Increase In Yield Stress Research Articles

Comparison of the Effect of Nano Cellulosic Additives on Rheological Parameters for 3D Printable Concretes

In 3D printable concretes, fresh requirements such as pumpability, workability, constructability are affected by the rheological properties of the mixture. High yield stress and high viscosity cause blockages in the transmission line, while low yield stress and low viscosity cause the mixture to not provide the required shape stability. Even if the mixture to be printed has suitable rheological parameters, it is very important in terms of workability that it can be preserved over time. Here, cellulose nanocrystals and cellulose nanofibers were used in cement paste mixtures in order to ensure sufficient workability time in 3D printable concrete mixtures, and also to ensure that the originally designed rheological properties can be maintained over time. The Bingham model, which was created as a result of the measurements made with the viscometer device, was used to evaluate the rheological parameters of the mixes containing cellulose nanocrystal and cellulose nanofiber additions in varied amounts. As a result of the study, it was determined that the yield stress of the reference sample increased by 290% 45 minutes after the preparation of the mixture, while this rate decreased to 135% in the use of cellulose nanocrystals and to 79% in the use of cellulose nanofibers. While it increased by 205% at the end, this rate decreased to 37% with cellulose nanofibers and to 68% with cellulose nanocrystals. When the different additives are compared, it was concluded that cellulose nanofiber may be more effective than cellulose nanocrystal in controlling the increase in viscosity and yield stress over time.

Resting time effect on the rheological behaviour of glass beads and cement suspension: The role of PCE size and ionic strength

AbstractStudies have shown that in an aqueous suspension of inert glass beads, the yield stress is increased as a function of resting time. This study investigates this unexpected phenomenon, focusing on solid surface potentials, PCE size in ionic solutions, and their possible relations to rheological behaviour with resting time. Results indicated that PCE in artificial pore solution will result in a higher yield stress with time. PCE with lower anionic charges and molecular weight can significantly increase the yield stress after 2 h of resting. The absolute value of the zeta potential grows to a certain degree after resting phase. Dynamic light scattering (DLS) results showed that the size of PCE molecules in ionic solutions increases over time, suggesting an enhanced depletion force or bridging force among glass bead particles, which could be the reasons for the increase in yield stress over time. Furthermore, the possible impacts of the depletion force or bridging force resulting from the PCE size change on the rheological properties of cement suspension are discussed.

Modeling meibum secretion: Alternatives for obstructive Meibomian Gland Dysfunction (MGD)

PurposeWhile changes in meibum quality are correlated with severity of meibomian gland dysfunction (MGD) and dry eye disease, little is known regarding the mechanics of meibum secretion. The purpose of this study was to develop a finite element model of meibum secretion and evaluate the effect of various factors that might impact meibum delivery to the ocular surface. MethodsA finite element analysis in COMSOL 6.0 was used to simulate the flow of meibum within the gland's terminal excretory duct. Historical normal human meibum rheology data taken over the meibum melting range from fluid (35–40 °C) to solid (25–30 °C) were then used to calculate the minimum yield stress and plastic viscosity of meibum. The effects of meibum melting state, eyelid pressure and terminal duct diameter on meibum flow rates were then systematically investigated. ResultsThe melting state of meibum from liquid to solid was associated with an increase in the minimum yield stress and plastic viscosity that caused an exponential decrease in meibum flow. Modeling also established that there was a linear correlation between meibum flow rate and eyelid pressure needed to express meibum and the 4th power of the terminal duct radius. ConclusionsOur results suggest that changes in the melting state of meibum from fluid to solid, as well as changes in the radius of the terminal excretory duct and the force exerted by the eyelid can lead to dramatic decreases in the flow of meibum. Together these findings suggest alternative mechanisms for meibomian gland obstruction.

Time-dependent clay gels: Stepdown shear rate behavior, microstructure, ageing, and phase state ambiguity

To study the time-dependent properties of clay gels in the structural rejuvenation mode, a combination of the stepdown shear rate method and an ageing test is advocated. In this mode, the structural development process at two different initial equilibrium states during the low stepdown shear rate and the rest period of the ageing test were followed. The roles played by the electric double layer (EDL) repulsive force in this structural development process with and without shear, for platelet sodium montmorillonite or NaMnt, hectorite and Laponite gels, and fibrous sepiolite gels were assessed. The response of the stepdown shear stress and ageing yield stress reflects the action of these forces on the structural rejuvenation kinetics and the type of structure formed. The platelet clay gels with their strong EDL repulsive particle interaction commenced the structural development process immediately and formed an open network structure. This process is short in shear and takes years when without. The fibrous sepiolite gel with its weak EDL repulsive interaction did not display structural rejuvenation in shear and only showed a significant yield stress increase after the first few hours of ageing. An additional issue concerning the stability of the isotropic liquid (IL) phase state of dilute Laponite dispersions was addressed. These dispersions aged for >9 years, displaying that yield stress and network structure confirmed that the IL phase is unstable. A low energy path available for Laponite particles to aggregate was discussed. A new method for determining the flocculate phase state was also presented.

Unveiling the role of Portland cement and fly ash in pore formation and its influence on properties of hybrid alkali-activated foamed concrete

Aiming to enhance the foam-stability and mitigate the drying shrinkage of alkali activated foamed concrete, the hybrid alkali activated materials are used to prepare foamed concrete. The role of Portland cement and fly ash in pore formation and its influence on performances of hybrid alkali-activated foamed concrete (HAAFC) are studied. Experimental results reveal that the addition of Portland cement promotes the hydration process, decreases the setting time, shortens the evolution time from bubbles to pores and prevents defoaming. In addition, the Portland cement in HAAFC decreases the bubble diameters and enhances the foam stability, which is owing to the increased yield stress of the matrix. The presence of Portland cement and fly ash in HAAFC can increase the crystalline phases and reduce the percentage of mesopores (2 nm ≤ r ≤ 50 nm) thus tempering the drying shrinkage of the HAAFC. The HAAFC with 20% cement, 60% GGBS, and 20% FA possesses the low drying shrinkage, high foam-stability and slightly low compressive strength which have the potential to replace alkali activated foamed concrete.

Development, performance research and shape memory multifunctional exploration of epoxy/PEG shape memory nanofibers

To improve the spinnability of thermoplastic epoxy resin and develop epoxy resin-based shape memory nanofiber materials, polyethylene glycol (PEG) was utilized to regulate the properties of thermoplastic epoxy. The process involved the preparation of thermoplastic epoxy/PEG nanofibers using melt-dispersion and electrospinning techniques. The addition of PEG has enabled the electrospinning of the thermoplastic epoxy, with EPP-5 having the smallest average diameter (227 nm) and EPP-10 showing the most uniform morphology. FTIR analysis confirmed sufficient polymerization of epoxy resin. The water contact angle analysis revealed that an appropriate amount of PEG reduced the hydrophilicity of the nanofiber membranes, with the EPP-10 showing a 14.9 ° increase in water contact angle compared to the EPP-5. The mechanical analysis demonstrated that an appropriate amount of PEG improved the nanofibers’ yield stress, breaking strain, and maximum tensile stress. The EPP-10 exhibited a 3.27 MPa increase in yield stress, a 19.54 % increase in breaking strain, and a 1.17 MPa increase in maximum tensile stress compared to the EPP-5. Thermal analysis showed that epoxy/PEG nanofibers decomposed mainly between 400 ℃ and 500 ℃. Thermomechanical analysis revealed a reduction in the Tg of epoxy/PEG films with the addition of PEG, among which the Tg of EPP-25 was 55.1 ℃ lower than that of EPP-0. The epoxy/PEG nanofiber membrane exhibits excellent shape memory performance, among which the EPP-10 exhibits the best shape recovery effect. In addition, the EPP-10 yarn stretched to 160 % of original length recovered within 25 s, and EPP-10 yarn tied in a temporary shape recovered to original shape within 15 s. In conclusion, by modulating the properties of thermoplastic epoxy through PEG, the spinnability of thermoplastic epoxy was achieved, and shape memory epoxy/PEG nanofibers with potential applications in the field of smart textiles were developed.

Impact of multi‐pin atmospheric cold plasma on dynamic rheological characteristics of kodo millet starch

AbstractKodo millet's popularity grows due to its diverse applications and health benefits, primarily as a starch source for industries like food processing, requiring modifications to overcome raw starch limitations. In this study, raw kodo starch underwent treatment with cold plasma, varying in voltage (10, 20, and 30 kV) and duration (10, 20, and 30 min). The treated samples were designated as S1 to S9. The results showed that the modified starch exhibited significantly higher G' and G" values compared to the control sample. Three different models were attempted to describe the flow behavior, with the most suitable model selected. This analysis led to an increase in yield stress from 86.55 to 217.98 Pa and an increase in the flow index from 0.31 to 0.56 for the modified starch. This understanding of the rheological characteristics of modified starch can be invaluable for innovating new products, blending novel ingredients, and optimizing manufacturing processes.Practical applicationsKodo millet has gained significant popularity in recent times owing to its high fiber content and low glycemic index. In this research, starch extracted from kodo millet underwent plasma treatment, resulting in a modified starch with enhanced functional properties. The study focused on exploring the rheological properties of this treated starch, and the findings revealed notable improvements in dynamic rheological parameters, including the storage modulus, loss modulus, and viscosity of the starch, all of which increased due to the plasma treatment. The enhanced viscosity makes kodo millet starch particularly suitable for a wide range of food applications. This modified starch finds practical use in the preparation of porridges, soups, and as a reliable filling agent in various food products. This emerging technology has captured the attention of food manufacturers in the grain processing sector, offering a means to elevate the quality and desirability of new food products.

Laminar planar hydraulic jump during free surface flow of Bingham plastic liquid

The hydrodynamics of laminar planar hydraulic jump during free surface flow of Bingham plastic liquid is investigated through numerical simulation and shallow flow analysis, both validated against experimental results. The singularity of Bingham plastic model at the yield surface is addressed by adopting the Bingham plastic Papanastasiou regularized model with the value of regularization parameter as 1000 s in order to mimic ideal BP flow. The study reveals that an increase in both yield stress and plastic viscosity creates a jump closer to the inlet with a higher film thickness and jump strength. Interestingly, a higher Froude number shifts jump towards channel exit with increased strength while a higher Reynolds number forms a jump of lower strength closer to exit. The jump types noted from simulations are presented as phase diagrams in terms of inlet Reynolds and inlet Froude number for different yield stress values.

Research on limestone calcined clay cement-based ultra-high performance concrete with high cement substitution: Mixture design and engineering properties

Limestone calcined clay cement (LC3) is believed to be able to substitute Portland cement and promote low-carbon development in the concrete industry. Ultra-high performance concrete (UHPC) features high proportions of cement. Herein, an LC3-based UHPC with high cement substitution was designed and optimized based on the D-optimal method, and its engineering properties, including workability, mechanical properties, hydration process, microstructural evolution, and carbon emission were evaluated. Results show that with an increase in LC3, the dynamic yield stress increases and the water film thickness decreases, which indicates that the workability of LC3-based UHPC reduces. When the substitution ratio is about 55 percent, the optimized LC3-based UHPC achieves the maximum compressive strength, about 146.05 MPa. XRD and TG results show that LC3 consumes portlandite by the pozzolanic reaction, which generates additional gel products. However, as the substitution ratio exceeds 55 percent, the degree of pozzolanic reaction drops, resulting in a decrease in the count of hydration products and compressive strength. The pore structure of LC3-based UHPC is refined, with fewer air voids and a larger percentage of gel pores compared to the normal UHPC. The optimized LC3-based UHPC with a substitution ratio higher than 50 percent in this study achieves high integrated performance with engineering applications and low carbon environmental value.

Properties of red mud blended magnesium phosphate cements: Workability and microstructure evolution

Alumina and iron oxide which constitute significant components of red mud can participate in the hydration reaction with phosphate. This paper partially replaced magnesia with red mud in the preparation of magnesium phosphate cement (MPC) and investigated the effects of red mud substitution on the performance of fresh and hardened properties of the MPC. The fresh properties of MPC were analyzed by measuring the setting time, fluidity, early hydration heat release and rheological properties, while the hardened mechanical properties were characterized by compressive strength measurement. The phase composition and micro-morphology were examined by XRD, SEM, EDS, and TG-DTA. The experimental results demonstrated that the incorporation of red mud prolonged the setting time, and postponed the occurrence of the peak temperature, and reduced the hydration reaction intensity. Regarding to the rheological properties, an increase in the dynamic yield stress was followed by a decline with gradually addition of red mud. Conversely, the plastic viscosity increased consistently. Concerning hardened properties, adding appropriate amount of red mud powders to MPC can significantly improve the compressive strength of hardened MPC mortars. The samples containing 15 % red mud yield the highest compressive strength, especially in the early strength, it was even more pronouncedly improved by 14.8 % comparing to the controlled samples at 3 h. Furthermore, detailed information about the modification effects of red mud on the MPC microstructure has been studied. The density of the MPC matrix was improved, microcracks and gaps appeared less, and some new hydrates could be determined, which is responsible for the improvement of the MPC’s mechanical properties.

Influence of Curing Time and Initial Moisture Content on Metakaolin-Based Geopolymer-Stabilized Soft Soil

This study thoroughly investigated the impact of initial moisture content and curing time on metakaolin (MK)-based geopolymer stabilized soft soil. The stabilized soft soil was characterized with an unconfined compressive strength test, scanning electron microscopy, energy dispersive X-ray spectroscopy (EDS), and compaction test. The geopolymer was used at different concentrations (5%, 10%, and 15% of the dry weight of the soil), and the soft soil was prepared at its initial moisture content, which ranges from 0.75 LL to 1.25 LL. The results of the test indicate that there was an increase in yield stress at low initial moisture content. Conversely, the yield stress experienced a decrease at high initial moisture content. Furthermore, the strength of geopolymer-stabilized soil increased as the curing time increased, regardless of the binder and initial moisture contents. The microstructure analysis confirmed that the stabilized soil had a denser microstructure, the formation of homogeneous gel, and fewer microcracks and pores. As the automatic compaction test revealed, the maximum dry density increased at higher binder contents while the optimum moisture content decreased. This research demonstrates that stabilizers made with metakaolin can efficiently stabilize soft soils. It is worthwhile to conduct further studies on the durability, shear tests, and cost of using geopolymer for soil stabilization under varying environmental conditions.

Strain-rate response of 3D printed 17-4PH stainless steel manufactured via selective laser melting

This research work aims at investigating the strain rate effects on the 17-4PH stainless steel produced by Selective Laser Melting (SLM), with the final goal of adding knowledge about its mechanical performance for applications under certain dynamic conditions. Mechanical characterization of 3D-printed samples was performed using a universal quasi-static electro-mechanical machine, a hydro-pneumatic machines, and a split Hopkinson Tensile Bar device for quasi-static, intermediate (5 s−1) and high strain-rates (100 s−1 and 300 s−1), respectively. The use of this technology represents the most interesting breakthrough of the paper, allowing to identify and to evaluate the performance of 3D-printed metals in relation to impacts or other dynamic conditions typical of different areas of engineering, which are impossible to be obtained by other technologies. The conducted tests allowed to identify the main information on the material behaviour, from the quasi-static to the high strain-rate regime, mainly in terms of stress–strain curves and fracture, and the behaviour after fracture was assessed by applying the Bridgman equations.The effect of horizontal and vertical printing directions, as well as of an annealing heat treatment, are also discussed. According to the evaluation of the main mechanical parameters, the strain-rate increase affects the mechanical behaviour both in the horizontal and vertical directions: samples built horizontally show increased yield stress and ultimate strain and decreased ultimate strain, whereas specimens printed vertically exhibit a remarkable increase in yield stress and a decrease of the ultimate stresses and strains. However, the better behaviour observed, with respect to the horizontally printed specimens, for vertically printed specimens in the quasi-static regime was no longer observed for intermediate and high strain rates. Moreover, all heat-treated specimens present higher strain-rate sensitivity than not heat-treated ones.Finally, the applicability of the existing Johnson-Cook material strength model to represent the mechanical behavior of 3D printed 17-4PH stainless steel is examined. The obtained results represent an important contribution to the current literature, because the dynamic behaviour of 17-4PH stainless steel produced by SLM, under the studied strain rates values, have been scarcely investigated so far.

The Effect of Viscous Drag on the Maximum Residual Stresses Achievable in High-Yield-Strength Materials in Laser Shock Processing.

In this paper, the experimentally observed significant increase in yield stress for strain rates beyond 104 s-1 (viscous regime) is explicitly considered in laser shock processing (LSP) simulations. First, a detailed review of the most common high-strain-rate deformation models is presented, highlighting the expected strain rates in materials subject to LSP for a wide range of treatment conditions. Second, the abrupt yield stress increase presented beyond 104 s-1 is explicitly considered in the material model of a titanium alloy subject to LSP. A combined numerical-analytical approach is used to predict the time evolution of the plastic strain. Finally, extended areas are irradiated covering a squared area of 25 × 25 mm2 for numerical-experimental validation. The in-depth experimental residual stress profiles are obtained by means of the hole drilling method. Near-surface-temperature gradients are explicitly considered in simulations. In summary, the conventionally accepted strain rate range in LSP (106-107 s-1) is challenged in this paper. Results show that the conventional high-strain-rate hardening models widely used in LSP simulations (i.e., Johnson Cook model) clearly overestimate the induced compressive residual stresses. Additionally, pressure decay, whose importance is usually neglected, has been found to play a significant role in the total plastic strain achieved by LSP treatments.

Experimental Investigation on Thermal Stability of Dual Particle Magnetorheological Fluid and Performance

Magnetorheological fluid and their properties are essential in Magnetorheological applications. The present study aims to obtain the thermally stable carrier fluid for Magnetorheological damper application through thermogravimetric analyses of three base fluids for higher stability fluid to synthesize Magnetorheological fluid. Scanning electron microscopic images of particles were also tested for their morphology. Magnetorheological fluid samples with 10%, 15%, and 20% by volume were prepared in-house with a 3% calcium base additive (base fluid). Sedimentation and thermal conductivity studies reveal that increasing particle concentration increases the settling time and thermal conductivity. The flow properties show an increase in yield stress with an increase in particle concentration and magnetic fields. The application part of the fluid consists of Magnetorheological damper fabrication and dynamic testing of 20% volume concentration particles at 10 mm amplitude, 2 Hertz frequency, and 0 Ampere and 0.5 Ampere currents, and the temperature of the system is captured with a K-type thermocouple. The results show an 8.2 °C rise at 0.5 Ampere with a 26.2% force decrease within 1000 cycles. The theoretical model based on the lumped parameter analysis predicts the temperature rise, similar to the experimental analysis with a 9.5% error.

Strengthening and permeability control in sand using Cr3+-crosslinked xanthan gum biopolymer treatment

Trivalent chromium (Cr3+), commonly used in reservoir conformance control, has recently been introduced as an innovative technique for enhancing the strength and durability of Xanthan gum (XG) biopolymer-based soil treatment via Cr3+-induced crosslinking. We investigated the effects of Cr3+-crosslinked XG (Cr-XG) biopolymer treatment on the strength, stiffness and hydraulic conductivity of sand through a comprehensive series of experiments, including unconfined compression, direct shear, constant-head permeability tests, and rheological yield stress measurements. The results revealed that gelation of Cr-XG hydrogel via crosslinking between cation and carboxyl groups in XG, leading to gel stiffening, enhances cohesion within the sand over time. Furthermore, the increased yield stress in the Cr-XG hydrogel, compared to clean XG hydrogel, contributes to a more enduring pore-clogging effect, particularly under elevated hydraulic gradient conditions. The addition of 1% Cr-XG biopolymer to the sand significantly increased the ultimate bearing capacity by 466% and resulted in a four-orders-of-magnitude reduction in hydraulic conductivity, in comparison to untreated sand. This study elucidated the soil strengthening mechanism and efficacy attributed to crosslinking-induced gelation in Cr-XG biopolymer treatment. It effectively addressed limitations inherent in previous biopolymer-soil treatments, thereby accentuating its potential as a rapid grouting material within geotechnical engineering.

Smart materials for experimental tectonics: Viscous behavior of magnetorheological silicones

Magnetorheological (MR) materials are a relatively new class of materials whose mechanical properties can be controlled by a magnetic field. In this work, we investigate the viscous behavior of MR silicones, i.e., mixtures of Polydimethyl Siloxane (PDMS) and carbonyl iron powder, for analog modeling purposes. The rheological properties were determined using a parallel plate rheometer equipped with a variable strength electromagnet. We measured viscosity, yield stress and response time of MR silicone samples in a wide range of strain rates as a function of concentration of carbonyl iron powder and applied magnetic field. We find that MR silicones in the unmagnetized state show a transition from Newtonian to shear-thinning behavior for increasing strain rates. In the magnetized state, MR silicones behave as Herschel-Bulkley, shear-thinning fluids. Viscosity, yield stress and power-law exponent increase as a function of carbonyl iron powder concentration and magnetic field. A simple experiment is presented to demonstrate how the rheology of the analog material can be easily controlled during an experimental run, a condition that is challenging to achieve with other materials commonly used in experimental tectonics.

Effect of resting time on rheological properties of glass bead suspensions: Depletion and bridging force among particles

AbstractThe effect of resting time on the rheological properties of cement suspensions is generally explained by early formed structure and overconsumption of polycarboxylate superplasticizers (PCEs). In this paper, we propose that the influence of resting time on the rheological properties is closely related to size variation of non‐absorbed PCE. To identify this, glass bead suspensions were prepared with various amounts of PCE and ionic solution, and their rheological properties were evaluated at various times. We found that the yield stress increases with time at higher PCE concentrations and higher ionic strength solutions. Adsorbed PCE during resting tends to bridge the particles rather than disperse them. In addition, it was found that hydrodynamic radius of PCE increased with resting time, and depletion forces resulting from non‐absorbed PCE size changes correlate well with the increased yield stress.

Impact of calcium sulfoaluminate cement on printability and early strength development of a slag-based 3D printing cementitious material

Blast furnace cement (BFC) as a binder for 3D printing cement (3DPC) has potential to lower carbon-footprints, but its unstable workability and strength development hinder its application. In this article, blast furnace cement (BFC), a mixture of ordinary Portland cement (OPC) and ground granulated blast furnace slag (GGBFS) based binder was designed toward low-carbon 3DPC, and the impact of calcium sulfoaluminate cement (CSA) content on printability and early strength of printed materials was discussed. The researched components of the materials, GGBFS, OPC and CSA, were first designed based on the 24 h early strength development. Slugs test was utilized to investigate 3D printability, and adiabatic hydration temperature, compressive strength, X-ray diffraction, thermal gravimetric analysis, and solid Si-nuclear magnetic resonance were used to analyze the early strength development and hydration products. For 1 h of hydration, CSA played a significant role in the hydration, due to generation of rod-shape Aft, which resulted in increase of yield stress as indicated by the slugs test. For 24 h of hydration, when CSA is less than 20%, it is C-S-H gel provided the strength; when CSA content is more than 20%, AFt constructs the skeleton, and unhydrated material and AH3 fill in the voids of the skeleton, resulted in high early strength. Strength response surface results indicated that GGBFS, OPC and CSA mass ratio of 5:4:1 is the best in this research. The 3DP opening time is in the range of 20–40 min, and the compressive strength at 24 h reaches 14.43 MPa.

Effect of the superplasticizer addition time on the fresh properties of 3D printed limestone calcined clay cement (LC3) concrete

Although 3D concrete printing (3DCP) is considered one of the most significant innovations in the construction sector, its large cement consumption remains an environmental concern. In response, limestone calcined clay cements (LC³) have emerged as a promising alternative to Portland cement (PC) due to their potential for reducing environmental impact. While previous studies have shown the influence of addition timing of superplasticizer (SP) on the fresh performance of PC, its impact on LC³, particularly in the context of 3DCP, remains less understood. This study assessed the effect of the SP addition time (either directly with the mixing water or with a 10-min delay) on the rheology and printing-related properties of LC³ and PC. Rotational rheometry and 3DP-related tests were conducted over two hours, besides hydration assessment through thermogravimetric analysis (TGA). The results showed that adding SP directly to LC³ led to significantly higher yield stress (1.2–2.5 times) and viscosity (14–59 % higher) values compared to delayed SP addition within the same time frame. However, LC³ exhibited slower rates of yield stress and viscosity increase over time in contrast to PC (with Athix structuration rate approximately 92 % lower for LC³). These tendencies influenced printability performance. While direct SP addition reduced PC's open time from 100 to 30 min, it had no impact on LC³ 's open time, which remained at 80 min. TGA analysis yielded additional insights, highlighting that hydrating LC³ samples required a greater free water content to achieve the same yield stress as PC and this difference was attributed to the presence of calcined clay. These findings make contributions to the technical and scientific communities by enhancing comprehension of LC³ cement fresh behavior and solidifying 3DCP as a disruptive construction method.

Effect of minor elements Al and Ti on dynamic deformation and fracture of CoCrNi-based medium-entropy alloys

Three texture-free CoCrNi-based medium-entropy alloys (MEAs) with similar grain sizes, CoCrNi, (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, are investigated under plate impact loading. Free surface velocity histories are obtained along with microstructure characterizations of postmortem samples. The influences of the addition of minor elements Al and Ti on the microstructures, dynamic mechanical properties and underlying deformation and damage mechanisms of MEAs are elucidated. Compared to the single-phase CoCrNi MEA, spherical nanosized L12 precipitates and micron sized body-centered-cubic phase are found in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively. The addition of minor elements Al and Ti leads to an increase in the dynamic yield stress approximately by 70% (to 1.0 GPa) and 130% (to 1.4 GPa) for (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5, respectively, due to the solid solution strengthening and precipitate strengthening. Multiple deformation mechanisms are observed. Planar dislocation glide and stacking faults are the dominant deformation mechanisms for all the MEAs. However, deformation twinning is deactivated in (CoCrNi)94Al3Ti3 and (CoCrNi)90Al5Ti5 with small additions of Al and Ti. For spallation, damage is ductile in nature for CoCrNi and (CoCrNi)94Al3Ti3, but brittle for (CoCrNi)90Al5Ti5. Under similar shock stress, the spall strength of (CoCrNi)94Al3Ti3 is comparable to that of CoCrNi, ∼ 150% higher than that of (CoCrNi)90Al5Ti5 because intergranular brittle fracture activation does not require substantial plastic deformation. The minor addition of Al and Ti in medium-entropy alloys significantly affects their dynamic mechanical properties, deformation and damage mechanisms.