Formation Of Grain Boundaries Research Articles

Evolution mechanism for a surface gradient nanostructure in GH4169 superalloy induced by an ultrasonic surface rolling process

In this study, the evolution mechanism of surface gradient nanostructure induced by the ultrasonic surface rolling process (USRP) in GH4169 superalloy was investigated. The gradient nanostructure, which was characterized using transmission electron microscopy, revealed that the repeated impacts induced by USRP produced a gradient nanostructure with a thickness of ∼330 μm at the surface of the material. Equiaxed nanograins with an average grain size of ∼30 nm were observed on the topmost surface, and the grain size gradually increased with depth. In the nanocrystallization mechanism, dislocations first formed as a result of the initial plastic strain introduced by USRP. Next, dislocation tangles and dislocation walls were formed and gave rise to the formation of low-angle and high-angle grain boundaries, which resulted in grain refinement. As plastic strain accumulated, nanoscale deformation twins were formed. The interaction between dislocations and deformation twins further refined the parent grains to produce equiaxed nanograins with high-angle grain boundaries. In addition, stacking faults were generated when the interaction between dislocations and deformation twins was initiated. A 9R structure was observed inside the nanograins at the topmost surface that significantly improved the work-hardening capacity of the superalloy by acting both as dislocation blockers and dislocation storage sites. The microhardness of the nanostructured GH4169 superalloy was improved by 67% as compared to the untreated material.

Role of prismaticdislocations in sub-zero temperature deformation of Mg-Gd-Y-Zr alloy

This work investigates the effect of activating and accumulating prismatic<a>dislocations on ductility of a Mg-7.37Gd-3.1Y-0.27Zr (GW83K) alloy at sub-zero temperatures. It is found that the prismatic <a> dislocations were more easily activated with decreasing temperature due to the lowering of CRSS prismatic<a>/CRSS basal<a> ratio in this alloy. Increasing the proportion of prismatic<a>dislocations shows limited effect on improving the ductility of GW83K alloy in the early-stage deformation at sub-zero temperatures, because the prismatic<a>dislocations cannot accommodate c-axis deformation. However, with the formation of low-angle grain boundaries rotating along [0001] axis induced by the accumulation of prismatic<a>dislocations, the GW83K alloy can perform a much better ductility in the late-stage deformation. Such a finding provides a new perspective on understanding the contribution of prismatic <a>dislocations to ductility in Mg alloys.

Synergistic Effect of Calcination and Sintering on the Reduction of Grain Boundary Resistance of LATP Solid Electrolyte.

The NASICON-type Li1.4Al0.4Ti1.6(PO4)3 (LATP) solid electrolyte is a promising candidate for next-generation lithium-ion batteries due to its high stability in air and moisture, as well as high bulk ion conductivity. However, the grain boundary resistance of LATP limits its overall ionic conductivity, which remains a major obstacle to the commercialization of all-solid-state batteries. In this study, we made efforts to solve the problem by promoting the minimization of voids and the formation of well-defined grain boundaries by controlling the temperature of two heat treatments during the synthesis process. The crystallization temperature was confirmed through thermogravimetric analysis/DTA analysis, and the degree of crystallization was confirmed using XRD analysis. The formation of grain boundaries and the presence of voids were evaluated by cross-sectional SEM images after sintering. After sintering, the LA_900 °C sample, characterized by a high degree of crystallization and well-formed grain boundaries without voids, demonstrated a low bulk and grain boundary resistance, which was confirmed by electrochemical impedance spectroscopy analysis. The result was an ionic conductivity of 1.72 × 10-4 S/cm. These results provide valuable insights into the facile synthesis of LATP.

Electrochemical CO2 reduction to C2+ products with Cu-oleylamine based nanoparticles synthesized by simple thermal treatment

Cu-based catalysts for CO2 electrochemical reduction reaction suffer from both activity and selectivity towards C2+ products. Here, Cu-Oleylamine (CuOAm) based catalysts with adjustable Cu chemical state are synthesized through thermal reducing method with oleylamine as the only surfactant, solvent, and reducing agent. The catalyst exhibits a high faradaic efficiency of 54% and 79% with a partial current density of 186 mA cm−2 and 245 mA cm−2 at −1.0 V (vs. RHE) for C2H4 and C2+ products, respectively. Further characterizations of the size, morphology and surface composition of CuOAm by XRPD, XPS, SEM, HRTEM, and EDS showed that the presence of different crystal structures (CuO and Cu2O) with comparable amounts were vital for the formation of grain boundary and lattice mismatch during CO2RR. The sequential reduction of CuOx and CO2 at abundant grain boundary and lattice mismatch could facilitate the formation of ethylene and C2+ products, which was confirmed by in-situ Raman measurements under varied potentials.

Microstructural Evolution and Distributions of Grain Boundary in SPD Processed Al–3 mass%Mg Alloy

SPD-processed Al alloys have an ultra-fine-grained microstructure and extremely high strength. The strength of SPD-processed Al alloy exceeds the estimated it by the grain size using Hall-Petch relationship. SPD-processed Al has high dislocation density because the excess introduced strain remains in dynamically recrystallized grains. The excess dislocation may form the low angle grain boundary during dynamic recrystallization. Al-Mg alloy represents especially large strengthening by dislocation hardening after SPD not only smaller grain size than pure Al. This is because the stacking fault energy (SFE) of the alloy is lowered by Mg solute atom. The dislocation movement become suppressed during DRX in low SFE alloys. As a result, SPD-processed Al-Mg alloys have extremely fine-grained (~0.25 µm) microstructure. SFE of the alloy may also affect the formation of low angle grain boundary and dislocation accumulation in the grain interior. The dislocation distributions could result the degree of extra-hardening. The distributions of dislocation and low angle grain boundary in ECAE processed Al-3 mass%Mg alloy were investigated by EBSD and XRD techniques. Annealing after ECAE for recovery the strain provides the relaxation of the strain hardening, in spite of the LAGB fraction remains static.

Temperature and Strain Rate Dependence on the Tensile Mechanical Properties, Constitutive Equations, and Fracture Mechanisms of MarBN Steel.

The effect of strain rate and temperature on the thermomechanical behavior and microstructure of MarBN steel is studied with the strain rates of 5 × 10-3 and 5 × 10-5 s-1 from room temperature (RT) to 630 °C. At high strain rates of 5 × 10-3 s-1, the Holloman and Ludwigson equations can better predict tensile plastic properties. In contrast, under low strain rates of 5 × 10-5 s-1, coupling of the Voce and Ludwigson equations appears to predict the flow relationship at RT, 430, and 630 °C. However, the deformation microstructures have the same evolution behavior under strain rates and temperatures. Geometrically necessary dislocations appear along the grain boundaries and increase the dislocation density, which results in the formation of the low-angle grain boundaries and a decrease in the number of twinning. The strengthening sources of MarBN steel include grain boundary strengthening, dislocation interactions, and multiplication. The fitted R2 values of these models (JC, KHL, PB, VA, ZA) to plastic flow stress at 5 × 10-5 s-1 are greater than 5 × 10-3 s-1 for MarBN steel. Due to the flexibility and minimum fitting parameters, the phenomenological models of JC (RT and 430 °C) and KHL (630 °C) give the best prediction accuracy under both strain rates.

Microstructural evolution and oxidation in α/β titanium alloy under fretting fatigue loading

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation, thus potentially causing unanticipated catastrophic fractures. The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous, which obstructs the understanding of fretting fatigue issues, and is unable to guarantee the reliability of structures for long-term operation. Here, fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of an α/β titanium alloy up to 108 cycles. Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone. The grain size in the topmost nanostructured layer could be refined to ∼40 nm. The grain refinement process involves the initial grain rotation, the formation of low angle grain boundary (LAGB; 2°–5°), the in-situ increments of the misorientation angle, and the final subdivision, which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays. The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary (HAGB) and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer, thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress. Eventually, it facilitates surface degradation and the formation of catastrophic fractures.

Method of Suppressing Solidification Cracking by Laser Surface Melting and Epitaxial Growth Behavior for Directionally Solidified 247LC Superalloy

In this study, the relationship between solidification cracking and epitaxial growth behavior with the high-speed laser surface melting of a directionally solidified 247LC superalloys was fundamentally and metallurgically investigated, to develop a successful welding procedure for the next generation of gas turbine blades. Under typical laser surface melting conditions (scan speed: 50 mm/s, heat input: 40 J/mm), severe solidification cracking phenomena occurs. The key metallurgical factors of solidification cracking have been identified as solidification segregation-assisted pipeline diffusion behavior at the solidification grain boundary, and in the randomly formed polycrystalline melting zone microstructure. In addition, under extremely low heat input and high-speed laser beam scan conditions (scan speed: 1000 mm/s, heat input: 2 J/mm), an effective surface melting zone can be obtained within a single directionally solidified grain under a relatively high-energy beam density (65 J/mm2) using the characteristics of single-mode fiber lasers. Results reveal that the laser melting zone successfully shows a 99.9% epitaxial growth achievement ratio. Because of the superior epitaxial growth ratio within the laser surface melting zone, and the rapid solidification phenomena, formation of a solidification grain boundary and solidification segregation-assisted pipeline diffusion behavior can be suppressed. Finally, a solidification crack-free laser melting zone can thus be achieved.

Electrochemical corrosion behavior in sulfuric acid solution and dry sliding friction and wear properties of laser-cladded CoCrFeNiNbx high entropy alloy coatings

316 stainless steel (SS) was coated with CoCrFeNiNbx (x: atomic ratio, x = 0, 0.1, 0.2, and 0.3) high entropy alloy coatings (HEACs) by laser cladding (LC). The impacts of Nb content on the phase composition, microstructure, microhardness, wear resistance and corrosion properties of CoCrFeNi based HEACs were examined. Different from the canonical dendritic (DR) - interdendritic (ID) microstructure presented by the other three HEACs, the ID of Nb0.3 HEACs also existed nanoscale Nb-rich Laves phase composed of NbNi and NbCo. The inclusion of Nb significantly reduced the average grain size of CoCrFeNi based HEACs from 59.43 μm to 11.38 μm. It also greatly diminished the formation of high angle grain boundaries (HAGBs), increasing the percentage of low angle grain boundaries (LAGBs) from 31 % to 61 %. The microhardness of Nb-containing HEACs was significantly promoted due to the solid solution strengthening and fine grain strengthening caused by large size Nb atoms. Affected by Laves phase with high coordination number and space filling ratio and higher proportion of LAGBs, the microhardness of Nb0.3 HEAC was even as high as 530 HV. The increase in Nb concentration also strengthened the HEACs' resistance to wear. Nb0, Nb0.1, Nb0.2, and Nb0.3 HEACs had corresponding specific wear rates of 0.36, 0.32, 0.23, and 0.20 mm3/Nm. Unlike 316 SS, Nb0 and Nb0.1 HEACs, which had both adhesive wear and abrasive wear, Nb0.2 and Nb0.3 HEACs were mainly wear by abrasive wear. The corrosion behavior of CoCrFeNiNbx HEACs in 0.5 M H2SO4 solution was similar to 316 SS, but the corrosion resistance was stronger than 316 SS. Nb0.3 HEAC with the Ecorr of −164 mV and Icorr of 6.40 × 10−7 A/cm2 showed the best acid corrosion resistance, which could be attributed to the stable and dense passivation film on its surface. Distinct from the intergranular corrosion of other coatings, the corrosion of Nb0.3 HEAC mainly occurred in DR, because the FCC phase acted as the anode site in it when FCC and Laves phase co-existed.

Dysprosium Incorporation for Phase Stabilization of Atomic-Layer-Deposited HfO2 Thin Films

The relatively low thermal stability of HfO2 films severely affects the performance of semiconductor devices. For instance, the low crystallization temperature of HfO2 (∼500 °C) leads to the formation of grain boundaries, which increases the leakage current. In this study, Dy incorporation leads to the phase transformation of HfO2 films from various directional planes to a main m(−111) plane by the crystallographic stabilization of HfO2 films, increasing the size of grains. Dy-doped HfO2 thin films with modulated doping content, prepared by plasma-enhanced atomic layer deposition (PE-ALD), are characterized by analysis of their chemical composition combined with electron microscopy and synchrotron X-ray techniques. The transformation from m(110), m(−111), m(111), m(020), and m(120) to a main m(−111) plane is observed through X-ray diffraction, which indicates that Dy plays a role for the phase stabilization of HfO2 films. The atomic-scale images of the cross section and top view obtained using an electron microscope demonstrate that the in-plane average grain size is increased by approximately 4 times due to Dy incorporation compared with that of single HfO2 films. The reduction in the area of the grain boundary of HfO2 due to Dy incorporation decreases the leakage current density of HfO2 by 1000 times and increased the breakdown strength. This result can aid future electronics by determining the effect of a dopant on the crystallographic structure of host thin-film materials.

Investigating the corrosion resistance of Cu-doped Ni-Mo low alloy steel through electrochemical tests

Corrosion resistance of Cu-doped Ni-Mo low alloy steel (Ni-Mo steel) in marine environments was systematically investigated through microstructure observation and galvanostatic polarization tests. Results show that solid solution Cu improved the corrosion thermodynamic stability of steel. The increasing Cu content promoted the formation of Cu-rich precipitates, martensite-austenite island, granular bainitic and high angle grain boundary, increasing the surface potential difference and the occurrence probability of local galvanic corrosion on steel surface. The addition of Cu improved the protectiveness of corrosion product film after galvanostatic polarization tests relying on the functions of CuFeO2 and CuO, whose formation was mainly related to the solid solution Cu in Ni-Mo steel.

Analysis of the Tensile Deformation Behaviors and Microstructure Characterization under Various Temperatures of MarBN Steel by EBSD

The uniaxial tensile behavior of MarBN steel with a constant strain rate of 5 × 10-5 s-1 under various temperatures ranging from room temperature to 630 °C was analyzed. This study aimed to identify the effect of the temperature on the tensile behavior and to understand the microstructure deformation by electron backscatter diffraction. The tensile results showed that the yield and ultimate tensile strength decreased with increasing temperature. Serrated flow was observed from 430 °C to 630 °C. The electron backscatter diffraction analysis showed that the low-angle grain boundaries decreased at the medium deformation and increased at the maximum deformation. In contrast, they decreased with increasing temperatures. In addition, the number of voids increased with the increasing plastic strain. As the strain increased, the voids joined together, and the tiny cracks became larger and failed. Three mechanisms were responsible for the tensile deformation failure at various temperatures: grain rotation, the formation and rearrangement of low angle grain boundaries, and void nucleation and propagation. Finally, the formation of the low-angle grain boundaries and voids under different degrees of deformation is discussed.

In Situ Observation of CGHAZ Microstructure Evolution of Hot‐Rolled Ship Plate Steel Containing 0.012 Yttrium under High‐Heat‐Input Welding

The laser scanning confocal microscope is used to simulate the microstructural evolution of a coarse‐grained zone composed of hot‐rolled ship steel with yttrium under high‐energy input welding. The results demonstrate that Y2O3 near the austenite grain boundary can effectively block the overall migration of the austenite grain boundary during the application of the welding heating process. Additionally, the presence of Y2O3 in the austenite grain can promote the formation of new grain and subgrain boundaries, thereby refining the austenite grain. During the postweld cooling process, the undercooling degree is high under a lower heat input, and the phase variation occurs along the direction at a large angle to the proto‐austenite boundary. Thus, a large amount of acicular ferrite (AF) is formed perpendicular to the proto‐austenite boundary. Based on the first principle, Y2O3 can be used as the nucleation site for AF phase transformation to promote the formation of a fine interlocking structure. However, its impact is limited because the cooling rate significantly affects the microstructure evolution of the coarse‐grain zone of the weld during welding with a high heat input.

Facile and Effective Band Gap Engineering of 2D Ruddlesden–Popper Perovskites with Improved Structural and Optoelectronic Properties

Incorporation of alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) into 3D metal-halide perovskite crystal structures has resulted in improved crystallinity and film morphology, reduced trap density, and enhanced charge carrier transport. However, due to intrinsic structural instability associated with 3D perovskites, their alkali metal ion-doped semiconductors are not a suitable option for real device optoelectronics. In recent years, Ruddlesden–Popper (RP)-phase quasi-two-dimensional (2D) metal halide perovskites have drawn considerable attention due to their tunable energy band gap, large electron and hole diffusion lengths (∼10 μm), and lower recombination rates compared to pristine 2D metal halide perovskites (n = 1). However, alkali metal ion-incorporated RP perovskite systems are not explored much; therefore, we demonstrate the incorporation of Rb+, using the RbI precursor, into the quasi-2D RP perovskite (BA)2(MA)Pb2Br7 (BA = C4H9NH3; MA = CH3NH3), which has also resulted in the successive replacement of Br– with I– in the PbBr6 network. Subsequent replacement of Br– with I– finely tuned the energy band gap from 2.92 to 2.43 eV, while keeping the phase of the pristine lead bromide RP perovskite intact, as confirmed by X-ray diffraction investigations. MA+ cations available at the outer edge sites got partially exchanged with Rb ions, which in turn has regulated the perovskite crystal growth, whereas the excess Rb+ alloyed with replaced Br– to form RbBr, which resulted in the formation of thinner grain boundaries and thus showed improved charge transport. Transient photocurrent measurements performed for RbI-doped perovskites (lateral configuration photodetector: fluorine-doped tin oxide (FTO)/RP-perovskite/FTO of channel width ∼150 μm), for 35 cycles under standard Xe-lamp (100 mW/cm2) illumination, have shown strong and fast photocurrent response with large photoresponsivity (3.92 × 10–4 A/W). Comparison of structural, optical, and optoelectronic properties of RbI-doped RP perovskites is carried out with the mixed RP perovskites and has further confirmed that Rb+ incorporation plays an important role in enhancing the crystal growth.

Tilt grain boundary stability in uranium dioxide and effect on xenon segregation

Grain boundaries (GBs) play a key role in the accumulation and release of xenon (Xe) in uranium dioxide (UO2). The stability and structure of tilt GBs with low sigma values in UO2 has been investigated by molecular statics/dynamics (MS/MD) simulations with three different empirical potentials. The atomistic modeling results confirm that Σ3 and Σ11 are more stable than both Σ5 and Σ9, consistent with previous work [J. Nucl. Mater. 517 (2019) 286]. In general, the higher distortion of atoms at a GB corresponds to a higher GB formation energy. Density functional theory (DFT+U) calculations indicate that a defective triangle-like pattern in the Σ5{310} is preferred rather than an oxygen-ring pattern. The energetics of Xe with or without vacancies at the most stable GBs have been studied by MS simulations. The incorporation energy profiles indicate that the Xe interstitial sites at the GBs are always energetically favorable in comparison to the bulk, while Xe incorporation in some Schottky defects (SDs) or uranium vacancy (VU) sites at the Σ5{210}, Σ5{310} and Σ9{221} boundaries are energetically unfavorable relative to the bulk. However, in general, interstitial Xe, Xe-SD, and Xe-VU complexes tend to segregate to the GBs. Notably, the symmetry degree of the distribution of excess atomic volume at a GB is correlated to the charge character of the GB. An asymmetric distribution of excess atomic volume results in the formation of electrostatic dipoles in GB systems, which significantly influences the segregation behavior of charged Xe-vacancy complexes at the GB. The segregation energy of Xe with or without vacancies is to some extent related to the GB excess volume.

Precipitate evolution and properties of an Al–Zn–Mg–Cu alloy processed by thermomechanical treatment

The precipitate evolution, strength and corrosion resistance of an Al–Zn–Mg–Cu alloy processed by cryorolling, warm rolling and final aging have been investigated. A large number of spherical Zn-rich GP zones formed after warm rolling. In-situ transmission electron microscopy (TEM) observation indicated that the GP zones are stable during aging treatment and can serve as nuclei for subsequent precipitates, leading to homogeneous distribution of precipitates in the matrix after aging. TEM results also showed that ultrafine subgrains were developed in the deformed microstructure during aging. The high density of precipitates and dislocations combined with grain refinement contribute to the higher strength of the thermo-mechanically treated alloy compared with the peak-aged alloy. On the other hand, the η phase with C15 cubic structure having ‧‧‧RR‧‧‧ stacking is found in the thermo-mechanically treated alloy and is different from one with C14 hexagonal structure having ‧‧‧RR−1‧‧‧ stacking that is commonly observed in conventionally aged alloys, where R and R−1 represent rhombic layer and the relatively 180°-rotated rhombic layer, respectively. The transition from ill-defined stacking to C15 stacking and further to C14 stacking is observed in the structure of a η phase. Besides, the combination of warm rolling and aging can avoid the formation of continuous grain boundary precipitates, which is beneficial to the corrosion resistance of the alloy.

Grain Structure Formation and Texture Modification through Multi-Pass Friction Stir Processing in AlSi10Mg Alloy Produced by Laser Powder Bed Fusion.

A new strategy is proposed to modify the grain structure and crystallographic texture of laser-powder bed fusion AlSi10Mg alloy using multi-pass friction stir processing (FSP). Accordingly, 1-3 passes of FSP with 100% overlap were performed. Scanning electron microscopy and electron backscattered diffraction were used for microstructural characterization. Continuous dynamic recrystallization and geometric dynamic recrystallization are the governing mechanisms of grain refinement during FSP. The stir zones have bimodal grain structures containing large and fine grains. The multi-pass FSP caused a considerable increase in the volume fraction of the large-grained area in the stir zone, which contained higher values of low-angle boundaries and sharp shear texture components of B(11¯2)[110] and B¯(1¯12¯)[1¯1¯0]. The formation of low-energy grain boundaries in the stir zone and alignment of the low-energy crystallographic planes with the surface of the sample made the strategy of using multi-pass FSP a promising candidate for corrosion resistance enhancement in future studies. Moreover, the detailed evolution of the grains, texture components, grain boundaries, and Si particles is discussed.

Improvement of severe plastic deformation realized by several passes rotary swaging in the microstructure and properties of Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca alloy

The industrial applications of Mg alloys are greatly limited by their trade-off of high strength and ductility. In this work, conventional extrusion and rotary swaging (RS) were applied to Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca (wt.%) alloy to obtain high strength-ductility synergy Mg alloys. The microstructure evolution and mechanical properties of as-extruded and as-swaged alloys at different (2, 6 and 10) RS passes were investigated. The results show that large quantity of dislocations occur in twin lamellae and original dynamic recrystallized (DRXed) grains after RS. Those dislocations promote the formation of low-angle grain boundaries (LAGBs) and subgrains. As the RS process continue, these subgrains transform into fine grains with high-angle grain boundaries (HAGBs), which result in grain refinement. Compared to the as-extruded alloys, the strength of as-swaged alloys increases significantly. The edge region of the as-swaged Mg-0.6Mn-0.5Al-0.5Zn-0.4Ca alloy after 10 passes exhibits comprehensive excellent mechanical performance with a yield strength (YS) of 403 MPa, an ultimate tensile strength (UTS) of 427 MPa and a fracture elongation (FE) of 9.1%. The high strength of as-swaged alloys is the results of the combination of dislocation strengthening, fine grain strengthening and dispersion strengthening. The deformed grains deflected to ED and the tensile twins produced during RS enhance the basal texture and reduce the plasticity of the alloy.

THEINFLUENCE OF MODIFICATION WITH YTTRIUM ON THE STRUCTURE AND PROPERTIES OF CASTINGS OBTAINED FROM THE ЖС6У-ВІ ALLOY RETURN

Purpose. To study the effect of modification of nickel-yttrium ligature with additives on the structure and physical and mechanical properties of ЖС6У-ВІ alloy, smelted using its own technological return in the charge.&#x0D; Research methods. On the УППФ-3М installation with the base crucible, experimental melting of standart technological return of the ЖС6У-ВІ alloy was carried out using high-temperature melt processing.&#x0D; Experimental samples for mechanical tests and determination of stress-rupture strength were cast from the obtained ingots, cut into measured batch blanks and cleaned in a shot blasting drum, using the method of equiaxial crystallization in ceramic molds.&#x0D; When pouring one ceramic mold, the metal melt at a temperature of 1540 °C was modified with a nickel-yttrium ligature of the ІтН1 brand (grain size 2...5 mm) in the amount of 0.136 % of the mass of the charge in a crucible with a holding time of 1 min 15 s ... 1 min. The second block was poured without modification.&#x0D; Cooling of the poured blocks was carried out at the melting site at normal ambient temperature.&#x0D; The samples underwent heat treatment according to ОСТ 1 90126-85: heating to a temperature of 1210 ± 10 °C, holding for 4 hours, cooling in air.&#x0D; The balance of the chemical composition of the experimental alloys was evaluated by the computational and analytical method.&#x0D; The chemical composition of the alloy of experimental variants was determined. The microstructure and mechanical properties at room temperature were studied. Stress-rupture strength tests were performed at 975 °C under a load of 230 MPa.&#x0D; Results. Experimental melting of the charge was carried out, which consisted exclusively of our own technological return of the ЖС6У-ВІ alloy with the use of high-temperature processing of the melt and modification with a nickel-yttrium ligature.&#x0D; The chemical composition, microstructure of the experimental alloy, its mechanical properties at room temperature, and heat resistance indicators were studied.&#x0D; A method of determining the degree of balance of the chemical composition of modern superalloys based on the total content of groups of alloying elements is proposed.&#x0D; Scientific novelty. Calculations carried out in accordance with the proposed method of determining the degree of balance of the chemical composition of the alloy show that for the experimental variants, phase separations may form along the grain boundaries. Studies of the microstructure confirmed the separation of the γ-γ' eutectic phase in the form of a “white” border along the thickened grain boundaries in the metal of the ЖС6У-ВІ alloy sample.&#x0D; According to the calculations of the metal alloying system balance for experimental melts, not only the thickening of the grain boundaries and the separation of the γ-γ' eutectic phase, but also a decrease in mechanical properties and stress-rupture strength can be observed.&#x0D; The application of modification with nickel-yttrium ligature in the amount of 0.136 % in the remelting process with the use of high-temperature processing of the ЖС6У-ВІ alloy return melt allows to ensure the formation of grain boundaries without visible discharges (pollution).&#x0D; It was established that in the metal of the experimental melting with the application of modification with nickel-yttrium ligature, carbides have globular and lamellar morphology. There is no separation of the γ-γ' eutectic phase.&#x0D; The mechanical properties and stress-rupture strength of the metal of the experimental melts with the use of high-temperature processing of the melt of the conditioned return of the ЖС6У-ВІ alloy, both with and without modification by nickel-yttrium ligature, meet the requirements of OST 1 90126-85, but at the same time, they are lower than the properties of the alloy cast with charge refreshment.&#x0D; Practical value. A computational and analytical method of determining the degree of balance of the chemical composition of modern heat-resistant alloys based on the total content of certain groups of alloying elements is proposed.&#x0D; It was established that the modification of nickel-yttrium ligature with additives significantly improves the microstructure of castings obtained from the technological return of the ЖС6У-ВІ alloy and creates conditions for a general improvement in the quality of the material of the responsible castings.&#x0D; The conducted studies show that the use of 0.136 % nickel-yttrium ligature allows to clean grain boundaries, change the morphology of non-metallic inclusions and counteract the segregation of eutectic inclusions in nickel-based superalloys alloys.

A novel microwall sliding test uncovering the origin of grain refined tribolayers

The tribology induced microstructure determines the tribological properties of metals. Yet the complex interaction of multiple microcontacts makes it impossible to decouple the fundamental plastic mechanisms. Single nano-/microasperity sliding tests allow to separate the microcontacts and to study the origins of plastic evolution during the initial stages of wear. However, the fundamental formation mechanisms of the sliding induced grain refinement are still largely unknown. In this work, we pre-remove material around the wear track and produce microwalls, which simplify the stress state and allow to directly inspect the contact zone. After sliding on the microwalls, a misorientation jump forms in the crystal orientation below the wear track. The crystallographic misorientation reaches 30° in some configurations, resulting in the formation of large angle grain boundaries. The misorientation jump formation depends on the grain orientation, wear direction and on twin boundaries in its path. We unravel the grain formation mechanism via slip-step and dislocation analysis and address dislocation arrangements at the grain boundary. This novel microwall sliding test has a potential to better understand the origins of microstructure modification in metals during the run-in stage of wear.