in the deformation temperature from 450 to 250 °C in 50 °C steps. The cumulative true strain at the end of the process was 22.5. In the case of the as-cast alloy, this resulted in a refined microstructure characterized by an average grain size of 2.7 µm and a fraction of high-angle boundaries (HABs) of 57.6%. The corresponding values for the annealed alloy were 2.1 µm and 68.2%. The predominant mechanism of structure formation was associated with discontinuous and continuous dynamic recrystallization acting in concert. MAD was also shown to lead to the formation of a rather sharp prismatic texture in the as-cast alloy, whilst in the case of the annealed one the texture was weakened. A displacement of the basal poles {00.4} from the periphery to the center of a pole figure was observed. These changes in the microstructure and texture gave rise to a significant improvement of the mechanical characteristics of the alloy. This included an increase of the ultimate tensile strength reaching 308 MPa for annealed material and 264 MPa for the as-cast one in conjunction with a twofold increase in ductility. A further important result of the MAD processing was a reduction of the rate of electrochemical corrosion, as indicated by a significant decrease in the corrosion current density in both microstructural states of the alloy studied.

The evaluation of microstructure, grain boundary character and micro texture of [Al/Si3N4/Al2O3] P nanocomposites fabricated through PM route and its influence on compressive and three-body wear properties

The compressive properties and 3 body wear characteristics of powder metallurgical (PM) processed [Al/Si3N4/Al2O3] P Nanocomposites with single and combined reinforcement of Al2O3 and Si3N4 reinforcing particles having different compositions (1%, 2% and 3%) were studied and evolution of microstructure, grain boundary character and micro texture of fabricated [Al/Si3N4/Al2O3] P Nanocomposites was investigated through EBSD in the present research work. The fraction of high angle boundaries (HAGBs) were observed more in combined reinforcing samples of Al2O3 and Si3N4 whereas a single reinforcing sample of Al2O3 and Si3N4 showed fewer HAGBs. Micro texture results showed the strong textures components near to {112}〈111 〉 Cu and {110}〈111 for pure sintered Al sample P and mixed reinforcement composites (M1, M2 and M3) > P whereas for single reinforcing sample showed weak textures near to transverse direction. Out of all fabricated composites, 2% mixed Al2O3 and Si3N4 reinforced composite revealed the maximum ultimate compressive strength (209.98 MPa) and least wear rate (0.1 mm3/min mm3/N-cm for 1 kg load and 3.5 mm3/N-cm for 2 kg load) attributing formation of nanocluster causing grain boundary pinning effect. The dominant failure mechanism for all samples was also detected and found to be a mixed-mode ductile failure mechanism for 2% mixed Al2O3 and Si3N4 reinforcement composite while other sample failed through ductile as well as mixed-mode mechanisms.

Cтруктура и прочность мелкозернистой меди после криогенной прокатки и обработки однократными импульсами тока различной мощности

The effect of a treatment combining isothermal cryogenic rolling and single electro-pulsing on the structure and hardness of M1 grade copper with an initial grain size of 10 –15 μm was investigated. Copper was deformed at liquid nitrogen temperature by multi-pass rolling with a total reduction of 90 %. Subsequent electro-pulse treatment (EPT) was carried out in the interval of integral current densities (Kj) from 3.5 ×104 to 8.1×104 A2 s / mm4. Nearly two-fold strengthening of Cu under rolling was found due to the formation of a heavily deformed (sub)grain structure with a crystallite size of the order of 1 μm and a fraction of high-angle boundaries of about 30 %. With further EPT with an energy of 3.5 ×104 A2s / mm4, the processes of recovery predominantly occur, resulting in slight softening with a simultaneous strong decrease in micro-distortions of the crystal lattice and dislocation densities. Processing with higher energies resulted in a sharp drop in the hardness of Cu owing to an activation of in-situ continuous static recrystallization, forming regions of new fine defect-free grains, whose fraction increased with Kj, and intensified the formation of annealing twins. As a result of EPT with Kj in the range of 5×104 – 7×104 A2s / mm4, a uniform fine-grained structure with a grain size of near 2 μm and a fraction of high-angle boundaries of about 90 %, a third part of which were twins of Σ3, was obtained. With a further increase in pulsing energy to 8.1×104 A2s / mm4, normal grain growth to 4 μm with minor changes in the angular parameters of the structure were observed. The nature of structural and mechanical behavior of copper is discussed. A conclusion is made on the viability of the use of combination of cryogenic rolling and EPT to manufacture ultrafine- and fine-grain sheets of different strength out of copper.

The effect of a new route on the microstructure and property of 6063 alloy during the ECAP deformation

The microstructures and hardness of 6063 Al alloy subjected to different ECAP route (Bc and the new route) were investigated. These two routes have different rotation scheme of a sample around its long axis between two consecutive ECAP passes: Bc route, by which sample were rotated in the same direction by 90°; the new route, by which sample were rotated in the same direction by 135°. The feature microstructures of the samples subjected to the two routes were investigated by the optical microscopy (OM) and transition electron microscopy (TEM), respectively. In comparison, the pressing by the new route produces more homogeneous microstructure with the higher fraction of high angle boundaries. Furthermore, the hardness results showed that work hardening can be observed and do not depend much on the ECAP route. However, distribution of hardness in cross-sections of samples revealed that processing to total passes number of eight by the new route improves deformation uniformity.

Effect of specific microstructures on hydrogen embrittlement susceptibility of a modified AISI 4130 steel

The focus of this study is to analyze hydrogen embrittlement susceptibility of a modified AISI 4130 steel by means of incremental step loading tests. Three different microstructures with a hardness of 40 HRC were analyzed: martensite with large and small prior austenite grains and dual-phase (martensite/ferrite). According to the results, the dual-phase microstructure presented the lowest hydrogen embrittlement susceptibility and martensite with large prior austenite grains, the highest. This behavior was attributed to the lower fraction of high-angle boundaries presented by the martensite with large prior austenite grains, which led to a higher diffusible hydrogen content. Moreover, the ferrite local deformation in the dual-phase microstructure enhanced its hydrogen embrittlement resistance by lowering the stress concentration. A synergic effect of decohesion and localized plasticity was identified on the hydrogen induced fracture of the tested microstructures leading to an intergranular + quasi-cleavage fracture in the martensite and quasi-cleavage in the dual-phase microstructure.

Влияние температуры равноканального углового прессования на формирование мелкозернистой структуры в сплаве Al-3 %Cu

Тhe effect of the temperature of equal channel angular pressing (ECAP) on the microstructural changes in a model alloy Al-3 %Cu was investigated. ECAP of the as- homogenized and preliminary overaged alloy with an initial grain size ranging from 200 to 400 μm was performed by route A up to effective strains 8 –12 under isothermal conditions in the temperature range 150 – 400°C (0.45 – 0.75 Tm). ECAP at 150°C led to the significant refinement of the grain structure of the alloy. New fine grains were predominantly formed due to fragmentation of the original grains by the development of mutually intersected deformation- / microshear bands, which initially had the medium-angle misorientations (from 5 to 15°). The number of bands, as well as their boundary misorientations, gradually increased during deformation, leading to the development of crystallites surrounded by high-angle boundaries in accordance with the mechanism of continuous dynamic recrystallization. After 8 ECAP passes, a partially recrystallized structure was obtained in the material, containing regions of (ultra)fine grains with an average size of about 1.2 μm and arrays of the fragmented structure. The average misorientation angle in such the structure was as high as 23.2° and the fraction of high-angle boundaries was about 50 %. Precipitates of Θ (Al2Cu)-phase present in the alloy hindered the dynamic and static recovery and grain growth at the elevated ECAP temperature, and so stabilized the forming grain structure. The increase of the ECAP temperature to 400°C postponed the formation of the fine-grain structure to higher strains and resulted in increase in the average size of new grains to 15 μm, as well as decreased the fraction of high-angle boundaries to 30 % and the average misorientation angle of intercrystallite boundaries to 16.2°. The main reasons for the suppression of the fine-grain structure formation with increasing the ECAP temperature were the occurrence of a more uniform deformation in the earlier stages of processing and the gradual dissolution of the Θ-phase that stabilized the alloy structure.

Влияние электроимпульсной обработки на структуру и твердость криокатаного алюминия

The influence of the energy of electric pulse treatment (EPT) in the range of integral current densities (Kj) from 0.06 ×105 to 0.29 ×105 A2s / mm4 on the structure and hardness of coarse-grained high-purity Al, isothermally rolled up to a total strain of 90 % at a liquid nitrogen temperature, was studied. It was found that EPT with an energy up to Кj = 0.104 ×105 A2s / mm4 practically did not affect the microhardness obtained in the cryorolled aluminum (45 – 50 HV). An increase in the EPT energy to Кj = 0.121×105 A2s / mm4 led to a rapid drop of the hardness to 30 HV followed by its gradual stabilization near 25 HV at higher Кj values. It was established that an enhanced microhardness of rolled Al resulted from formation of a well developed cellular structure with a crystallite size of about 2 μm, containing less than 10 % of ultrafine grains of about 4 μm in size. The minor hardness changes after EPT with Кj up to 0.104 ×105 A2s / mm4 were related to occurrence of recovery and continuous recrystallization, resulted only in improving the deformation structure without noticeable changes in its type and the crystallite sizes. Therewith the softening caused by a partial decrease in the scalar dislocation density and the microdeformation of the crystal lattice was compensated by increasing the high angle boundaries fraction. At EPT with Кj = 0.121×105 A2s / mm4, the deformation structure was severely replaced by the fine-grain recrystallized one with the grain size of 19 μm, and resulted in the loss of the strenthening effect, caused by rolling. With further increase in the EPT energy, an extensive grain growth was observed, leading to the formation of a non-uniform structure grains and to appropriate material softening, owing to the grain coarsening. It was concluded that the restoration processes that took place during EPT were similar in nature to those that occur during furnace annealing of heavily deformed materials. Therewith, the short time of the thermal exposure on the deformed metal during EPT was compensated by high applied energies.