Fine Microstructure Research Articles

Effects of Quasi-Static Strain Rate and Temperature on the Microstructural Features of Post-Processed Microstructures of Laser Powder Bed Fusion Ti6Al4V Alloy

This study documents an investigation of the flow stress properties and microstructural features of Ti6Al4V (ELI) alloy produced using laser powder bed fusion (LPBF). Selected heat treatment strategies were applied to the material to obtain different microstructures. The influence of quasi-static strain rates and temperature on the obtained microstructures of this material and their strain hardening properties are documented in this study. All microstructures of the alloy formed in this study were found to be sensitive to quasi-static strain rates and temperatures, where their flow stresses increased with increasing strain rate and decreased for tests conducted at elevated temperatures. The strain hardening rates of the fine microstructures were found to be high compared to those of coarse microstructures. The strain hardening rates for the various forms of LPBF Ti6Al4V (ELI) examined here were found to diminish with increasing test temperature. Though the deformed surfaces of the built samples were largely dominated by adiabatic shear bands (ASBs), the absence of ASBs was noted for all samples tested at a temperature of 500 °C and an imposed strain of 30%.

Synergistic effect of Nb and Mo on the microstructural formation of the Ti(C,N)-high chromium ferrous-based cermets

In this study, Ti(C,N)-Fe-based green cermets with different metallic alloying elements have been consolidated by pressureless liquid-phase sintering. The addition of different metallic binders on Ti(C,N)-based cermet such as Nb and Mo on high chromium Ferrous based binder has been investigated. Detailed analysis of the phase constitution was conducted using thermodynamic calculations and experiments, as well as a systematic study of the microstructure evolution and room temperature mechanical properties including hardness and fracture toughness was conducted. The Nb and Mo addition to the binder system affects the sintering temperatures and can significantly affect the phase formation and microstructural development. Scanning electron microscope (SEM) and Energy dispersive spectroscopy (EDS) technique were used to examine the microstructure, composition, and fracture surface of cermets. The addition of the Mo, and Nb reveals lower porosity and finer microstructure as compared to the reference material (Ti(C,N)-Fe-Cr). The refinement of microstructure improves mechanical properties such as hardness and fracture toughness of Ti(C,N)-Fe-Cr-Mo-Nb-based cermets. Further, the addition of these binder elements may reduce the formation of Fe-Cr-based intermetallic complex carbides, allowing cermets to perform better in terms of toughness and corrosion resistance. As a result of the experiments, it is evident that Nb and Mo dissolve in Ti(C,N) and form solid solutions during sintering. The increased number of coreless grains, spinodal decomposition, and crack deflection in cermet further enhance the fracture toughness.

Achieving highly variable mechanical behaviors in TiBw/TA15(Si) composite by microstructure manipulation

Discontinuously reinforced titanium matrix composites (DRTMCs) are competitive candidates for aero-space applications. However, the high processing temperature of DRTMCs often yields fully lamellar microstructures, making their properties hard to be tailored by heat treatment. In our previous work, a TiBw/TA15(Si) composite with fine microstructure was fabricated, showing a good potential for microstructure and mechanical properties manipulation. In the present work, heat treatments of different conditions were conducted to this TiBw/TA15(Si) composite. Controllable microstructures from bimodal to fully laminated, and varied mechanical properties, with the ultimate strength ranging from 1136 MPa to 1347 MPa and the elongation ranging from 6.7% to 12.9%, were obtained. The softening induced by microstructure coarsening was balanced by the strengthening of the lath microstructure, and the deformation behaviors of heat-treated composites were determined to be influenced by the strain softening and the constraint of Burgers orientation relationship in the lath microstructure. The present work provided an insight into the deformation and strengthening mechanism of TiBw/TA15(Si) composite, and revealed the possibility of DRTMCs to fulfill the requirements from different service conditions without changing the chemical composition and fabrication procedures.

Influence of Equal Channel Angular Pressing Technique on Mechanical Properties, Corrosion Resistance, and Microstructural Behavior of Al-Alloy 5083

Introduction: During severe plastic deformation (SPD) processing, aluminum alloys exhibit moderate strength and ductility. Nevertheless, the materials' ductility, as determined by tensile testing, does not accurately represent their capacity for plastic deformation. After the tensile test, its material, aluminum alloy 5083, was observed to be super ductile. Method: The results of a more thorough plasticity analysis—applied for the first time to material following SPD processing—are presented in this paper. The aluminum alloy 5083 is examined in the patent both before and after equal channel angular pressing (ECAP). A highly effective aluminum alloy 5083 specimen preparation was suggested. Under heated temperatures, the process involves severe plastic deformation. Optical microscopy and scanning electron microscopy (SEM) were used for micro-structural research in order to look at the distribution of second-phase particles and the evolution of grain structure. Furthermore, Vickers micro-hardness testing was utilized to assess the mechanical characteristics of the alloy after processing. The outcomes showed that after ECAP processing, there was a significant decrease in grain size and an increase in micro-hardness. Result: Additionally, the production of a finer microstructure with a more uniform distribution of strengthening precipitates was clarified by electron microscopy (SEM). The micro-structural evolution and mechanical behavior of aluminum alloy 5083 under ECAP are well-explained in this patent, which may lead to improved performance in structural applications. This method can be used to forecast fractures in plastic deformation processes and estimate the final plasticity of the materials for various stress-strain states after ECAP processing, as well as the mechanical properties discussed. Conclusion: Due to its unique mechanical qualities, aluminum alloy 5083 shows great potential in a variety of structural applications. The effects of Equal Channel Angular Pressing (ECAP) on the hardness and microstructure of aluminum alloy 5083 were examined in this patent work. result: The specimen's mechanical characteristics are at their finest following a single deformation when the temperature reaches 300°C. Compared to the original specimen, the tensile strength observed enhanced after the fourth pass, and breaking elongation (28.3%) was improved. Through second-phase strengthening and fine-tuning, the recrystallized grains and second-phase particles increase the material's strength.

Silver and silicon doped βTCP scaffolds with gentamicin or ceftazidime loaded P(3HB) coatings as multifunctional biomaterials for bone regeneration

The risk of bacterial infections is a significant challenge faced frequently in the use of implants or scaffolds for bone regeneration. Therefore, this study focusses on the development and characterisation of novel β tricalcium phosphate (βTCP) scaffolds co-doped with silver and silicon, along with composites coated with antibiotic-loaded poly(3-hydroxybutyrate) (P(3HB)) layers. The successful incorporation of silver and silicon dopants while maintaining the formation of βTCP phase was confirmed using X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) analysis. The developed materials demonstrated comparable total and open porosity (∼64–70 vol%), suggesting high interconnectivity between pores conducive to nutrient transport and tissue repair. The increase in compressive strength was achieved for both doped (4.73 ± 0.79 MPa) and P(3HB) coated (5.79 ± 0.92 MPa) scaffolds due to the fine bioceramic microstructure and polymeric coating. Silver and silicon-modified βTCP demonstrated enhanced growth inhibition of Gram-negative (Pseudomonas aeruginosa, Escherichia coli) and Gram-positive (Staphylococcus aureus) bacterial strains in vitro compared to the pure βTCP. P(3HB) coatings, enriched with gentamicin or ceftazidime, exhibited burst and sustained release of the antibiotics from the scaffolds up to 120 h further intensifying the bacteria-killing capability, with evident inhibition zones observed in vitro. Moreover, the composites exhibited apatite-forming ability, suggesting their bioactive potential. In vivo evaluation using Caenorhabditis elegans demonstrated the lack of toxicity of the tested materials. The simultaneous incorporation of the dopants and antibiotic-loaded P(3HB) coatings not only offer a dual antibacterial approach but may also facilitate bone regeneration. However, further in vitro and in vivo investigations are needed to assess their potential in clinical application.

Micro and nanostructured carbon-phenolic ablators modified by PVP addition

Carbon-phenolic ablators can efficiently protect space vehicles from the extreme temperatures typical of the reentry phase in a planet’s atmosphere. Their performances are attributed to the low thermal conductivity and to the decomposition of the phenolic resin. These phenomena are strongly influenced by the materials microstructure. In the present work, standard and polyvinylpyrrolidone (PVP)-modified carbon-phenolic ablators were manufactured and characterized: the influence of PVP on the final microstructure and chemistry of the ablators was studied through SEM-EDS, X-ray Microscopy (XRM) analysis and FTIR technique; the mechanical properties were evaluated through compression tests on virgin and charred samples while ablative performance of the ablators were evaluated with an oxyacetylene flame exposure test. Weak bonds between phenolic resin chains and PVP were observed. The microstructure of the ablators, both before and after the exposure to the oxyacetylene flame, is strongly influenced by the PVP addition, furthermore the addition of 10 and 20 %wt of PVP can guarantee a reduction of about 30 % of the back temperature during the oxyacetylene flame test with respect to the standard carbon-phenolic ablator. Compression tests on the manufactured ablators enlighten also an improvement in the mechanical properties for PVP-enriched ablators, in particular considering their charred state.

Effect of martensitic variant selection on the crystallographic features of bainite/martensite multiphase structure of 42CrMo steel

The multiphase structure of bainite and martensite with different proportions was obtained in 42CrMo steel by controlling the isothermal temperature and isothermal time, and the mechanism of martensitic variant selection on the crystallographic features of materials was studied. The results show that the final microstructure after austempering is a multiphase structure of bainite and martensite, which conforms to the K–S orientation relationship with the parent phase austenite. Under the redistribution of carbon elements and plastic confinement effect, the martensite formed by the post-transformed austenite has stronger variant selectivity than bainite. As the isothermal temperature decreases, the dislocation density of the material increases, which improves the strength of the material. At the same time, the increase of phase transformation driving force and austenite strength weakens the variant selection, resulting in the decrease of grain size and the increase of high angle grain boundaries (HAGBs), further improving the strength and plasticity of the material. At a shorter isothermal time, more martensite with stronger variant selectivity reduces HAGBs and decreases the plasticity of the material. Finally, the best matching of strength and plasticity was obtained under the heat treatment process of isothermal temperature and 30 min isothermal time. This study provides a theoretical basis for further improving the product of strength and elongation of 42CrMo multiphase steel.

Investigation of segregation behaviour in 2060 aluminium-lithium alloy during twin-roll casting

The utilization of the twin-roll casting process to fabricate aluminium (Al) - lithium (Li) alloys is regarded as an efficient and energy-saving strategy to meet the aerospace industry’s demands for reducing processing steps, cost-effectiveness, and corrosion resistance. This study successfully achieved the preparation of Al-Li alloy twin-roll cast slabs across various processing ranges to investigate the corrosion resistance differences in different processing conditions. Remarkable macro segregation phenomena were observed in the fabrication under high twin-roll casting speeds for TRC-3. By combining temperature and flow rate calculations, we comprehensively analysed the results of this segregation mechanism. The segregation is inherited into the final T6 state microstructure, exacerbating the precipitation of the T1 phases and deteriorating the ultimate corrosion resistance performance. We delved into a profound discussion regarding the mechanistic impact of segregation on corrosion behaviour. Ultimately, this study postulates a process parameter range for twin-roll casting of Al-Li alloys, aimed at attaining minimal segregation and maximal corrosion resistance.

Design of heat-treatment and its effect on Ni-rich matrix of a W-Ni-Co alloy

The aim of this investigation is to optimize the heat-treatment for tailoring a suitable microstructure, especially that of the matrix phase in a W-Ni-Co alloy. Different microstructural features that have been focused on are grain size, fine W particles, intermetallics and short-range ordering. 92W-5.3Ni-2.7Co alloy prepared through powder metallurgy route was imparted a cyclic heat-treatment to obtain a microstructure comprising finely distributed tungsten particles in an FCC matrix that envelopes the large tungsten grains. While the fine W particle influences the mechanical behaviour of W-Ni-Co alloys, the grain size of the matrix and short-range ordering are also expected to play a significant role. The effect of different heat-treatments on the microstructure of the matrix was studied using transmission electron microscope (TEM) and electron back scatter diffraction (EBSD). It is observed that increasing the quenching rate diminishes short-range ordering in the matrix. The formation of intermetallic phase at intermediate temperature range (between 800 °Cand 900 °C) and its subsequent dissolution at higher temperature influences the final microstructure of the alloy. In the present study, the effect of the intermetallic phase on the formation of randomly orientated fine equiaxed grains in the matrix has been elucidated.

Modeling the effect of calendering pressure on the microstructure and properties of Li-ion battery electrodes

The microstructure changes of lithium-ion batteries significantly influence the performance, with calendaring being a crucial manufacturing step linked to the final microstructure. However, the current adjustments of compression parameters are empirical. This work aims to explain the impacts of calendaring force on the microstructure and electrical characteristics by a comprehensive model combining Discrete Element Method (DEM) and Finite Element Analysis (FEA), giving a validated model. A worthy correspondence between simulation and experimental findings was achieved. This work offers a validated combined model, intertwining the microstructure and electrochemical performance, furnishing a numerical simulation foundation for optimizing calendaring parameters for the electrode.

Rapid L-PBF printing of IN718 coupled with HIP-quench: A novel approach to manufacture and heat treatment of a nickel-based alloy

There has been a growing interest in additive manufacturing in recent years, with researchers actively working on improving component quality through adjustments to printing parameters, lead time, and productivity. Rapid L-PBF printing has emerged as an attractive approach to expedite component manufacturing. However, employing a rapid building strategy may lead to additional internal flaws, which can be addressed during a Hot Isostatic Pressing (HIP) cycle. In this study, Inconel 718 was L-PBF printed using two distinct strategies: one involving the creation of a dense 1 mm shell with loosely packed powders in the core and the other utilizing different sets of printing parameters for the shell and core, respectively. These strategies resulted in a 60% and 45% printing time reduction, respectively, compared with the time requirement for printing same-size cubic samples with optimized parameters. Additionally, full densification and porosity elimination were achieved through a HIP-quench approach, obviating the need for further heat treatment. The study presents the final microstructures and retained flaws, along with assessing the degree of recrystallization via EBSD analysis and evaluating mechanical properties using hardness measurements and compression tests.

Effect of Hot Isostatic Pressing on the Stellite 6 Alloy prepared by Directed Energy Deposition

The directed energy deposited (DED) alloys show higher hardness values than the welded alloys due to the finer microstructure following the high cooling rate. However, defects such as microcracks, pores, and the residual stress are remained within the DED alloy. These defects deteriorate the wear behavior so post-processing such as heat treatment and hot isostatic pressing (HIP) are applied to DED alloys to reduce the defects. HIP was chosen in this study because the high pressure and temperature uniformly reduced the defects. The HIP is processed at 1150oC under 100 MPa for 4 hours. After HIP, microcracks are disappeared and porosity is reduced by 86.9%. Carbides are spherodized due to the interdiffusion of Cr and C between the dendrite and interdendrite region. After HIP, the nanohardness (GPa) of carbides increased from 11.1 to 12, and the Co matrix decreased from 8.8 to 7.9. Vickers hardness (HV) decreased by 18.9 % after HIP. The dislocation density (10-2/m2) decreased from 7.34 to 0.34 and the residual stress (MPa) changed from tensile 79 to a compressive -246 by HIP. This study indicates that HIP is effective in reducing defects, and the HIP DED Stellite 6 exhibits a higher HV than welded Stellite 6.

Mechanisms of microstructural evolution in high-volume TiC reinforced steel composites: Insights into particle coarsening, morphology, and thermochemical reactions

Titanium carbides reinforced steel matrix (TiC-steel) composites exhibit a range of excellent properties, including increased strength-to-weight ratio, high-temperature strength, and outstanding wear resistance. Prior research has demonstrated that the mechanical properties of TiC-steel composites are significantly affected by particle coarsening and the associated core-rim structure. Nevertheless, the fundamental mechanisms of thermochemical reactions that govern the formation of the final microstructure in these composites, including the core-rim structure, remain incompletely explored. This study elucidates the microstructure evolution mechanism of high-volume TiC-steel composites, particularly particle coarsening, through thermodynamic calculations and chemical analysis. The investigation unveils the coarsening process, shedding light on the formation of more stable phases and associated compositional changes, including the incorporation of substitutional elements and carbon depletion (resulting in low stoichiometry, x < 1) within the newly developed (Ti,M)Cx. Additionally, the increased carbon content contributes to pearlite emergence within the steel matrix. Notably, key factors influencing the distinct morphologies observed in growing TiC particles are identified, attributed to variations in Mo, V, and W concentrations, as elucidated by rigorous thermodynamic and first-principles calculations. This study represents significant progress in understanding the intricate microstructure development of high-volume TiC-steel composites.

Elucidating nickel oxide reduction in a Ni-YSZ solid oxide cell via in-situ X-ray nano holo-tomography

Reducing nickel oxide in solid oxide cell (SOC) electrodes is a critical step during the early stages of cell operation. It is considered crucial in shaping the final microstructure of the fuel electrode. In this work, we investigate the NiO reduction process in a state-of-the-art Solid Oxide Cell (SOC) using in-situ X-ray nano-holo-tomography. Results show a fast reaction kinetics and a reaction front from the outer to the inner regions of the sample. NiO reduction is complete in the first few seconds and the metallic Ni particles present a sponge-like structure, with many nanocrystallites and internal nano porosity. After reduction, the Ni network undergoes coarsening, increasing its particle size. Based on the acquired data, we propose a new chain of reaction mechanisms for forming the Ni nanocrystallites observed after reduction.

Effects of h-BN content and heat treatment temperature on the tribological behavior and toughness of laser-clad Mo2FeB2/WC/h-BN coatings

In this study, h-BN-modified Mo2FeB2/WC composite coatings were fabricated via laser cladding. The Mo2FeB2/WC/10 wt% h-BN composite coatings were heat-treated at 400, 600, and 800 °C. The microstructure, microhardness, tribological properties, residual stress, fracture toughness and fracture morphology of the coatings were investigated. The results showed that h-BN addition combined with heat treatment promoted the distribution of elements and finer microstructure. The Mo2FeB2/WC/10 wt% h-BN coating heat-treated at 600 °C showed the highest microhardness (1416.8 HV0.5) among the prepared coatings. The pristine and 600 °C heat-treated Mo2FeB2/WC/10 wt% h-BN coating showed excellent wear rates (1.97 and 1.84 × 10−5 mm3/Nm, respectively). The composite coatings modified with h-BN exhibited an abrasive wear mechanism after heat treatment. As the h-BN content and heat treatment temperature increased, there was a notable decrease in residual stress. Specifically, the coating subjected to heat treatment at 800 °C demonstrated the lowest residual tensile stress recorded at 168 MPa. Conversely, when heat-treated at 600 °C, the coating exhibited an optimal fracture toughness of 14.96 MPa⋅m1/2, with fracture behavior consistent with typical brittle fracture characteristics.

Microstructure and fracture toughness of 850°C quenched and aged Zr-2.5Nb alloy

The Zr-2.5Nb alloy pressure tubes manufactured by Water Quenched and Aged (WQA) fabrication route has reportedly exhibited superior resistance against in-reactor dimensional changes. WQA route comprises three consecutive thermo-mechanical treatments viz., solution heat treatment (SHT), cold working and vacuum aging, which govern the final microstructure and therefore fracture toughness of the pressure tube material. This work describes the influence of SHT soaking duration, degree of cold working and aging temperature on the microstructure and fracture toughness of WQA Zr-2.5Nb alloy. WQA pressure tubes are expected to have better resistance to in-reactor dimensional changes. However, if proper care of hydrogen absorption during thermo-mechanical processing is not taken, though resistance to creep will improve expectedly but fracture toughness at ambient temperature will deteriorate.

Evolution of microstructure and texture and their influence on the strength of an accumulative roll bonded (ARBed) Al-Sc-Zr-Er-Ti alloy

Accumulative roll bonding (ARB) is an effective approach to improve the strength of metal sheets. This work studied the evolution of microstructure and texture and their influence on the strength of an ARBed Al-Sc-Zr-Er-Ti alloy, produced by multi-microalloying Sc, Zr, Er, and Ti into pure Al. A high-quality interface and a fine microstructure with an average grain size of ∼0.6 μm were achieved as by the ARB process. The Dillamore {4411} <11118> and the Copper {112} <111> components were sharpened in final texture as is common in ARB. However, after the 5th cycle, there was a new texture component along the α-fiber, at (φ1, Φ, φ2) ≡ (60°, 45°, 0°) in Euler space. The yield strength (YS) and tensile strength (UTS) increased with increasing the number of ARB cycles. The 7th ARBed Al-Sc-Zr-Er-Ti alloy had increased YS by 45% and UTS by 57%, respectively, compared to the initial cold rolled sheet. Strengthening was mainly attributed to dislocation strengthening, grain refinement strengthening and precipitation strengthening. There was little texture strengthening because of a strengthening and softening alternation with the change of ARB cycle.

Effects of two-step austempering processes on the wear resistance and fatigue lifetime of high-carbon nanostructured bainitic steel

The previous research on two-step austempering processes have been mainly focused on the bainitic transformation kinetics, with few details the relationships between the parameters of two-step austempering process and the wear properties and fatigue lifetime. In this paper, the effects of two-step austempering process on wear resistance and fatigue life of high carbon nano-bainite steel were investigated. The results show that the first-step bainite with high volume fraction prolongs the total bainite transformation time. The two-step austempering samples contain finer bainitic ferrite plate and higher volume fraction of retained austenite, as well as resent excellent hardnesses, tensile properties, wear resistance and fatigue lifetime. In particular, the wear resistance of the 50%-290 °C sample is better than that of the 210 °C sample, this is caused by the transformation of the high fraction of retained austenite in the two-step austempering samples into hard martensite and the microstructure refinement zone that forms on the surface of the sample during the wear process. The 50%-290 °C sample have better fatigue properties at low total strain amplitudes (0.7% and 0.8%). When the total strain amplitude is 0.7%, the fatigue life of the 50%-290 °C sample is 57.5% higher than that of the 210 °C sample, which is because of the higher yield strength and finer microstructure of the 50%-290 °C sample. These results support the later study of controlling two-step austempering process for high-carbon nanostructured steels.

Superior oxidation resistance of a Y-Hf co-doped Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy at 1100–1300 °C

We report a study on the oxidation behavior of Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy co-doped with reactive elements (RE) Y and Hf at 1100–1300 °C. The highly stable eutectic microstructure of γ’/β phases within this alloy contributes to a low coefficient of thermal expansion, thus a low residual stress in the Al2O3 scale. The spinel with a low thickness is formed at the top of Al2O3 scale at 1100 °C due to the low Al diffusion coefficient of γ’-phase and low aspect ratio of β-phase in maze-like eutectic region. As the oxidation temperature increases, the oxidation products transition to exclusive Al2O3 at 1200 °C and 1300 °C, facilitated by the increased Al diffusion coefficient. The Al2O3 scale exhibits a dominated columnar grain microstructure at 1100–1300 °C, suggesting that the inward O diffusion plays more critical role than Al diffusion and thus leads to the low oxidation rate. Besides, the segregation of S to scale/metal interface is inhibited by the uniform RE distribution in the alloy resulting from the fine eutectic microstructure in the alloy. Overall, this alloy exhibits low residual stress in the oxide scale, slow oxidation rate, and the lack of interfacial sulfur segregation, thus contributing to its superior oxidation resistance. The strong oxidation resistance of Y-Hf co-doped Al18Co30Cr10Fe10Ni32 eutectic high-entropy alloy makes it a potential candidate for advanced oxidation-resistant alloy or coating applications.

Effect of remelting strategy on forming mechanism of selective laser melted AlSi10Mg

Selective laser melting (SLM) technology is widely used in various fields for manufacturing high-precision and complex parts. However, SLMed parts still exhibit some microstructural defects. The remelting strategy provides a new approach for defect elimination in SLM process, named as selective laser remelting (SLRM). In this study, the effects of remelting strategy on the melt pool were revealed through a finite element model. The porosity, microstructure, surface roughness, microhardness and tensile behavior of the specimens were also evaluated. The links between the formation mechanism of the melt pool and various experimental results were explored. The simulation results showed that the SLRMed melt pool had a significantly lower temperature and a shorter liquid phase time. The experimental results demonstrated that the remelting strategy effectively reduced the porosity, improved the density of the specimens, and provided a finer microstructure. The remelting strategy effectively reduced the surface roughness of the top surface of the specimens, while slightly adversely affecting the surface roughness of the side surface. Additionally, the microhardness and tensile properties of the specimens increased correspondingly due to the finer microstructure formed inside the SLRMed melt pool. These findings contribute to a better understanding and application of the remelting strategy for producing high-quality parts.