Low Boundaries Research Articles

Ecological-hydrological aspects of the delta Volga meadow vegetation dynamics

Many years ecological monitoring results of the Volga delta meadow plant cover are presented in this work. Temperature conditions and territory humidity conditions were distinguished as leading ecological factors. Geographical methods and mathematical statistics methods were used as analysis tools. It was revealed that watersoluble salts amount sharp increase in soils, also their toxicity level and soil salinization type change from chloride-sulfate to sulfate-chloride under aridization degree rapid increase, that influences on plant cover: biomass decrease, foliage cover degree decrease on all deltoid landscape high levels, carexes and gramens participation degree decrease and forbs role increase is noticed. Such processes are blocked and go to the reverse stage under the following sharp water availability conditions increase. The greatest and more rapid changes are observed within special alluvial low level natural boundaries. The main forecast of the Volga delta climate development can be considered general average temperature and air humidity increase with abnormal humid amount increase and abnormal dry years amount decrease.

Effects of hot extrusion deformation on microstructure and mechanical properties of the spray-deposition 17 vol.% SiCp/7055Al composite

In this study, the microstructure evolution and mechanical properties under hot extrusion of 17 vol.% SiCp/7055Al composite fabricated through the spray-deposition was investigated by employing the Microscope (SEM), the x-ray diffraction (XRD) and the electron back scattering diffraction (EBSD) under a range of deformation conditions. The extrusion temperature reached 400 °C, 450 °C, 500 °C, and the extrusion ratio was at 9, 13, 20 respectively. As revealed from the results the microstructure evolution and mechanical properties of the composite were affected by the extrusion and the precipitated phase. As indicated from the Optical microscope (OM) and the EBSD analysis, the structure variation of the hot extrusion deformation at 450 °C was mainly the dynamic recovery (DRV). However, the dynamic recrystallization (DRX) was dominating at the extrusion temperature of 500 °C and a ratio of 20. In the DRX process, the dislocation substructures appeared within the grains, to reduce low angle grain boundaries (LAGBs) progressively. The textures with {110} 〈111〉 paralleling to TD and the recrystallization textures {001} 〈100〉 and {124} 〈211〉 were characterized. As indicated from the tensile tests, the extrusion eliminated the preparation defects and accommodated the thermal expansion (CTE) mismatch between the matrix and the SiC particles, thereby improving the performance of the composite.

Microstructure, mechanical behavior, and crystallographic texture in a hot forged dual-phase stainless steel

In this work, the hot forging behavior of a dual phase stainless steel in the temperature range of 850–1250 °C was investigated. The study revealed the occurrence of a significant cracking phenomenon for processing temperatures below 950 °C that was attributed to the combined effect of intermetallic precipitation and severe deformation. EBSD examination highlighted the occurrence of continuous dynamic recrystallization in both ferrite and austenite microstructures for processing temperatures above 1050 °C. Increasing the hot forging temperature to 1250 °C increased the low angle grain boundaries fraction and lowered the one of the high angle grain boundaries. This was accompanied by a gradual change in the crystallographic texture of the material. The mechanical behavior investigation showed that the steel plasticity, sharply dropped after forging at 850°, was gradually recovered after hot forging at temperatures above 1050°C. This was confirmed by nanoindentation measurements that revealed a remarkable increase of the hardness and Young’s modulus of the steel after hot forging at 850°C and 950°C due to the dislocation nucleation and the σ phase precipitation at γ/δ interface. The enhancement of dislocation movement at the vicinity of the grain boundaries in the temperature range of 1050–1250 °C improved the global mechanical properties of the hot forged steel.

Dynamic microstructure evolution in cold sprayed Ni[sbnd]Ti composite coatings

The severe plastic deformation inherent to cold spray provides a unique opportunity to study dynamic microstructure evolution under extremely high strain rates, not achievable in conventional laboratory experiments. In this study, the correlation between dynamic deformation and microstructure evolution is investigated for a heterogeneous system, using cold spray of blended NiTi powder mixtures. EBSD analyses of the Ni and Ti particles show different degrees of dynamic recrystallization (DRX) especially at particle boundaries, which are attributed to different materials properties and dissimilar materials interaction. A heterogeneous structure of NiTi composite was achieved with Ni particle being embedded in an ultra-fine grain Ti matrix with an average grain size of 500 nm. Results show hierarchical structures including twins with a thickness of about 500 nm, micro-bands and low angle boundaries, which are associated with a grain subdivision mechanism through continuous dynamic recrystallization (CDRX). The results also demonstrate a drastically different response of Ti and Ni to the same loading conditions, hence provide an insight to the role of intrinsic material properties in DRX at extremely high strain rates.

Characterization of inhomogeneous microstructure and mechanical property in an ultra-thick duplex stainless steel welding joint

Ultra-thick duplex stainless steel welding plates are widely used in various industries. However, one of the significant concerns is the inhomogeneous microstructure and mechanical properties along the thickness, which reduces welding efficiency and degrade service performance. In this study, a 55 mm thick duplex stainless steel welding joint with an X-type weld groove was prepared to study the variation of microstructure and mechanical properties along the through-thickness direction by both the miniature tensile specimen and cross-weld tensile specimen. The correlation between the microstructure and mechanical property was also studied. The results showed that the weld metal was characterized by abundant columnar grains with decreasing austenite content from surface to mid-thickness. Besides, the total fraction of low angle grain boundaries increased more than one-fold, while the total fraction of high angle grain boundaries decreased from 48.03% to 4.52% with increasing depth to the mid-thickness. Furthermore, substantial substructured grains and recrystallized grains were dominated in the weld metal and base metal. Along the thickness direction, the mechanical strength almost decreased and the elongation of welding joint increased with the increasing distance from the surface. Microhardness had a similar trend to that of strength, which all can be attributed to the variation of austenite content and grain boundary character distribution. Along the transverse direction, weld metal had the highest strength of 597.67 MPa, followed by the heat-affected zone of 549.42 MPa and base metal of 510.20 MPa, while the trend of elongation was opposite to that of strength. The microhardness distribution was not consistent with that of strength and had a maximum at the heat-affected zone.

Grain size effect on tensile deformation behaviors of pure aluminum

Grain refinement is a very effective method to improve the mechanical properties of materials and attracts widespread interests among researchers. However, the grain size effect on the mechanical properties is still unclear due to the undesirable microstructure in ultrafine grained (UFG) materials. In the present work, series of ideal materials with average grain sizes range from 0.7 μm to 30.0 μm containing high fraction of high angle grain boundaries (HAGBs), equiaxed grains and low density of dislocations were produced by friction stir processing (FSP). It was found that the Hall-Petch relationships could be classified into three stages as the grain size reduced from coarse grain to UFG regimes, which were decided by the strengthening mechanisms during tensile deformation. The strengthening effect of HAGBs (71 MPa⋅μm1/2) was almost three times of low angle grain boundaries (25 MPa⋅μm1/2), resulting in the positive deviation of Hall-Petch slope in fine grain regime by the increased specific surface area of HAGBs. The further positive deviation of the Hall-Petch slope in UFG regime was affected by the occurrence of an extra dislocation source limited strengthening mechanism, which was up to 29 MPa and reached to about 20% of the yield strength. The increased recovery rate of dislocations at HAGBs contributed to the decrease of mobile dislocation density, leading to the losing of work hardening in UFG regime during tensile deformation.

Novel Gradient Alloy Steel with Quasi‐Continuous Ratios Fabricated by Selective Laser Melting: Microstructure and Corrosion Behavior

In this article, a quasi‐continuous ratio gradient alloy steel (GAS) with a 24CrNiMo high‐strength low‐alloy steel matrix for high‐speed railway brake discs is fabricated by selective laser melting (SLM). The composition of GAS is X% 24CrNiMo + (1–X)% wear‐resistant stainless steel (WSS), in which X is 100, 65, 35, and 0. The relationship between the microstructure and the corrosion resistance is intensively studied by electron backscatter diffraction (EBSD) and potentiodynamic electrochemical equipment. With the increase in the content of WSS, the low angle grain boundaries of the GAS increases, and the microstructure refines and the texture intensity decreases. The passivation film of the external WSS strengthening layer is dense; the corresponding corrosion potential and corrosion current density are −0.109 V and 2.08 × 10−8 A cm−2, respectively. The corrosion current density is about 1% of the substrate 24CrNiMo, and the corrosion resistance of the material is significantly improved.

New high strength ODS Eurofer steel processed by mechanical alloying

A new improved oxide dispersion strengthened (ODS) Eurofer steel was produced by a novel two step mechanical alloying route. Starting from atomized Eurofer powder, two batches of fine and coarse particles with average sizes of ~60 μm and ~120 μm were milled separately after the addition of Ti and nanosized Y2O3. The final blend of the two powder batches was synthetized by hot isostatic pressing (HIP) to obtain a fully dense material with a microstructure characterized by two distinct zones: zones with high density particles (HDPZ) and zones with low density particles (LDPZ). The mechanical properties of the processed material in the as−HIP state and after tempering have been improved significantly compared to various ODS Eurofer and ODS Fe–Cr steels treated by different thermomechanical processing routes (TMP). The stress−strain curve at 600 °C manifested softening while a steady state constant stress was observed at higher temperatures. The stress exponent was two times lower compared to the base Eurofer material; i.e. ~12 vs ~24. Dynamic grain refinement was observed together with ferrite to martensite transformation resulting in an increase in ferrite−martensite interfaces and formation of low angle grain boundaries (LAGBs). Electron back scatter diffraction (EBSD) technique was used to characterize the microstructure and analyze the deformation mechanism.

Improvement in corrosion resistance of wire arc additive manufactured Inconel 625 alloy through heat treatment

In this study, an Inconel 625 component was fabricated by gas tungsten arc welding-based additive manufacturing and the as-deposited specimens were heat treated at 980 and 1100 °C for 1.0 h, respectively. The effects of heat treatment on the corrosion resistance and microstructure were investigated. Potentiodynamic polarization tests showed that the as-deposited Inconel 625 alloy had disparities in corrosion resistance compared with wrought Inconel 625 alloy. The corrosion resistance deteriorated after heat treatment at 980 °C due to needle-like δ phases provided more sites to pitting initiation. While the corrosion potential (Ecorr) increased by 32%, passivation current density (Icorr) decreased by 52% after heat treatment at 1100 °C, which was comparable with that of wrought Inconel 625 alloy. Detailed microstructural examination demonstrated the recrystallization occurred with the dissolution of Laves and δ phases, weakening of 〈001〉 orientation, decrease of low angle grain boundaries and formation of large numbers of stable twin grain boundaries. All the evolution of the crystal and microstructure contributed to the striking corrosion resistance of the 1100 °C heat-treated Inconel 625 alloy.

Effect of multiple laser shock peening on microstructure, crystallographic texture and pitting corrosion of Aluminum-Lithium alloy 2060-T8

This study investigated the influence of multiple laser shock peening on microstructure, crystallographic texture, and pitting corrosion behavior of Aluminum-Lithium alloy 2060-T8 (in 3.5% NaCl) using EBSD, SEM and XPS techniques. Results show that microstructure and crystallographic texture were noticeably influenced by the peening passes. At single peening, plastic strain accommodation was homogenous, resulting in higher fraction of low Σ CSL boundaries, and significant crystallographic texture transformation from fibre (unpeened) into Cube {001}〈100〉 component (corresponding to dynamic recovery as plastic deformation mode) was induced. In contrast, repetitive peening (3 & 5 passes) induced heterogeneous plastic strain accommodation, developed random and low angle grain boundaries, and texture transformation from fibre to Goss {011}〈100〉, implying recrystallization as corresponding plastic deformation mode. As a result of dense atomic structured microstructures/textures, pitting corrosion after single peening was significantly lowered than that of multiple peening. Major reason for the improved corrosion behavior at single peening pass was the formation of dense and uniform α-Al2O3 type oxide film, which significantly lowered the materials dissolution. Whereas, multiple peening led to the formation of porous γ-Al2O3, θ-Al2O3 oxides and Cu-rich regions, which activated the deeper and wider pitting. Besides, the effect of precipitates (Al2CuLi, θ′ (Al2Cu) and β′ (Al3Zr)) on pitting corrosion is quantitatively examined.

Real-Time Estimation of Aerobic Threshold and Exercise Intensity Distribution Using Fractal Correlation Properties of Heart Rate Variability: A Single-Case Field Application in a Former Olympic Triathlete.

A non-linear heart rate variability (HRV) index based on fractal correlation properties called alpha1 of Detrended Fluctuation Analysis (DFA-alpha1), has been shown to change with endurance exercise intensity. Its unique advantage is that it provides information about current absolute exercise intensity without prior lactate or gas exchange testing. Therefore, real-time assessment of this metric during field conditions using a wearable monitoring device could directly provide a valuable exercise intensity distribution without prior laboratory testing for different applied field settings in endurance sports. Until of late no mobile based product could display DFA-alpha1 in real-time using off the shelf consumer products. Recently an app designed for iOS and Android devices, HRV Logger, was updated to assess DFA-alpha1 in real-time. This brief research report illustrates the potential merits of real-time monitoring of this metric for the purposes of aerobic threshold (AT) estimation and exercise intensity demarcation between low (zone 1) and moderate (zone 2) in a former Olympic triathlete. In a single-case feasibility study, three practically relevant scenarios were successfully evaluated in cycling, (1) estimation of a HRV threshold (HRVT) as an adequate proxy for AT using Kubios HRV software via a typical cycling stage test, (2) estimation of the HRVT during real-time monitoring using a cycling 6 min stage test, (3) a simulated 1 h training ride with enforcement of low intensity boundaries and real-time HRVT confirmation. This single-case field evaluation illustrates the potential of an easy-to-use and low cost real-time estimation of the aerobic threshold and exercise intensity distribution using fractal correlation properties of HRV. Furthermore, this approach may enhance the translation of science into endurance sports practice for future real-world settings.

Hierarchical structure in Al-Cu alloys to promote strength/ductility synergy

Experimental evidence demonstrated that equal channel angular pressing (ECAP) at cryogenic temperature, in comparison with ECAP at room temperature, led to promoted strength-ductility synergy in an Al-2.5wt.%Cu alloy. The simultaneous improvement is related to microstructural hierarchy of multimodal grains, low angle grain boundaries, and inter/intragranular precipitates, which was tuned by aging treatment in match with the low-temperature ECAP. The artificial aging could maintain multimodal grain size distribution, introduce a large number of low angle grain boundaries and produce intragranular precipitates to improve strength/ductility. A minor 0.3wt.% Sc addition was effective in optimizing the precipitations and further boosting the strength/ductility combination. The underlying mechanisms for higher strength and greater ductility were rationalized in terms of the low-temperature ECAP.

Effect of electrolytic plasma polishing on microstructural evolution and tensile properties of 316L stainless steel

Electrolytic plasma polishing (EPP) is an advanced technique for high-quality surface finishing of metal and alloy materials. Austenitic 316L stainless steel (SS) is extensively used in various industrial applications due to its excellent comprehensive performance. Therefore, it is widely studied to improve its properties by specific means and methods and expand its scope of application. In this paper, the influence of EPP generated dynamic composite energy on the microstructural evolution and tensile properties of machined 316L SS is investigated. A decrease in the surface roughness (from 0.103 μm to 0.046 μm), average grain size (from 5.65 μm to 5.21 μm), and low angle grain boundaries (LAGBs) are observed, along with an increase in geometrically necessary dislocation density in post EPP samples. Post EPP, grains are found to be predominantly oriented in 〈101〉. Schmid factor variation (from 0.475 to 0.45) shows a reduction in plastic deformability of the material. The yield strength and tensile strength of the material is increased by 8.98% and 4.32%, approximately, after EPP treatment. The dynamic composite energy produced during the EPP process may induce a cavitational effect, causing (i) rupture of the vapor-gaseous envelope, (ii) high-energy plasma bombardment, and (iii) development of annealing stress inside the material, resulting in microstructural evolution of 316L SS.

The microstructure and mechanical properties of novel cryogenic twinning-induced plasticity steel welded joint

Thick Fe–23Mn-1.5Al-0.45C high-Mn steel plates were multi-pass metal inert gas (MIG) welded with 307Si stainless steel wire. The microstructure, grain orientation and grain boundary evolution of the joint were investigated. The toughness impact at ambient and cryogenic temperatures, the microhardness and tensile strength were also measured. The weld metal (WM) is mainly composed of columnar dendrites, and the growth of grains in WM has obvious preferred orientation and significant stress concentration exists in the heat-affected zone (HAZ) and WM. In comparison to base material (BM), lower cryogenic impact toughness was obtained in WM, which was attributed to the precipitation of rod-shaped M7C3 carbide and the high proportion of low angle grain boundaries in WM. The microhardness is higher in HAZ than in BM, and the average microhardness of each pass weld is very different. The joint strength and efficiency are 669 MPa and 91.5%, respectively. With the decrease of impact test temperature (25 °C, 0 °C, −40 °C, −196 °C), the impact toughness of BM and WM both decreased and the fracture mode of WM diverted from ductile to intergranular brittle.

Dynamic recrystallization by subgrain rotation in olivine revealed by electron backscatter diffraction

We document how dynamic recrystallization by subgrain rotation (SGR) develops in natural olivine-rich rocks deformed in extension to up to 50% bulk finite strain (1473 K, confining pressure of 300 MPa, and stresses between 115 and 180 MPa) using electron backscatter diffraction (EBSD) mapping. SGR occurs preferentially in highly deformed grains (well-oriented to deform by dislocation glide) subjected to local stress concentrations due to interactions with hard neighboring grains (poorly-oriented olivine crystals or pyroxenes). Subgrains (misorientation <15°) are mainly delimited by tilt walls composed of combinations of dislocations of the [100](001), [001](100), [100](010) and [001](010) systems, in order of decreasing frequency. The activation and prevalence of these systems agree with a Schmid factor analysis using data for high-T dislocation creep in olivine. The development of closed 3D subgrain cells by SGR recrystallization requires the contribution of at least three different slip systems, implying the activation of hard slip systems and high (local) stresses. The transition from subgrain to grain boundaries (misorientation ≥15°) is characterized by a sharp change in the misorientation axes that accommodate the difference in orientation between the two subgrains or grains. We propose that this change marks the creation and incorporation of new defects (grain boundary dislocations with different Burger vectors and, likely, disclinations or disconnections) at the newly-formed grain boundaries and that this might be favoured by stress concentrations due to increasing misalignment between slip systems across the boundary. Finally, we document the development of strong misorientations within the parent grains, due to the accumulation of low angle grain boundaries, and between them and the recrystallized grains. SGR recrystallization may thus produce strong dispersion of the crystal preferred orientation without the need for grain boundary sliding.

Effect of Deformation Rate on the Elastic-Plastic Deformation Behavior of GH3625 Alloy

GH3625 alloy is a typical polycrystalline material. The mechanical properties of a crystal within the alloy depend on the single crystal properties, lattice orientation, and orientations of neighboring crystals. However, accurate determination of single crystal properties is critical in developing a quantitative understanding of the micromechanical behavior of GH3625. In this study, the effect of deformation rate on the elastoplastic deformation behavior of GH3625 was investigated using in situ neutron diffraction room-temperature compression experiments, EBSD, and TEM. The results showed that the microscopic stress–strain curve included elastic deformation (applied stress, σ ≤ 300 MPa), elastoplastic transition (300 MPa 350 MPa) stages, which agreed with the mesoscopic lattice strain behavior. Meanwhile, the deformation rate was closely related to the crystal elastic and plastic anisotropy. The results of the lattice strain, peak width, and intensity reflected by the specific hkl showed that the deformation rate had little effect on the elastic anisotropy of the crystal, but had a significant effect on the plastic anisotropy of the crystal. With the increase in the deformation rate, the high angle grain boundaries gradually changed to the low angle grain boundaries, and the proportion of twin boundaries gradually reduced. Also, the grains transformed from uniform deformation to nonuniform deformation. Moreover, with the increase in deformation rate, the total dislocation density (ρ) of the alloy first decreased and then increased, whereas the geometrically necessary dislocation density (ρGND) monotonically increased, and the statistically stored dislocation (SSD) density (ρSSD) monotonically decreased. Meanwhile, the abnormal work hardening behavior of the sample at a deformation rate of 0.2 mm/min was mainly related to the SSD generated by uniform deformation. Additionally, the contribution of dislocation strengthening and TEM observation confirmed that the dominant deformation of GH3625 was dislocation slip, and its work hardening mechanism was dislocation strengthening.

The effect of pearlite morphology and crystallographic texture on environmentally assisted cracking failure

Mechanism of failure by environmentally assisted cracking was extensively investigated in the present work to find the effect of pearlite morphology and crystallographic texture on crack nucleation and propagation sites. The macrotexture analysis revealed that (001) crystallographic texture components parallel to the normal direction was predominant in the investigated sample. Crack nucleated at surface pits and propagated in low angle grain boundaries. The refined microstructure prevents crack growth and can deflect crack propagation. High angle grain boundaries were found to suppress crack propagation. In the crack-free region (223) [11¯0] and (1¯11) [11¯2] texture components were found. Therefore, the control of crystallographic orientation and engineering the grain boundary characteristics can effectively improve the environmentally assisted cracking resistance.

Low temperature electrogalvanization: Texture and corrosion behavior

Electrogalvanization at room temperature using aqueous bath is extensively used for protection of steel in highly corrosive environments. Here, it is shown that low temperature galvanization conducted at ~2 °C using aqueous electrolyte bath produces Zn coating which exhibits significantly lower corrosion current density (icorr) value when compared to the icorr values of coatings deposited at room temperature (~28 °C) and high temperature (~60 °C) when these coatings are exposed to a corrosive environment (3.5 wt% NaCl solution). The icorr values for coatings deposited at 2 °C, 28 °C and 60 °C were 8.94 µA/cm2, 24 µA/cm2 and 182 µA/cm2 respectively. Micro-texture analysis revealed that the texture (grain orientation and grain boundary constitution) of the coatings is also highly sensitive to the deposition temperature. Higher corrosion resistance observed in case of 2 °C deposited coatings was due to factors such as growth of grains oriented with relatively inert basal plane on the coating surface which retards the transgranular corrosion and presence of higher fraction of low energy special boundaries (such as low angle grain boundaries, coincidence site lattices and symmetrical tilt grain boundaries) which retards the intergranular corrosion significantly.

Development of grain size/ texture graded microstructures through friction stir processing and subsequent cold compression of a rare earth bearing magnesium alloy

The present work deals with the effect of mechanical post-treatment on the microstructure and texture evolution of friction stir processed Mg-4.37 wt%Y-2.9 wt%RE-0.3 wt%Zr alloy. In this respect, the stir processed specimens were compressed down to engineering strain of 10 and 20%, and the resultant microstructures were characterized. Through applying subsequent compressive loading, fraction of low angle boundaries was increased, substructure was developed, the grain size was refined, and specified misorientation angles/texture components were faded or appeared. The degrees of such evolutions were different between the surface layer and stir zone, and resulted in development of graded microstructures. The fraction of strain-less and fresh grains was considerably decreased; but the fraction of coarse grains holding high grain orientation speared (GOS) was remarkably increased specially by moving away from material surface. Schmid analysis indicated the higher probability of non-basal slip within the unstable coarse grain in the course of subsequent loading. This was led to the occurrence of post-dynamic recrystallization which was traced through the sequence of substructure development and transformation of the low angle boundaries in to the high angle ones. Such gradual microstructure/texture evolution indicate the stir processing plus compression loading as capable method for fabrication of micro-scale functionally graded materials. • Mechanical post-treatment of friction stir processed WE43 magnesium alloy. • Fabricating graded microstructures regarding grain size, substructure and texture. • Contribution of continuous dynamic recrystallization during friction stir processing. • High potential of unstable coarse grains for refinement during subsequent loading. • Post-dynamic/Meta-dynamic recrystallization activates during subsequent compression.

On elliptic problems in domains with low dimensional boundaries

We study a solvability for some concrete boundary value problems in domains with a low dimensional boundary. Such boundaries are considered as a limit situation of a volume problem for which one or more parameters of a domain tend to their endpoint values. The solvability conditions in Sobolev-Slobodetskii spaces are given using the wave factorization concept for an alliptic symbol..