Microstructural Characterization Research Articles

Tuning Microstructure of Mesophase Pitch Carbon Fiber by Altering the Carbonization Ramp Rate

The microstructure of mesophase pitch carbon fibers (CFs) are tuned by varying ramp rates from 1 to 50 °C min−1 up to 1000 °C to study the effect of ramp rate on CFs’ microstructure, thermal and mechanical properties with the goal of offsetting the cost by decreasing cycle time. The ramp rates represent carbonization times ranging from 16.4 to 1.17 h, not including cool down. Differential scanning calorimetry, thermogravimetric analysis, and derivative thermogravimetry are used to investigate the impact ramp rate has on the thermal properties of mesophase pitch. It is found that lower ramp rates are endothermic in nature with a lower temperature onset and maximum weight loss. Higher ramp rates possess an exothermic nature with higher temperatures resulting in maximal weight loss over a smaller range of temperatures. Mechanical testing shows varying CF strengths and moduli dependent on ramp rate and an optimized process is developed to produce the strongest CF. Microstructural characterization revealed that faster ramp rates lead to smaller interplanar spacings and larger crystallites but possessed greater disorder.

Understanding the mechanisms behind ultrasonic vibration in resistance spot welding of aluminum and steel

An ultrasonic-assisted resistance spot welding process, which can improve the quality of aluminum/steel spot-welded joints, has been developed. However, the mechanism of the thermal-electrical-mechanical behavior caused by the ultrasonic longitudinal vibration to welding process has not been clarified. In this study, the role and impact mechanisms of ultrasonic longitudinal vibration during the welding process to form joints were confirmed by combining the microstructural characterization of the formed joints with the signal analysis of the process, as well as the analysis of the independent finite element numerical calculation results. Additionally, the interface metallurgical reaction behavior, welding defects, fracture behavior, and joint strength characteristics of ultrasonic-assisted aluminum/steel resistance spot welding joints were comprehensively analyzed, and the results indicated that the sizes of the aluminum/steel nuggets decreased. It could be attributed to the competition among multiple ultrasonic effects. Specifically, ultrasonic vibration decreased the contact resistance between the aluminum/steel sheets, whereas the acoustic cavitation and acoustic streaming effects increased the electrical conductivity of molten steel and thermal conductivity of molten aluminum, respectively. Moreover, ultrasonic vibration promoted the radial creep of molten and near-molten aluminum, which expanded in the effective bonding area between the two workpieces. Furthermore, ultrasonic vibration effectively reduced the thickness of the intermetallic compound layer (<3.0 µm), concurrently inhibited the formation of interfacial welding defects. Finally, the peak tensile-shear load of aluminum/steel joints is significantly increased under multiple ultrasonic optimization mechanisms.

Utilization of Slip Casting Process for Recycling CAD/CAM Dental Zirconia Wastes

This study aimed to find the ideal parameters for shaping waste zirconia powders from dental laboratories using the slip-casting process. Additionally, the qualities of ceramic products created in this manner were evaluated using microstructural characterization and physical-mechanical tests. Various dental laboratories provided the waste CAD/CAM zirconia powder used in the investigation. Wastes in powder form were first calcined. Afterward, an attritor mill was used to grind the grain size until it was usable, following the completion of the grain size distribution analysis. Waste and commercial zirconia powders were combined using various dispersants to create slip-casting slurries. The rheological characteristics of these slurries were then ascertained. By evaluating the rheological properties of slip-casting slurries prepared in this way, the most suitable casting parameters were determined, and ceramic products were formed by slip-casting technique from the slurries to be prepared in accordance with these parameters. The shaped samples were dried and sintered at two different temperatures, 1400-1450°C, and samples were designed for physical, mechanical, and microstructural characterization. The pore percentages, bulk densities, and water absorption of the sintered samples, according to Archimedes’ principle, as well as their strengths, were determined by the three-point bending strength test. Phase analysis was performed with XRD (X-ray diffractometer) microstructure studies with SEM (Scanning Electron Microscopy). It has been concluded that waste zirconia can be used in dental applications.

Microstructural Characterization of AlCrCuFeMnNi Complex Concentrated Alloy Prepared by Pressureless Sintering.

A significant and increasing number of studies have been dedicated to complex concentrated alloys (CCAs) due to the improved properties that these metallic materials can exhibit. However, while most of these studies employ melting techniques, only a few explore powder metallurgy and pressureless sintering as production methods. In this work, a microstructural characterization of AlCrCuFeMnNi CCA samples obtained by powder metallurgy and pressureless sintering using mixtures of powders with different compositions was carried out. One batch of samples (B1) was prepared using commercial powders of Al, Cr, Cu, Fe, Mn, and Ni. Another batch (B2) used mixtures of CrFeMn, AlNi, and Cu powders. A third set of samples (B3) was obtained by adding 1% at. of Mg to the B2 powder. The samples were characterized by X-ray diffraction, scanning and transmission electron microscopy, energy dispersive spectroscopy, density measurements, and hardness tests. Thermodynamic calculations were also used to complement the microstructural characterization. All the obtained samples exhibited high relative density and hardness values. However, B3 samples showed a higher hardness, attributed to the finer distribution of oxide particles, which was promoted by the presence of Mg during sintering. These last samples presented a hardness/density ratio of 62 HV/(g cm-3), surpassing that of some martensitic stainless steels and nickel-titanium alloys.

Effect of SiC particle content on the mechanical behavior and deformation mechanism of SiCp/Al composite under high-frequency dynamic loading

SiCp/Al composites exhibit excellent structural properties but suffer from inherent brittleness, which has brought rigorous challenges on the materials processing. Ultrasonic-vibration-assisted approach offers a promising solution to improve the processability. However, the underlying mechanisms of mechanical response and defect suppression under ultrasonic action have been rarely studied for SiCp/Al composites, hindering the development of high-efficiency processing. This work aims to investigate the mechanical behavior and microstructural evolution in this class of composites with varying SiC volume fractions up to 45% under high-frequency dynamic load induced by ultrasonic vibration. The results showed that the deformation stress of SiCp/Al composites under ultrasonic compression was significantly reduced to only about 1/10 of that under conventional quasi-static loading. Moreover, as the SiC fractions increasing, this ultrasonic softening effect of SiCp/Al became more pronounced. The plastic limit of SiCp/Al with 45% SiC fraction nearly doubled under ultrasonic compression without any macroscopic cracks. The subsequent nanoindentation tests revealed a transition in work-hardening behavior of SiCp/Al with different SiC fractions under ultrasonic loading. Microstructure characterization was conducted to analyze the grain refinement, recrystallization, and deformation textures within the deformed areas, revealing the deformation mechanism of SiCp/Al under different loading mode. TEM observations confirmed the distribution of dislocations in Al matrix and dense stacking faults in SiC particles after ultrasonic compression, indicating that the plastic behaviors were enhanced in both Al matrix and SiC phase under the high-frequency cyclic stress. This research revealed the distinct deformation mechanisms of SiCp/Al composites under high-frequency dynamic loading, exhibiting extremely low deformation stress accompanied with significantly enhanced plasticity. The results provide theoretical insights for the high-efficiency processing and rapid property regulation of particle-reinforced-composites based on ultrasonic-vibration-assisted technologies.

The role of Mg content in regulating microstructures and mechanical properties of Al–Mg–ZnO composites fabricated via in-situ reaction sintering

Al–Mg-oxides composite system exhibits great potential for achieving aluminum matrix composites (AMCs) with exceptional mechanical properties. However, the effects of Mg element on in-situ reaction mechanism and precipitation behavior remains largely unknown. In this work, Al–Mg–ZnO composite was successfully fabricated by using segmented ball milling, reaction sintering and heat treatment, resulting in an ultimate tensile strength of ∼760 MPa and fracture elongation of ∼3.5 %. The Mg content-dependent reaction pathway and precipitation evolution were systematically investigated through thermodynamic analysis and microstructural characterization. The results revealed that the relatively high Mg content promotes the in-situ generation of the hybrid reinforcements composed of MgAl2O4 and MgO. Additionally, the semi-coherent reinforcement-matrix interface facilitates interfacial precipitation by reducing the energy barrier for nucleation. Consequently, solute-rich/vacancy-rich Guinier-Preston (GP) zones are activated to form η′ and T′ precipitates. These high-density nano-sized secondary phases contribute to the considerable strengthening effect of the composite. The present work provides valuable theoretical insight into the effect of Mg content on the microstructure evolution of Al–Mg–ZnO composite system, which offers promising avenues for achieving AMCs with superior mechanical properties.

Microstructure and mechanical properties of cast, eutectic Al-Ce-Ni-Mn-Sc-Zr alloys with multiple strengthening mechanisms

The effects of Ni additions on microstructure and microhardness of near-eutectic Al-9Ce-xNi-0.75Mn-0.18Sc-0.12Zr (x = 2.5, 3.2, and 3.9 wt.%) alloys are investigated for various cooling rates. The as-cast microstructure consists of fine, micron-scale Al11Ce3 and Ni-rich phases formed during eutectic solidification. Two Ni-rich phases are observed: (i) Al27Ce3Ni6 at higher Ni contents and slower solidification rates, and (ii) Al9(Ni,Mn,Fe)2 at lower Ni contents and faster solidification rates. While these Ni-containing alloys have higher microhardness than a Ni-free control alloy, varying the Ni concentration in the 2.5–3.9 wt.% range does not significantly affect the microhardness, indicative of competing strengthening and weakening effects from adding Ni. The alloy with intermediate Ni content (3.2 wt.%) is selected for further microstructural and mechanical characterization. It contains four coarsening-resistant strengthening constituents: (i) micron-scale Al11Ce3 and (ii) Al27Ce3Ni6 or Al9(Ni,Mn,Fe)2 platelets, all formed during eutectic solidification, (iii) L12-Al3(Sc,Zr) nanoprecipitates formed on aging, and (iv) Mn atoms in solid solution in the α-Al matrix. The combined strengthening mechanisms impart high strength at ambient and elevated temperatures, as measured by microhardness (as a function of aging time), by compression and tensile experiments, and by compressive creep measurements. Alloys previously reported in the literature - with various combinations of one, two or three of these four strengthening phases - show lower hardness and creep resistance, indicating that cumulative strengthening can be achieved when combining mechanisms; the present alloy containing all four phases shows a remarkably high creep threshold stress of 62 MPa at 300 °C.

Cyclic fatigue behavior of Y‐TZP nanostructures with non‐homogeneous yttria distribution

AbstractNanostructured Y‐TZP samples with Non‐Homogeneous Yttria Distribution (NNHYD) were prepared by mixing co‐precipitated 3Y‐TZP powder (Tosoh TZ‐3Y) with yttria‐free monoclinic powder (Tosoh TZ‐0), yielding an overall yttria content of 2 mol%. The cyclic fatigue behavior of NNHYD was evaluated by Step‐Stress Accelerated Life Testing (SSALT) and mechanical cycling in a biaxial flexural strength test (BFST) setup and compared to Conventional Submicrometric 3Y‐TZP (CS). Physical, structural, microstructural, and mechanical characterizations of all groups were also evaluated and compared before and after artificial aging in autoclave. NNHYD presented a higher capacity for damage accumulation and maintenance of phase transformation potential than CS, associated with elevated resistance to the propagation of Low‐Temperature Degradation (LTD). A nanostructured 3Y‐TZP surface layer introduced in NNHYD by dip coating (DC‐NNHYD) improved resistance to LTD at the cost of reduced reliability under cyclic fatigue by increasing the stability of the tetragonal phase. This surface alteration highlights the impact of surface properties on mechanical performance. The results of this study present practical possibilities to help achieve the ideal balance between strength, toughness, and LTD resistance to improve the lifetime of zirconia‐based biomaterials.

Investigation on microstructural and mechanical integrity of GTAW dissimilar welded joint of IN 718/ASS 304L using Ni-based filler IN 625 using EBSD and DHD techniques

The current study investigates the relationship between the microstructural and structural characteristics of dissimilar gas tungsten arc welding (GTAW) between Inconel 718 (IN 718) and austenitic stainless steel 304L (ASS 304L) using Inconel 625 (IN 625) as a filler material. The microstructural characterization of the dissimilar weld includes the application of an optical microscope (OM), field emission scanning electron microscopy (FE-SEM) including energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The optical and FESEM analysis of the base metals (BM) indicates the austenitic nature of the IN 718 BM matrix, which incorporates strengthening precipitates γ′ and γ'' distributed throughout the Nickel matrix. In contrast, the ASS 304L BM exhibits an austenitic microstructure along with twins. Moreover, IN 625 weld metal exhibits austenitic microstructure in the form of cellular, columnar, and equiaxed dendrites. Elemental segregation demonstrates the existence of Nb, and Mo-rich carbides, including the laves phases at the interdendritic regions of the IN 625 weld zone and IN 718 Heat-Affected Zone (HAZ). Ferrite stringers were noticed in the HAZ of ASS 304L. EBSD analysis shows the improved texture of the weld zone exhibiting fine equiaxed recrystallized microstructures. EBSD/Kernel average misorientation (KAM) micrographs reveal the existence of high strain at the weld metal grain boundaries. Vickers microhardness assessment of the dissimilar weld demonstrates the high hardness of 257.5 HV for the IN 625 weld zone because of the occurred NbC, laves phases, and Mo-rich carbides. Room temperature tensile characterization shows the failure of tensile specimens from ASS 304L BM reflecting the high strength of the IN 625 weld metal (662 MPa). Tensile tests at high temperatures were performed at 550 °C, 600 °C, and 650 °C. High-temperature tensile result exhibits a higher tensile strength of IN 625 weld zone than the ASS 304L BM, as the tensile specimens fails from 304L BM. The specimen exposed at 550 °C exhibits the highest tensile strength of 388 MPa. The Charpy impact toughness test was conducted, with notches at the HAZ of ASS 304L, HAZ of IN 718, and the weld center being evaluated. The toughness value of the weld metal decreases (99 J) due to occurrence of the secondary precipitates. To analyze residual stress, the deep hole drilling (DHD) technique was used, which revealed that the peak residual stress (230 MPa) is induced at 6 mm from the weld surface and falls into the tensile stress category.

An approach for identifying the deformation-induced strain from tensile tests and x-ray diffraction measurements

ABSTRACT Non-uniform plastic deformation and phase transformation introduce residual stress (RS) in a material. The present study focuses on the influence of tensile strain on RS development in 304L austenitic stainless steel, wherein strain-induced martensite (SIM) transformation influences deformation behaviour. Room temperature tensile tests were interrupted at various tensile strains between the material yield and ultimate tensile strength at 5.21 × 10−4 and 1.3 × 10−2 s−1 strain rates. Microstructural characterisation of as-received and deformed samples by electron backscatter diffraction provides evidence of SIM formation and shear bands. Analysis of tensile flow and work hardening behaviour is discussed in correlation to the microstructural and dislocation density variations. In addition, RS in the austenite and martensite phases was measured using the x-ray diffraction method, and the RS balance that occurs between the longitudinal and transverse components and the two phases after tensile deformation is presented. It is shown that a combined analysis of RS and x-ray diffraction peak width could be utilised to find the deformation level in tensile fractured, press brake bent, and roll bent samples.

Microstructural Characterization, Mechanical Performance, and Anti-Corrosive Response of Zinc Multifaceted Coating on Mild Steel

Zinc has attracted significant attention in research due to its cost-effective use as an electrodeposited material, effectively protecting various types of steel from corrosion and wear. However, despite its advantages, zinc has limitations in fully guarding steel against corrosion. Recent studies propose that blending zinc with other metals during the coating process can proficiently shield mild steel from deterioration. The motivation for this study stems from recognizing the restrictions of zinc electrodeposition and the limited exploration of zinc multi-facet composite coatings for mild steel. In this study, the electrodeposition technique was employed to apply a coating to mild steel using zinc and nanoparticles of calcium oxide (CaO) and manganese oxide (MnO2). The coating bath's chemical composition included mass variations of 0-12 g/L for CaO and MnO2, along with 10 g/L each of boric acid, thiourea, and Na2SO4, and 15 g/L of K2SO4 and ZnSO4. The coating process occurred over a twenty-minute period, with a pH of 4.8, voltage set at 3.2V, current density at 1 A/cm2, temperature at 47°C, and stirring rate at 200 rpm. Results obtained from the coated mild steel demonstrated that Zn-6CaO-6MnO2 exhibited the greatest coating thickness at 0.2308 mm, and it showcased impressive corrosion resistance at 2.0618 mm/year. The Zn-CaO-MnO2 coating displayed a substantial deposit of crystallites in its microstructure, assisted by the presence of manganese, contributing to a smoother surface texture.

Sequential hybridisation of wire and powder-based additive manufacturing of Inconel 718: Mechanical and microstructural characterization

The present study combines the Wire-based DED (W-DED) and Powder-based DED (P-DED) to achieve a high deposition rate and higher feature resolution, respectively, within the single component. The research puts forward a novel Wire-Power (WP) Hybrid DED process, which is realized by sequential deposition of feedstock in Wire and then in Powder form. Based on the deposition-extraction combination, three sample configurations, C1 (Y-X), C2 (X-X) and C3 (Y-Z), were fabricated and characterized for the mechanical properties and microstructural aspects. OM images revealed defect-free P-DED and W-DED interface, while the EBSD analysis showed grain size variations owing to differences in the cooling rates. The Ultimate Tensile Strength (UTS) values of C1 and C2 configurations are about 132.2 and 139.7 % higher in comparison to C3. Low cycle fatigue results showed that the C2 sustained a higher number of completely reversed cycles to failure in comparison to the other configurations. The impact energy absorbed by C3 is the highest, affirming the strong W–P interface.

Interfacial structure and formation mechanism of 90 W-7Ni-3Fe/steel using co-sintering method

In this study, we proposed a powder metallurgy method to develop an interlocking interface in tungsten heavy alloy/steel composites, which is achieved by co-sintering technology using tungsten heavy alloy and steel powders with a large difference in particle size. The interfacial microstructure evolution and mechanical properties of the co-sintered joint as a function of co-sintering temperature were investigated systematically. Results showed that a well-bonded interlocking interface was formed along the steel particles as a result of the tungsten heavy alloy particles filling in the gaps between two neighboring steel particles. The co-sintered joint contains four regions: the W alloy matrix, the Fe3W3C layer, the Ni-affected layer, and the steel matrix. Based on the microstructural characterization, the interface formation mechanism and reaction layer growth behavior are discussed in depth. Benefitting from the interlocking structure, the highest tensile strength of the co-sintered specimen is 265 MPa, which is 66% higher than that of the non-interlayer joint obtained by diffusion bonding method. Moreover, its performance is better than that of some W alloy/steel with an interlayer. This technique offers a potential method for the efficient preparation of a W alloy/steel composite structure with outstanding properties.

Advanced microstructural characterization in high-strength steels via machine learning-enhanced high-speed nanoindentation and EBSD mapping

This research investigates the nanoscopic features of Advanced High-Strength Steels (AHSS) through a bottom-up approach employing high-speed nanoindentation mapping (HSNM) to elucidate structure-property relationships. The influence of grain boundaries on nanomechanical properties was documented, highlighting the challenge of SEM-EBSD analysis in differentiating phases with identical crystal structures (BCC, FCC, etc.). Integrating SEM-EBSD with HSNM in the same region of interest is essential for detailed insights into phase/microstructure distribution and accurate grain boundary identification. A modular four-step analysis protocol, designed and validated on ferritic-bainitic TRIP steels (TBF), leverages machine learning-enhanced HSNM for significant advancements in AHSS design. The initial phase involves the application of the expectation-maximization algorithm for probability distribution fitting of HSNM data, deriving primary mechanical phase statistics. This exclusively facilitates the correlation of elastic modulus and hardness for each phase/microstructure using nanoindentation data. Further refinement of phase/microstructure to mechanical property correlations was achieved through a supervised machine learning approach, ensuring precise association between EBSD and nanoindentation data. This includes detailed image analysis and clustering of nanoindentation data, enhancing the precision in phase recognition. This methodology addresses the critical challenges in developing 3rd Generation AHSS, aiming to fill the gap in accurately identifying and quantifying phases such as martensite, austenite, bainite, and ferrite, thereby reducing classification and measurement uncertainties. The approach contributes to the fundamental understanding of AHSS microstructures and provides a scalable framework for the comprehensive characterization of structural materials.

Microstructural Characterization of Alluvial Sand Containing Cohesive Soil Lumps During Loading and Inundating

Microstructural Characterization of Alluvial Sand Containing Cohesive Soil Lumps During Loading and Inundating

Polymer solid electrolytes with ultra-stable cycles and high-capacity retention for all-solid-state Li-metal battery

Polymer electrolytes have become widely popular on account of their low expenses, reduced weight, simple processing, and excellent safety features. However, polymer electrolytes have yet to find use in commercial batteries because of their limited mechanical durability and poor ionic conductivity. In this paper, an ultra-stable cycles and interfaces blending polymer electrolyte (BPE) suitable for all solid-state Li metal batteries was prepared by blending three polymers: polymethyl methacrylate (PMMA), polyvinylidene fluoride hexafluoropropylene (PVDF-HFP), and polyethylene oxide (PEO). Microstructure characterization showed numerous amorphous areas with strong interfacial strength of the BPE. The BPE3 exhibited a maximum of 15.65 MPa in tensile strength. BPE2 have excellent electrochemical performance, such as wide electrochemical windows (up to 4.7 V vs Li/Li+), high lithium ion transference numbers (up to 0.75), high ion conductivity (up to 1.87 × 10-4 S cm−1 at 60 °C), and excellent interface stability. By assembling all state-solid LiFePO4/BPE2/Li batteries for electrochemical testing, BPE2 exhibit high electrochemical characteristics. BPE2 discharge specific capacity is as high as 154.7 mAh g−1 at 1C. The BPE2 underwent 200 cycles at 0.5C and 400 cycles at 1C, with capacity retention rates of 95.7 % and 96.6 %, respectively. BPE is a promising electrolyte candidate for flexible lithium metal batteries.

Microstructure and Phase Characterization of Alkali-Activated Slag–Fly Ash Materials with Tetrasodium of 1-Hydroxy Ethylidene-1, 1-Diphosphonic Acid (HEDP·4Na)

The effect of tetrasodium of 1-hydroxy ethylidene-1, 1-diphosphonic acid (HEDP·4Na) on the microstructure and phase characterization of alkali-activated fly ash–slag (AAFS) materials is not clear or well documented. In this study, XRD, DTG, TAM-air, and SEM analyses of AAFS were used to identify the microstructural changes in AAFS made with HEDP·4Na. Meanwhile, the workability and compressive strength of AAFS were evaluated. The results demonstrated that the early-age alkaline-activated reactions were retarded due to the addition of HEDP·4Na in the AAFS mixture. However, the degree of gel formation was relatively increased at a later age in the AAFS made with HEDP·4Na compared to the plain AAFS mixture. Additionally, in comparison to the control group, the incorporation of HEDP·4Na in AAFS specimens resulted in improved flowability, with increments of 5%, 15%, and 24% for concentrations of 0.1%, 0.2%, and 0.3%, respectively. The initial and final setting times were prolonged by 5% to 50%, indicating a beneficial impact on the rheological properties of the AAFS fresh mixture. Furthermore, the addition of HEDP·4Na led to an improvement in compressive strength in the AAFS mixtures, with enhancements ranging from 13% to 16% at 28 days compared to the control group. With the presence of HEDP·4Na, the increase in the degree of reactions shifted to the formation of gel phases, like C-S-H, through the combined measurement of TGA, XRD, and SEM, resulting in a denser microstructure in the AAFS matrix. This study presents novel insights into the intricate compatibility between the properties of AAFS mixtures and HEDP·4Na, facilitating a more profound comprehension of the potential improvements in the sustainable development of AAFS systems.

Data-driven composition design and property optimization of solid solution and precipitation simultaneously strengthened non-oriented silicon steel

Limitations on data volume and quality are key bottlenecks in using machine learning for property prediction and property-oriented composition design of non-oriented silicon steel. In response, this study employed a Gaussian Mixture Model (GMM) to generate virtual samples for enhancing the in-house experimental data, by which the generated virtual data well captured the distribution of the raw experimental data. As a result, compared with the model without data augmentation, the enhanced prediction model (composition → property) fitted by virtual samples improved its R2 value from 0.52 to 0.86. Based on this model, a multi-properties-oriented (yield strength σs, magnetic induction B50, and iron loss P10/400) composition prediction model (property → composition) was established to rapidly discover high-performance non-oriented silicon steel. Experimental characterization and theory calculation of the explored candidate alloys exhibited that their yield strength was above 750 MPa, which primarily results from solid solution strengthening (50 %) and precipitation strengthening (36 %). Moreover, the alloys possessed magnetic induction B50 of over 1.72 T and low core losses with P10/400 of 12.1 W/kg (0.2 mm) and 17.0 W/kg (0.35 mm). Further microstructural characterization exhibited that such satisfactory performance is associated with copper (Cu)-rich nanoprecipitates that dramatically improved the yield strength without deteriorating the magnetic properties.

Effect of Alloying Elements on the High-Temperature Yielding Behavior of Multicomponent γ'-L12 Alloys.

The exceptional mechanical properties of Ni-based high entropy alloys are due to the presence of ordered L12 (γ') precipitates embedded within a disordered matrix phase. While the strengthening contribution of the γ' phase is generally accepted, there is no consensus on the precise contribution of the individual strengthening mechanisms to the overall strength. In addition, changes in alloy composition influence several different mechanisms, making the assessment of alloying conditions complex. Multicomponent L12-ordered single-phase alloys were systematically developed with the aid of CALPHAD thermodynamic calculations. The alloying elements Co, Cr, Ti, and Nb were chosen to complexify the Ni3Al structure. The existence of the γ' single phase was validated by microstructure characterization and phase identification. A high-temperature compression test from 500 °C to 1000 °C revealed a positive temperature dependence of strength before reaching the peak strength in the studied alloys NiCoCrAl, NiCoCrAlTi, and NiCoCrAlNb. Ti and Nb alloying addition significantly enhanced the high-temperature yield strengths before the peak temperature. The yield strength was modeled by summing the individual effects of solid solution strengthening, grain boundary strengthening, order strengthening, and cross-slip-induced strengthening. Cross-slip-induced strengthening was shown to be the key contributor to the high-temperature strength enhancement.

Application of Intercritical Austenitizing in a Commercial CMnSi Steel as a Tool for Optimizing a One‐Step Quenching and Partitioning Cycle Aiming to Achieve a Third‐Generation Advanced High‐Strength Steel

Advanced high‐strength steels (AHSS) have been extensively studied to develop its third generation from relatively low‐cost alloys, but seeking optimized mechanical properties. In this context, the effects of quenching heat treatments are evaluated in this work after intercritical austenitizing (IA) on microstructure, alloying elements partitioning, and Ms temperature for a commercial CMnSi alloy. Applying dilatometric data and microstructural characterization, different heat cycles are evaluated. The obtained experimental results are compared with those ones obtained by computer simulations at equilibrium conditions. Based on this characterization, an optimized one‐step quenching and partitioning (Q&amp;P) cycle (after IA) is proposed and applied to tensile test specimens. Then, the specimen microstructure is characterized by scanning and transmission electron microscopy as well as by X‐ray diffraction. Using the proposed methodology, it is possible to produce a Q&amp;P steel with a product of tensile strength and elongation of 29.5 GPa% and a 7% volumetric fraction of retained austenite. It is concluded that the precise knowledge about the IA effects on the phase transformations and microstructure evolution in a commercial CMnSi alloy has great potential to be applied in defining a one‐step Q&amp;P heat treatment that favors a multiphase microstructure, which meets mechanical properties requirements for a third‐generation AHSS.