Angular Grain Research Articles

Research on the Mechanism of Oxygen-Induced Embrittlement Fracturing in Industrial Electrolytic Nickel.

In this study, severe cracking occurred during an investigation of the direct hot rolling of industrial electrolytic nickel plates. To determine the cause of hot-rolling cracking, the microstructure phase composition was analyzed through the utilization of various techniques, including optical microscopy, scanning electron microscopy, electron backscattering diffraction, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and electron probe micro-analysis. The comparative microstructural analysis took place between specimens heat treated in atmospheric and vacuum environments. The characterization and analysis of the hot-rolled plates considered the crack microstructure and fracture morphology. It was shown that holes appeared along the large angular grain boundaries after annealing at 1100 °C for 8 h. Possible reason: In a high-temperature environment, the decomposition of residual additives in the electrolytic nickel releases oxidizing gases, which oxidizes the grain boundaries. The reaction with carbon diffused into the grain boundaries and produced carbon monoxide gas, which induced holes and severely reduced the grain boundary plasticity. The heat treatment time did not need to be very long for severe grain boundary degradation to occur. After severe cavitation, the electrolytic nickel was severely cracked at grain boundaries cracks due to a shear force, and brittle fractures occurred along grains with very low plasticity.

Enhancing oxidation resistance via grain boundary engineering in L12-strengthened medium entropy alloys

The concept of grain boundary engineering (GBE) has been successfully applied to L12-strengthened (CoCrNi)94Al3Ti3 medium entropy alloy, with the aim of improving the oxidation resistance by increasing the ratio of special boundaries and suppressing discontinuous precipitation. Surprisingly, our results reveal that GBE treatment not only slows down the oxidation kinetics and but also alters the oxide scale from TiO2 and multi-defect Cr2O3 to continuous and protective Cr2O3 and Al2O3, thereby contributing to an enhanced oxidation and anti-spalling resistance. The GBE treatment reduces the oxidation weight gain of the current alloy from 1.950 mg cm–2 to 1.211 mg cm–2 after 100 h of cyclic oxidation at 800 °C. The findings show that the extensive outward diffusion of Ti accelerates ion transport and promotes microporosity, thus leading to more defects being formed in the oxide film. The GBE treatment suppresses the discontinuous precipitation of the Ti-bearing L12 phase and breaks the random large angular grain boundaries network, inhibiting the diffusion of Ti and ultimately enhancing the oxidation properties of the alloy. The current work provides an idea of oxidation resistance enhancement for Ti-bearing L12-strengthened alloys without changing the alloy composition.

Analysis of properties and microstructure of ASTM 5% Ni steel used for cryogenic pressure vessel

The influence of the QLT (quenching + lamellarizing + tempering) heat treatment process on the structure and properties of ASTM 5% Ni steel is analyzed. Using QLT heat treatment is to improve the cryogenic toughness of steel, which is generally just related to the reversed austenite. The present study deals with the analysis of the role of QLT heat treatment on structure transformation by researching the relationship among QLT, reversed austenite, and cryogenic toughness. After QLT heat treatment, the structure of ASTM 5% Ni steel transforms from coarse F + B to uniform and fine-tempered sorbite. Grain refinement and high dislocation density bring excellent comprehensive mechanical properties. Alloying elements influence the process and results of the phase transformation of austenite and martensite by getting into them to form new segregation structural units. The oscillographic impact results show that unstable crack growth does not occur during the fracture process. The fracture is upper platform type, which indicates that 5% Ni steel has good cryogenic crack arrest performance. Austenite is not found in the structure under X-ray and transmission electron microscope. The results of the electronic probe indicate the condition of creating conversed austenite is not met. So, the good low-temperature toughness is not from austenite but grain refinement and the increase of the ratio of the large angular grain boundary. This study not only provides proof that the QLT process is not the only factor in the forming of reversed austenite, but also gives inspiration that cryogenic toughness should not rely too much on the reversed austenite.

Cracks propagation mechanism of intergranular corrosion in Al–Cu–Li AA2195 alloys governed by constant relative humidity

Atmospheric corrosion of Al–Cu–Li AA2195 alloys was investigated in constant relative humidity (RH) controlled by saturated salt solutions. Corrosion process was governed by RH, especially for the transformation of corrosion products and crack growth. The deceleration corrosion kinetics and maximum pit depth were found to be in accordance with the power laws. The corrosion products gradually transformed from γ-Al(OH)3 and γ-AlOOH into NaAlCO3(OH)2 and γ-Al(OH)3 affected by RH and exposure time. The localized formation of pitting, filiform and exfoliation corrosion was induced by the presence of Cu/Mg/Li-containing secondary precipitates, including T1(Al2CuLi), θ'(Al2Cu) and S'(Al2CuMg) phases. Secondary precipitates with lower potential were prone to triggering intergranular corrosion. The primary cracks penetrated into the internal matrix along large angular grain boundaires. Secondary cracks propagated along corrosion channels of the stress-concentrated triple grain boundaries with higher schimd factors, larger Kernel average misorientation and higher geometrically necessary dislocation densities. A crack propagation mechanism controlled by RH was proposed.

Microstructure evolution and mechanical property of Mg-Gd-Y-Zn-Zr alloy wires prepared by hot drawing

This study examined the effects of hot drawing deformation on the microstructure, texture, and mechanical properties of an Mg-9.5Gd-4Y–2Zn-0.3Zr alloy. A Φ6 mm extruded alloy bar was hot drawn down to a Φ1.2 mm wire through multi-passes. The results revealed that the intergranular 18R-long period stacking ordered (LPSO) phase was gradually changed into elongated shapes distributing along the drawing direction with increasing deformation strain, while the 14H-LPSO phase precipitates in the α-Mg matrix are progressively fragmented. The average grain size in the wire first increased from 4.6 μm to 25.9 μm and then decreased to 6.7 μm, and the volume fraction of dynamic recrystallisation first decreased from 95.5% to 8.6% and then increased to 48.7%. The initial weak <0001>//ED anomalous texture gradually changed and strengthened into a <01-10>//DD dual basal texture. The Φ1.2 mm wire achieved a tensile strength of 553.5 MPa, a yield strength of 473.6 MPa, and an elongation of 2.5%. Calculations indicated that the high yield strength in Φ1.2 mm wire was mainly attributed to the high content of the large aspect ratio 18R-LPSO phase, high density of low angular grain boundaries, and strong basal texture.

Effect of laser power on microstructure and properties of laser assisted cold sprayed copper coatings on steel

Laser assisted cold spraying (LACS) is a promising technology for preparing high performance copper-steel composite materials for grounding electrode applications. Laser power is a critical LACS parameter, however, in literature there is a lack of studies on the influence of laser power on coatings. In this study, the effect of laser power on microstructure and properties of LACS copper coatings on steel is investigated. The results show that with the increase of laser power, the copper coating generally shows reduced porosity and microhardness, as well as increased bonding strength. However, the grains are first refined and then coarsened, while the electrical conductivity and corrosion resistance both increased and then decreased. This is because when the laser power exceeds 4 kW, the grain is coarsened and the amount of large angular grain boundaries (HAGBs) increases because excessive temperature increases the recrystallization driving force, ultimately leading to decrease in coating electrical conductivity and corrosion resistance. 4.0 kW, 5 MPa, 800 °C is considered as optimal LACS parameters for preparation of pure copper coatings on Q235 steel. The coating has a porosity of <0.04 %, grain size of 0.634 μm, microhardness of 99.4HV0.1, bonding strength exceeds 62 MPa, electrical conductivity of 99 % IACS, and corrosion current density of 6.092 × 10−8 A/cm2. In addition, mechanisms of LACS particle deposition and corrosion of copper coatings in simulated soil solution were discussed. With the increase of laser power, more heat absorbed by the powder and the substrate promotes thermal softening and plastic deformation, resulting in stronger mechanical bonding. The corrosion mechanism gradually changes from pitting corrosion to uniform corrosion due to the reduced porosity and tighter grain boundaries, and finally to intergranular corrosion due to grain coarsening and high HAGBs content. Overall, this study provides guidance and reference for the practical application of LACS.

Effect of phase transformation sequence on the low cycle fatigue properties of bainitic/martensitic multiphase steels

The three microstructures of bainite-fresh martensite, pre-martensite-bainite and carbide-free bainite were obtained by modulating the phase transformation sequence by the heat treatment process in the medium-carbon multiphase steel. The effects of martensitic and bainitic phase transformation sequences on the low cycle deformation behavior of multiphase steels were investigated. The results showed that the sample of the fresh martensite transformation process (295 °C–15 min) exhibited the best fatigue life among all processes under strain control. The samples of the preformed martensite transformation process (250 °C–295 °C) and the carbide-free bainite transformation process (295 °C–120 min) exhibited similar fatigue life. The sample of 295 °C–15 min possesses good resistance to plastic destabilization, resulting in higher fatigue life. The results of the EBSD crack propagation path analysis demonstrate that high carbon M/A islands block crack propagation in the sample of 295 °C–15 min, and high angular grain boundaries skew the crack propagation path. Fracture scans observed that secondary cracks in the sample of 250 °C–295 °C extend inward with aggregation of voids, which are more likely to form crack sources.

Study on periodic immersion and infrared aging corrosion behavior of bridge weathering steel Q420qNH welded joint and plate in NaHSO3 solution

AbstractThe periodic immersion corrosion behavior and corrosion mechanism of Q420qNH steel weld, heat‐affected zone and plate sample in simulated industrial atmospheric medium were studied. The corrosion type of the three samples is the non‐uniform total corrosion, and the corrosion products at different stages are different. Ferric oxide and lepidocrocite (γ‐FeOOH) are generated in the immersion stage and the initial stage of infrared drying, respectively. However, lepidocrocite (γ‐FeOOH), ferrosoferric oxide (Fe3O4) and goethite (α‐FeOOH) generated in the later stage of infrared drying. The infrared drying process is conducive to the formation of goethite (α‐FeOOH). The corrosion rate of weld is always lower than that of plate, and the Iα‐FeOOH/Iγ‐FeOOH ratio of the weld is always higher than that of plate and heat‐affected zone sample. In the later stage of corrosion, the self‐corrosion current density of the three samples from small to large is weld, heat‐affected zone and plate, and self‐corrosion potential from large to small is weld, heat‐affected zone and plate. The resistance of the rust layer of the weld and heat‐affected zone sample is 1.81 times, 1.29 times of that of the plate, respectively. This is because the weld is composed of a large number of small angular grain boundary structures dominated by tiny acicular ferrite in the original austenite grain, which is conducive to the improvement of corrosion resistance. In addition, the higher chromium/carbon (Cr/C), copper and nickel elements of the weld can promote the conversion from lepidocrocite (γ‐FeOOH) to goethite (α‐FeOOH), and improve the density and stability of weld rust layer.

Evolution of Microstructures, Texture and Mechanical Properties of Al-Mg-Si-Cu Alloy under Different Welding Speeds during Friction Stir Welding

The effects of different welding speeds on the microstructures and mechanical properties of Al-0.75Mg-0.75Si-0.8Cu alloys were investigated using optical metallographic microscopy (OM), X-ray diffraction (XRD) analysis, an ETM105D electronic universal testing machine and field emission electron microscopy (SEM). The results reveal that during the friction stir welding process, the welded joint forms the base material (BM), heat-affected zone (HAZ), thermomechanically affected zone (TMAZ) and nugget zone (NZ), under the action of shear force and friction heat. The textures present in the BM are mainly C ({001}&lt;100&gt;) recrystallised cubic texture and P({110}&lt;112&gt;) recrystallised texture, Goss (G) texture, brass (B) texture and {112}&lt;110&gt; rotating copper texture. The organisation of the NZ undergoes dynamic recrystallisation, forming fine isometric crystals with large angular grain boundaries, accounting for more than 75% of the total. The geometrically necessary dislocations (GNDs) in the NZ grow as the welding speed rises. Moreover, {111}&lt;11−0&gt; and {111}&lt;1−10&gt; shearing textures, {001}&lt;110&gt; recrystallisation textures and fibre textures are mainly present in NZs. The average grain size in the NZ was the smallest, and the mechanical properties were the best at a welding speed of 125 mm/min. The grain size and the tensile strength and elongation of the NZ were 2.945 µm, 200.7 MPa and 12.7% for the joint at a welding speed of 125 mm/min, respectively.

Syn-Bushveld “granite sheets” associated with the Molopo Farms Complex intruding into Transvaal Supergroup strata in southern Botswana

Abstract LA-ICPMS U-Pb isotope analyses are presented for zircons from a thin granite sheet intersected in a borehole drilled into the upper Transvaal Supergroup wall rocks to the Molopo Farms Complex in southern Botswana. Many of the zircons have irregular or angular grain margins, and some have rounded cores. Approximately half of the analysed grains yielded concordant 207Pb/206Pb ages ranging between 2 282 ± 29 and 2 113 ± 16 Ma. Assuming that these grains were inherited from the surrounding upper Transvaal Supergroup sedimentary strata, the youngest zircon age provides a maximum depositional age for these sediments. Importantly, these zircon ages coincide with dates from detrital zircons in upper Transvaal Supergroup strata in the eastern Kaapvaal Craton and in the Magondi Supergroup in western Zimbabwe. Hf isotope analyses of the rounded zircons suggest that these grains were sourced from a variable mixture of an old basement and more juvenile material. A younger age of 2 060 ± 12 Ma was obtained from a single subhedral zircon grain from the granite sheet. This is interpreted as the granite’s emplacement age, making it coeval with the adjacent Molopo Farms Complex that forms part of the Bushveld Large Igneous Province.

In-Situ Imaging of CO2 Trapping and Oil Recovery in Three-Phase Systems: Dependence on Pore Geometry and Wettability

SummaryCarbon dioxide (CO2) injection into reservoirs can be combined with enhanced oil recovery. Pore geometry and wettability are critical factors in CO2 trapping and oil recovery. However, their relationship has not been tackled and the influence of wettability is always controversial. To elucidate this intrinsic link, we prepared simple models using glass beads and angular grains, changed wettability by aging, and imaged the three-phase displacement process at high resolution. The results showed that the oil flow occurred through the ways of direct drainage and double imbibition, which is caused by the trapped CO2 under immiscible conditions. More residual CO2 was trapped in the angular grain sample and the water-wet sample, and more residual oil was trapped in the angular grain sample and the oil-wet sample. Under water-wet conditions, CO2 was easier to be trapped as the state of gas-in-oil-in-water or gas-in-water. While under oil-wet conditions, more CO2 was directly surrounded by oil and more small-size residual oil dispersed near the throats. The statistics of the residual clusters showed that the capillary-trapping capacity Ctrap of the angular grain sample was high (12.97% &amp;gt; 8.69%), but the oil recovery R2 was low (89.89% &amp;lt; 93.85%). The altered wettability (water-wet to oil-wet) increased the number of isolated CO2 clusters and the proportion of oil and CO2 in the smaller pores while decreasing the Ctrap (8.93% &amp;lt; 10.34%) and R2 (84.48% &amp;lt; 91.04%). The dissolution trapping and capillary trapping were maximized in the angular grain sample due to the increased surface-to-volume ratio of CO2 clusters and higher capillary force. The effect of wettability on dissolution trapping was slight, but the residual CO2 saturation was higher under water-wet conditions, thus the water-wet media were more conducive to CO2 trapping. We suggested that reservoirs with strong heterogeneity should be selected as CO2 storage sites, and the wettability of oil-wet rocks should be changed by using tertiary oil recovery technology to enhance CO2 storage efficiency and oil recovery. Our study provided direct evidence of the influence of pore geometry and wettability on residual CO2 and oil distribution, and clarified that complex pore geometry and water-wet systems were more conducive to CO2 trapping under immiscible conditions.

Study on the Effect of Temperature on the Microstructure and Properties of AZ31 Magnesium Alloy Sheet Rolled in Single Pass under High Pressure

The single-pass rolling test was carried out on AZ31 magnesium alloy sheet with a thickness of 1.3mm at room temperature and heated to 350°C with different reduction rates. The microstructure and mechanical properties of AZ31 magnesium alloy were contrastively analyzed, and the nucleation process of twinning dynamic recrystallization was analyzed. The results showed that the twin dynamic recrystallization was the main mechanism to coordinate the large deformation with the increase of the reduction rate, and the dynamic recrystallization mechanism at the grain boundary was involved in the coordination deformation at high temperature. When the reduction rate was large, a shear band would appear, which had a significant weakening effect on the material, and the shear band would appear earlier at high temperatures. The reason for the nucleation of dynamic recrystallization was that the formation energy of dynamic recrystallization was lower in the twin, and only the formation of short small angular grain boundary could induce the nucleation.

Geochemical, mineralogical and textural nature of beach placers, north-east Sri Lanka: Implications for provenance and potential resource

The Pulmoddai placer sands in Sri Lanka (9°3′23.65″–8°51′38.83″N and 80°55′22.91″–81°3′32.65″E) is considered to be one of the major rare earth element (REE) prospects world-wide. This deposit has a global significance in terms of strategic economic resources and can provide valuable insight for resource estimation and for its economic use. A detailed study of the geochemical, mineralogical, and textural properties of the heavy minerals of this region is undertaken applying the scanning electron microscopy–electron dispersive X-ray spectroscopy (SEM–EDS), inductively coupled plasma–optical emission spectroscopy (ICP–OES), X-Ray diffraction (XRD), and X-Ray fluorescence (XRF) techniques. The results from these analyses are modeled to delineate the source(s) which contribute to the formation of the mineral deposits and their subsequent enrichment. The beach sediment from this region, exhibited elevated concentrations of major elements like silicon (Si) and manganese (Mn) and high abundance of minor and trace elements like yttrium (Y), strontium (Sr), lanthanum (La), barium (Ba), which indicates that the provenance is similar to calcium-silicate rocks. The highest (total) concentration of Y, La, scandium (Sc) and cerium (Ce) are mainly associated to fine grained sediment, signifying that their occurrences are governed by the grain size distribution and ambient hydrodynamic conditions primarily from lagoonal input. Monazite, zircon, ilmenite, rutile, and xenotime are more abundant than garnet in the study area, as is evident from the higher abundance of thorium (Th) and uranium (U) and the enrichment of zirconium (Zr), Ce, vanadium (V), chromium (Cr), and Y. Textural analysis of sediment shows sediment particles are poorly sorted and have more angular grain boundaries, indicating their lower textural maturity. This suggests that the sediment is fluvially derived from a close source rock of charnockite specific to the hinterland lithology. The sediment transported by perennial rivers and the hydrodynamic conditions of the Kokkilai Lagoon estuary, are the two primary sources of heavy mineral enrichment in the study area, which is quite unique in terms of the placer formation. Subsequently, these sediment particles formed the beach placer deposits enriched in REE–Th rich heavy minerals, which are of immense strategic importance.

Sub-surface granular dynamics in the context of oblique, low-velocity impacts into angular granular media

Oblique, low-velocity impacts onto non-terrestrial terrain are regular occurrences during space exploration missions. These are not only a necessary component of landing and sampling maneuvers, but can also be used as impact experiments to reveal characteristics of the interacting surfaces. We conduct two-dimensional discrete simulations to model such impacts into a bed of triangular grains. Finite element method provides the basis for simulation, enabling the angular grain geometry. Our findings re-create the three classes of impact behavior previously noted from experiments: full-stop, rollout, and ricochet (Wright et al., 2020). An application of Set Voronoi tessellation assesses packing fraction at a high resolution, revealing how grains shift relative to each other during an impact event. We also assess how packing fraction at the point of impact influences different impact behavior types. Calculation of Von Mises strain distributions then reveal how grains shift relative to the overall system, leading to the notion of the ‘skin zone’. Intuition would suggest that the region of perturbed grains would grow deeper with higher velocity impacts, results instead show that increasing velocity may evoke a change in the grains’ dissipative response that dispatches energy predominantly laterally from the impact site instead of deeper into the bed. Finally, we consider how sub-surface response could link with impactor dynamics to deepen our understanding of oblique, low-velocity impact events, one day helping to improve mission outcomes.

DEM Analyses of Cemented Granular Fault Gouges at the Onset of Seismic Sliding: Peak Strength, Development of Shear Zones and Kinematics

Fault zones usually present a granular gouge, coming from the wear material of previous slips. This layer contributes to friction stability and plays a key role in the way elastic energy is released during sliding. Considering a mature fault gouge with a varying amount of mineral cementation between particles, we aim to understand the influence of the strength of interparticle bonds on slip mechanisms by employing the discrete element method. We consider a direct shear model without fluid in 2D, based on a granular sample with angular and faceted grain shapes. Focusing on the physics of shear accommodation inside the granular gouge, we explore the effect of an increase of cementation on effective friction (i.e. stress ratio) within the fault. We find that brittleness and the overall shear strength are enhanced with cementation, especially for dense materials. For the investigated data range, three types of cemented material are highlighted: a poorly cemented material (Couette flow profile, no cohesion), a cemented material with aggregates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R), and a highly-cemented material behaving as a brittle material (with several Riedel bands followed by fault-parallel shear-localization Y). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that the effective friction of a cemented fault cannot be directly predicted from Mohr–Coulomb criteria because of the heterogeneity of the stress state and kinematic constraints of the fault zone.

Compressive Strength of MICP-Treated Silica Sand with Different Particle Morphologies and Gradings

Microbially-induced calcium carbonate precipitation (MICP) can enhance the stiffness and bulk strength of sand aggregates via calcium carbonate cementation. This study explores the mechanical strength of silica sand aggregates with different particle morphologies and sizes following MICP treatment. Specifically, unconfined uniaxial compressive strength (UCS) of MICP-treated spherical, near-spherical, and angular aggregates are measured for four separate size fractions (18–25, 25–40, 40–60, and 60–80 mesh). Scanning electron microscope (SEM) imaging is performed on post-treatment samples to investigate the difference in cementation morphology and failure mechanisms. The experimental results indicate: (1) given identical cycles of MICP treatment, the UCS of treated spherical and near-spherical sands peaks at 25–40 mesh particle size, while the treated angular sands show increasing UCS with decreasing particle size; (2) spherical sands have the highest calcium carbonate (CaCO3) content given identical MICP cycles; and (3) higher post-treatment CaCO3 content does not correlate to higher UCS—implying that the distribution and morphology of CaCO3 precipitation exert crucial control. SEM analysis shows that CaCO3 fully encapsulates spherical sand particles uniformly, forming point contact bonds. In contrast, CaCO3 precipitations show a patchy distribution on near-spherical and angular sand grain surfaces. Unlike treated spherical sands, treated near-spherical sands feature a mixture of point and planar contact bonds while treated angular sands feature predominantly planar contact bonds. Planar contact bonds provide a larger effective cementation area between sand particles and thus result in higher bonding strength. The particle morphology and resultant inter-particle bonding morphology help reconcile higher ensemble UCS with lower CaCO3 content of treated angular sands.

Controlled substitution of coarse aggregate with waste glass and syringe needle in M20 concrete

Certain types of inorganic waste can be reused by substituting the primary materials of M20 concrete in controlled percentage. Inorganic wastes like waste glass and syringe needle are some of its examples which are currently being sent to landfill sites in a large share. This study considers the waste glass (toughen glass, flat glass) generated from the construction site &amp; glass market and autoclaved syringe needle generated as a hospital waste. Waste glass was used to partially replace up to 20% and syringe needle 2% of coarse aggregate in combined form for all concrete mixes. The workability test, compressive strength test and Flexural strength test was conducted to evaluate the effectiveness of the mix. The use of combination of waste glass and syringe needle resulted in decreasing the slump value due to angular grain shape of glass and reinforcing property of needle, whereas the compressive strength noticeably enhanced. With increasing the waste glass and syringe needle percentage the strength gradually increases up to a given limit beyond which it started to decrease. The maximum compressive strength of concrete was obtained at 10% of waste glass and 1% of syringe needle and the value of the compressive strength of the for 28 days of curing is 32.29 N/mm2. The Flexural strength for the sample 2 was observed 3.07 N/mm2.

A simultaneous enhancement of both strength and ductility by a novel differential-thermal ECAP process in Mg-Sn-Zn-Zr alloy

Equal-channel angular pressing (ECAP) technique has emerged as a desirable strategy to fairly improve the properties of magnesium (Mg) alloys. However, poor workability of Mg alloys results in the failure of ECAP processing. Herein, a novel differential-thermal ECAP (DTECAP) process, simultaneously endowing Mg-Sn-Zn-Zr (TZK) alloys with good deformability and superior mechanical properties, was developed. Analysis of deformation microstructures of TZK alloys revealed that the excellent comprehensive properties were mainly associated with the activation of non-basal slip, grain refinement, and texture weakening. The results indicated that prismatic<a>slip was predominant deformation mode of DTECAP TZK alloys, which contributed significantly to the improvement of strength and ductility (tensile yield strength of ~311 MPa and elongation of ~20.5%). Moreover, the presence of astonishingly submicron scale grains (down to ~1.35 µm) was responsible for the high strength in DTECAP TZK alloys. In particular, high angular grain boundaries contributed to the highest strengthening effects. Besides, after DTECAP processing, a strong basal<0001>// ND-preferred texture was weakened from 13.48 to 9.38 (m.r.d.), and exhibited ED-split components. The current study can provide beneficial insights into the development of high-performance Mg alloys via DTECAP processing.

Alteration of volcanic ash glass chemistry due to lightning

Volcanic lightning generates high temperatures (>1500 °C) that melt volcanic ash particles, which re-solidify into lightning-induced volcanic spherules (LIVS). These LIVS have been documented within several ashfall deposits from multiple explosive volcanic eruptions. While several studies have analyzed the morphology and formation processes of LIVS, there is limited research on the chemical composition of these glassy particles. High-current (7 and 25 kA) impulse experiments were performed on very fine volcanic ash (<32 μm) produced from andesitic pyroclasts to create lightning-induced volcanic particles. The pre- and post-experimental ash was analyzed using a combination of wavelength and energy dispersive spectroscopy. The unaltered post-experimental particles, in addition to the pre-experimental samples, are characterized by angular grain shapes and euhedral mineral microlites within a glass matrix. Conversely, the 29 analyzed lightning-induced particles have a rounded morphology and are composed of chemically distinct and highly variable glass with no preserved microlites. Observed within the post-experimental particles are chemically diverse flow structures that are compositionally different than the surrounding glass, indicating mingling between molten components derived from the initial mineral and glass. Transformation of volcanic ash particles into LIVS will provide evidence of lightning during explosive eruptions and may increase the leaching rate of elements into the environment following ash deposition.

Pore Space and Fluid Phase Characterization in Round and Angular Partially Saturated Sands Using Radiation-Based Tomography and Persistent Homology

The multiphase flow transport properties find ubiquitous applications in partially saturated granular media, particularly in the vadose zone due to alternate drying and wetting cycles. The spatial heterogeneity in the degree of saturation at the pore scale has a significant impact on the transport properties of the porous materials. In the present work, three-dimensional (3D) computed tomography (CT) images of the two-phase flow experiments are used to visualize and investigate the pore space and fluid phase microstructure of sands with rounded and angular grain morphology. The dual image modality utilizing X-rays and neutrons is used to obtain 3D information on the arrangement of solids and water in a sand sample. Advanced analysis of the experimental data is carried out using a mathematical framework known as persistent homology. This approach allows quantification of geometry as well as connectivity of pore space and fluid phase directly from the CT images with powerful implications in exploring pore-scale physics of multiphase granular materials. Persistent homology predicted percolation length of 52.5 µm and 41.86 µm consistent with the hydraulic conductivity measurements for rounded sand and angular sand, whereas the value of global void ratio equal to 0.512 and 0.69, respectively, suggests the contrasting response. The fluid distributions obtained from numerical predictions using the pore morphology method (PMM) are compared with results from experiments utilizing persistent homology. These results demonstrate the need for incorporating pore-scale physics in boundary value problems related to multiphase flow in granular materials.