Microstructural Evolution Characteristics Research Articles

Understanding the microstructure evolution characteristics and mechanical properties of an AlCoCrFeNi2.1 high entropy alloy fabricated by laser energy deposition

In the present work, an AlCoCrFeNi2.1 high entropy alloy is fabricated by laser energy deposition. Specimens are heated to 700 °C (2#), 850 °C (3#) and 1000 °C (4#), respectively, held for 1 h and cooled in water to investigate the effect of heat-treating conditions on microstructure evolution and mechanical properties of the alloys. FCC(L12) + B2 dual-phase microstructure is acquired from all the four deposits. Ordered FCC (L12) disappears and transforms into B2 particles in 3# and 4#. Original B2 phase and newly formed B2 particles show K–S orientation relationship (OR) with adjacent FCC grains. Nano-sized phase m particles, which contain monoclinic lattice structure and have a composition similar with FCC phase, exist on FCC-BCC phase boundary in all the four deposits. FCC-m phase boundary is coherent. Existence of phase m in 4# indicates that it is stable at temperatures as high as 1000 °C. Precipitate hardening plays an important role in strengthening the materials, while the coarsening of precipitate particles weakens their strengthening effect. The high strength of this alloy group is also attributed to the two-phase interface strengthening effect. Possible reasons for the formation of m particles and their effect on the strength of the material are analyzed.

Effect of Heat Treatment on Microstructure and Properties of Improved 40CrNi2Si2MoV Steel

Ultrahigh‐strength steel (2000 MPa) is a key material widely used in national defense, aerospace, and various other sectors. This kind of steel is more likely to break as its strength increases. To improve its toughness, it is crucial to fully understand the strengthening and toughening mechanisms, microstructural evolution characteristics, and heat‐treatment parameters of ultrahigh‐strength steel. Herein, an improved 40CrNi2Si2MoV test steel is designed based on the composition of the 40CrNi2Si2MoV ultrahigh‐strength martensitic steel and is subjected to quenching at 860/900/930/1050/1150 °C. The effects of microstructural changes on the test materials are analyzed using scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, electron backscatter diffraction, transmission electron microscopy, and other characterization methods. The results show that at a quenching temperature of 1050 °C, the material exhibits an elongation of 8.2% and a strength as high as 2200 MPa owing to the coupled effects of complex multicomponent laths, substructures, dislocations, and nano‐precipitates.

Characterization of Microstructural Evolution in Heat-Affected Zone of Cu-Bearing Ultra-High-Strength Steel with Lamellar Microstructure.

The advanced lamellar microstructure significantly improves the toughness of Cu-bearing ultra-high strength steel by delamination toughening (yield strength: 1370 MPa, impact toughness at -40 °C: 60 J). The lamellar microstructure affects the microstructure evolution of heat-affected zone (HAZ), resulting in separate distributions of lath martensite and granular bainite in the complete austenitizing zone and the formation of cluster fresh martensite in the partial austenitizing zone. The grain refinement and decrease in dislocation density are predominant features, especially for the complete austenitizing zone, where the grain is refined to 4.33 μm, and dislocation density is decreased by 27%. With the degree of austenitizing increase, the dissolution of Cu-rich precipitates (CRPs) aggravates during welding. A small fraction of CRPs in the complete austenitizing zone implies the onset of reprecipitation of CRPs. The reason for softening in HAZ is attributed to a combined effect of granular bainite forming, dislocation density decreasing, and CRPs dissolving. After PWTH, large numbered reprecipitation of coherent CRPs occurs, contributing to the hardness recovery of HAZ. Meanwhile, due to the high density of dislocation of lamellar microstructure inherited by partial austenitizing zone, coarsening of coherent CRPs is easy to occur, and various incoherent structures are observed.

Nanostructured bainite transformation characteristics in medium-carbon steel subjected to ausforming and isothermal holding below martensite start temperature

The influence of martensite on the transformation rate of bainite as well as enhancement of various properties is well established, yet the combined influence of both the primary martensite and low temperature ausforming below the martensite start temperature (MS) on the characteristics of microstructural evolution comprising ultrafine/nanostructured bainite, martensite and retained austenite (RA) in medium C steels has not been unambiguously clarified. A series of physical simulation experiments involving isothermal holding at various temperatures below and above the MS was conducted with or without prior ausforming (∼0.5 strain, 0.5 s−1 strain rate) in order to assess the influence of ausforming on subsequent transformation behaviour. The results suggest that when ausforming was applied above MS, it not only accelerated the bainite transformation kinetics, but also initiated strain-induced bainite/martensite formation. At temperatures below MS, prior ausforming did hinder the onset of bainite transformation, but the transformation rates were faster suggesting the strain-induced acceleration in kinetics. Due to a remarkable refinement of the bainitic structure achieved by prior ausforming in samples held below MS, the Vickers hardness improved appreciably unlike in the case of samples held above MS. Ausforming promoted significant refinement and uniformity of the bainitic plates, and also the orientations of the plates became more randomized. The dislocation density of RA decreased with the reduced content of secondary (fresh) martensite that formed during final cooling to room temperature.

Microstructural evolution and energetic characteristics of TiZrHfTa0.7W0.3 high-entropy alloy under high strain rates and its application in high-velocity penetration

Energetic structural materials (ESMs) integrated a high energy density and rapid energy release with the ability to serve as structural materials. Here, a novel triple-phase TiZrHfTa0.7W0.3 high-entropy alloy (HEA) was fabricated and investigated as a potential ESM. A hierarchical microstructure was obtained with a main metastable body-centered-cubic (BCC) matrix with distributed Ta-W-rich BCC precipitates of various sizes and interwoven hexagonal close-packed (HCP) lamellar nano-plates. The compressive mechanical properties were tested across a range of strain rates and demonstrated a brittle-to-ductile transition as the strain rate increased while maintaining a high ultimate strength of approximately 2.5 GPa. This was due to the phase transformation from metastable matrix BCC to HCP structures. In addition, during the dynamic deformation, metal combustion originating from the failure surface was observed. Furthermore, the composition of the fragments was studied, and the results indicated that the addition of tungsten promoted combustion. Finally, the potential application of this HEA was evaluated by high-velocity penetration tests, and the results were compared to other typical structural materials for penetrators and bullets. A comparison was conducted by assessing the geometries of the penetration channel employing two dimensionless parameters normalized by the projectile size, representing longitudinal and lateral damage, respectively. The normalized depth of the TiZrHfTa0.7W0.3 HEA projectile was comparable to those of the other investigated materials, but the normalized diameter was the largest, showing an excellent ability to deliver lateral damage.

Multiscale Characterization of Microstructural Evolution in Powder Metallurgy and Ceramic Forming Processes

The microstructural evolution of materials during powder metallurgy and ceramic forming processes is a complex phenomenon that spans multiple length scales. In this study, we present a comprehensive multiscale characterization of the microstructural changes occurring during these processes. We employ a combination of advanced experimental techniques, including high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD), to investigate the microstructural features at various length scales. Our results reveal the intricate interplay between grain growth, phase transformation, and defect formation during sintering and forming processes. We observe a strong correlation between the initial powder characteristics, such as particle size and morphology, and the resulting microstructure. Furthermore, we employ phase-field modeling to simulate the microstructural evolution and validate our experimental findings. Our simulations provide insights into the kinetics of grain growth and the role of interfacial energy in governing microstructural changes. The results of this study have significant implications for the design and optimization of powder metallurgy and ceramic forming processes, enabling the tailoring of microstructures for specific applications. This work contributes to the fundamental understanding of microstructural evolution in these processes and paves the way for the development of advanced materials with tailored properties.

Investigation on microstructure evolution of clayey soils: A review focusing on wetting/drying process

Variability in moisture content is a common condition in natural soils. It influences soil properties significantly. A comprehensive understanding of the evolution of soil microstructure in wetting/drying process is of great significance for interpretation of soil macro hydro-mechanical behavior. In this review paper, methods that are commonly used to study soil microstructure are summarized. Among them are scanning electron microscope (SEM), environmental SEM (ESEM), mercury intrusion porosimetry (MIP) and computed tomography (CT) technology. Moreover, progress in research on the soil microstructure evolution during drying, wetting and wetting/drying cycles is summarized based on reviews of a large body of research papers published in the past several decades. Soils compacted on the wet side of optimum water content generally have a matrix-type structure with a monomodal pore size distribution (PSD), whereas soils compacted on the dry side of optimum water content display an aggregate structure that exhibits bimodal PSD. During drying, decrease in soil volume is mainly caused by the shrinkage of inter-aggregate pores. During wetting, both the intra- and inter-aggregate pores increase gradually in number and sizes. Changes in the characteristics of the soil pore structure significantly depend on stress state as the soil is subjected to wetting. During wetting/drying cycles, soil structural change is not completely reversible, and the generated cumulative swelling/shrinkage deformation mainly derives from macro-pores. Furthermore, based on this analysis and identified research needs, some important areas of research focus are proposed for future work. These areas include innovative methods of sample preparation, new observation techniques, fast quantitative analysis of soil structure, integration of microstructural parameters into macro-mechanical models, and soil microstructure evolution characteristics under multi-field coupled conditions.

Gradient multi-layered aluminum sheet with excellent corrosion resistance

Diffusion is a very important phenomenon in the thermal processing, including Cu diffusion, Si diffusion and Zn diffusion in the sheet during brazing. Specifically, the migration of Si affects the amount of liquid metal that can be used to form joint, and further affects the solubility of solid substrate at the liquid–solid interface between the clad alloy and core alloy. In addition, Cu accumulates on Al(Fe,Mn)Si precipitates and in Al–Si eutectic at the grain boundaries. Compared with Al matrix, the formation of Al(Fe,Mn,Cu)Si enhances the potential difference between these materials. These events greatly damaged the corrosion resistance of the sheets. Due to the existence of the interlayer, Zn in the interlayer diffuses to the core layer, and the concentration of dissolved zinc decreases with the increase of depth, resulting in an increasing potential of the Al matrix towards the core layer. With depth from the surface to the core layer, the Al matrix is more cathodic, which promoting lateral corrosion within the interlayer region, prevents through failure and which protects the core alloy. Diffusion during brazing leads to microstructure evolution and local electrochemical characteristics of different layers. The optimized potential gradient of the clad layer-interlayer-core layer can greatly improve the corrosion resistance of the alloy.

Microstructure evolution and hot deformation characteristics of 15Cr-22Ni iron-base superalloy

The hot deformation behavior and microstructure evolution of a newly designed 15Cr-22Ni iron-based superalloy containing 0.6 wt %Nb was studied. The strain-compensated constitutive equations were established, which can accurately predict the high-temperature flow behavior of the superalloy. The dynamic recrystallization kinetic model was developed. The degree of dynamic recrystallization gradually increases with decreasing strain rate and increasing deformation temperature. The hot processing map of designed superalloy was developed and the optimum hot processing range has been determined to be 1050–1150 °C/0.01 s−1. Hot deformation induces the precipitation of nano-sized NbC at grain boundaries and dislocations in the designed superalloy. Nano-sized NbC particles act as nucleation sites for dynamic recrystallization. The dynamic recrystallization behavior is compared with that of 15Cr-22Ni-1 Nb alloy. The starting of dynamic recrystallization of designed superalloy is later than that of 15Cr-22Ni-1 Nb alloy, which is attributed to the lack of active sites in the early recrystallization nucleation. However, the recrystallization kinetics of designed superalloy is faster than that of 15Cr-22Ni-1 Nb alloy. Due to the weak pinning force of nano-sized NbC precipitates to the grain boundaries and dislocations, the movement of grain boundaries and dislocations is less hindered, which is beneficial to the growth of dynamic recrystallization grains.

Additive manufacturing and characterization of microstructure evolution of Inconel 718 superalloy produced by vat photopolymerization

Vat photopolymerization is a potential method to prepare metal parts. This research reports the first manufacturing of nickel-based superalloy Inconel 718 components by vat photopolymerization. The photocurable pastes with a high-volume loading of metal powder of 55 % were prepared, and it has the advantages of excellent stability and fluidity at a constant shear rate, which is suitable for fabricating green body. Inconel 718 complex green parts with high precision were successfully printed by vat photopolymerization. Subsequently, the green body was debinded to remove resin and sintered at different temperatures to obtain metal parts and retain their specific characteristics. The microstructure evolution of the printed parts during sintering was studied comprehensively, and the sintering mechanism was discussed in detail. The microstructure and phase compositions were characterized by SEM, EDX, and XRD, respectively. The results indicate that oxides were formed in the sintered parts, hindering further densification of Inconel 718 metal components. Nickel-based superalloy with complex structures was obtained using vat photopolymerization technology.

Microstructure evolution and interface characteristics of diffusion bonded dissimilar titanium alloys

Microstructure evolution and interface characteristics of dissimilar TC4/TB8 titanium alloys subjected to diffusion bonding and aging treatment are investigated. TC4/TB8 alloys plates can be metallurgically bonded at 830°C for 2h under 18MPa. The microstructure of TC4 alloy is composed of fine intergranular β grains and equiaxed α grains due to recrystallization. The α grains characterized by lath-like, granular, and continuous layer morphology are precipitated from β matrix in the TB8 alloy during furnace cooling and aging treatment. Element distribution mapping and line-profile data indicate that Mo atoms diffuse from the enrichment area (TB8) to the depleted area (TC4), while a small amount of Al and V atoms diffuse along an opposite direction. In addition, the behavior of grain growth across the bonded interface is not obvious under the current parameters.

Shear-governed microstructural variation and evolution of PPTA in dry-jet-wet spinning process

A kind of aramid fiber with excellent mechanical properties, PPTA often shows process-induced, position-dependent molecular chain alignment that greatly impacts its mechanical performance. Direct observation of the chain reorientation along with other microstructural variation and evolution characteristics during fiber spinning has been a challenge for experiments. This study develops a coarse-grained molecular dynamics (CG-MD) model to investigate the shear-governed microstructural evolution of PPTA during dry-jet-wet spinning considering various shear deformation and rates. By using MD-based X-ray scattering, the shearing effects on microstructures are revealed systematically for PPTA both in the solution and after drying. The MD results are further coupled with a macroscopic analysis assuming Poiseuille flow to demonstrate how PPTA chain alignment varies in the cross section of a fiber, and how geometry of the fine orifice of a spinneret governs the shear deformation and chain alignment. These insights are expected to guide future process design to improve the quality and properties of PPTA fibers made via dry-jet-wet spinning.

Stress-assisted corrosion behaviour of Hastelloy N in FLiNaK molten salt environment

The use of molten-salt–based energy production and storage systems requires high-temperature corrosion- and creep-resistant structural materials. This study investigated the microstructure evolution and corrosion characteristics of Hastelloy N under working stress in a molten salt environment. Selective diffusion of Cr at the grain boundaries (GBs) of the Hastelloy N degraded the properties of the GBs, making them preferred locations for corrosion crack invasion. Stress further promoted Cr diffusion and accelerated GB carbide precipitation, thereby forming a corrosion couple between the carbide and matrix that facilitated the expansion of intergranular corrosion cracks into the grains. These findings are useful for providing advances in the design of novel structural alloys with extraordinary property-microstructure stability combinations for resistant to complex molten salt environments.

Characteristics of microstructure evolution of surface treated IN718 superalloy by warm laser shock peening during long-term aging at high temperatures

The microstructure evolution behavior and its mechanism of the surface treated IN718 superalloy by laser shocking peening (LSP) and warm laser shocking peening (WLSP) followed by long-term aging at high temperature ranging from 600 °C to 680 °C were investigated in the present study. It was found that the substructure evolution behavior, compressive residual stress reduction and γ″ phase coarsening of the hardened surface layer of WLSP sample could be effectively inhibited at high temperature, in comparison with those of LSP counterpart. Owing to the formation of complex structure of γ″ phase and high-density dislocation with stacking faults (SFs) and micro-twins in γ″ phase in the hardened layer of WLSP alloy, more stable and higher density geometrically necessary dislocations (GNDs) were developed around γ″ phase, which changed the micro-stress distribution of the hardened layer and produced more stable hetero-deformation induced strengthening effects. The findings of the present study provide a new cognition for further deepening the micro-mechanism of strengthening and toughening of hardened surface layer in WLSP treated IN718 superalloy at high temperature, promoting the more practical application of WLSP technology in the manufacturing field of key components of superalloys. • High temperature aging was performed on WLSP strengthened IN 718 alloy. • Strengthening effect of WLSP showed excellent high temperature stability. • Complex Structure of γ″ phase/high-density dislocation were formed in WLSP alloy. • Stacking faults and micro-twins were included in γ″ of this complex structure. • This particular structure and GNDs density were characterized by HRTEM and EBSD.

Microstructure evolution and defect characteristics of multilayer Fe-Cr alloy coatings fabricated by laser melting deposition

To clarify the microstructure evolution, defect characteristics and suppression methods for multilayer cladding by laser melting deposition (LMD), experimental research on multilayer LMD of Fe-Cr alloy with different laser energy densities was carried out. The phase composition and the evolution rules of the phase during the melting-solidification process were analyzed. The crystal growth trend, dendritic transition and dendrite spacing of multilayer Fe-Cr coatings were investigated. The formation mechanism of crack, porosity and surface spheroidization defects were revealed, the effects of laser energy densities on the defects were analyzed, and the defect suppression methods were proposed and verified. The results showed that the primary and secondary remelting zones of multilayer Fe-Cr coatings were fine equiaxed dendrites and irregular cellular structures, respectively. When the laser energy density increased from 22.74 J/mm2 to 55.70 J/mm2, the dendrite spacing increased gradually, and the porosity and spheroidization defects showed a trend of improvement and then deterioration. The cracks were considered cold cracks with transgranular characteristics. Meanwhile, when the residual stress exceeded the theoretical fracture strength of 1.6 GPa, the multilayer cladding experienced brittle fracture and formed cracks. Selecting an appropriate laser energy density and preheating treatment can effectively improve the forming quality and inhibit the formation of defects.

Microstructure characteristics and thermodynamic properties of A357-SiCp/A357 layered composites prepared by semi-solid vacuum stirring suction casting

A357-SiCp/A357 layered composites were prepared using a semi-solid vacuum stirring suction casting method. The microstructures, mechanical properties, and thermal conductivities of the composites fabricated under different suction casting processes were compared. Additionally, the microstructural evolution characteristics and performance enhancement mechanism of the A357-SiCp/A357 layered composites were discussed. The results demonstrate that suction casting at 610 °C with a low solid phase ratio can significantly enhance the material density and reduce the agglomeration of SiCp. The A357-SiCp/A357 interface is clear and straight with good bonding. With an increase in the suction casting temperature, the bending resistance and thermal conductivity of the A357-SiCp/A357 layered composites exhibit a trend of significantly increase at first and then slowly decrease owing to casting defects, interface bonding, and SiCp distribution. Compared with SiCp/A357 composites, the bending strength, deflection, and thermal conductivity of the A357-SiCp/A357 layered composites increase from 257 MPa, 1.07 mm, and 155.72 W·(m·K)−1 to 298 MPa, 2.1 mm, and 169.86 W·(m·K)−1, respectively. This study provides a reference for improving the rheological casting of aluminum matrix layered composites.

Microstructure evolution of bituminite and anthracite modified by different fracturing fluids

The intrusion of fracturing fluid into coalbed methane (CBM) reservoirs changes the microstructure of coal and affects CBM recovery. To study the microstructural evolution characteristics of bituminite and anthracite modified by different fracturing fluids, we prepared pure water (F1), slickwater (F2), and CO2 foam fracturing fluids (F3) to treat coals, and low-temperature N2 adsorption (LT-N2A), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) tests were conducted to characterize the variation in the microscopic pore structure, functional group structure, and mineral components of the modified coals, respectively. The results show that different fracturing fluids can significantly change the pore structure characteristics of bituminite and anthracite. The pore volume of bituminite and anthracite modified by F1, F2, and F3 decreases successively, while the average pore size and the percentage of macropore (>50 nm) volume increase. The average pore sizes of bituminite and anthracite modified by F3 are 16.78 nm and 19.38 nm, respectively, which increase by 18.59% and 30.89% compared with that of bituminite modified by F2 and F1, respectively; the average pore sizes of anthracite increases by 3.20% and 332.47%, respectively. The pore fractal dimension of coal was calculated based on the low-temperature N2 adsorption curve. It is found that the seepage pore fractal dimension (D1) of both bituminite and anthracite modified by F2 and F3 is smaller than that of F1 modification, with D1 of F2 modified coals being the smallest. Therefore, F2 and F3 can make the seepage pores of coal smoother and reduce the complexity. F2 and F3 can reduce the aromaticity (I) and the degree of condensation of aromatic rings (DOC) of bituminite and anthracite and increase the structural parameter of the oxygen-containing functional groups (‘C’). The three fracturing fluids have little effect on the aliphatic chain length (L). F2 and F3 can dissolve the carbonate and clay minerals of coal to generate new pores to increase porosity and permeability. The results provide theoretical guidance for the fracturing of medium- and high-rank CBM reservoirs.

Comprehensive characterization of irradiation induced defects in ceria: Impact of point defects on vibrational and optical properties

Validation of multiscale microstructure evolution models can be improved when standard microstructure characterization tools are coupled with methods sensitive to individual point defects. We demonstrate how electronic and vibrational properties of defects revealed by optical absorption and Raman spectroscopies can be used to compliment transmission electron microscopy (TEM) and x-ray diffraction (XRD) in the characterization of microstructure evolution in ceria under non-equilibrium conditions. Experimental manifestation of non-equilibrium conditions was realized by exposing cerium dioxide (CeO2) to energetic protons at elevated temperature. Two sintered polycrystalline CeO2 samples were bombarded with protons accelerated to a few MeVs. These irradiation conditions produced a microstructure with resolvable extended defects and a significant concentration of point defects. A rate theory (RT) model was parametrized using the results of TEM, XRD, and thermal conductivity measurements to infer point defect concentrations. An abundance of cerium sublattice defects suggested by the RT model is supported by Raman spectroscopy measurements, which show peak shift and broadening of the intrinsic T2g peak and emergence of new defect peaks. Additionally, spectroscopic ellipsometry measurements performed in lieu of optical absorption reveals the presence of Ce3+ ions associated with oxygen vacancies. This work lays the foundation for a coupled approach that considers a multimodal characterization of microstructures to guide and validate complex defect evolution models.

Microstructure evolution of Si nanoparticles during the melting process: Insights from molecular dynamics simulation

Si nanomanufacturing has attracted considerable attention in the manufacturing of advanced thermoelectric materials and microelectronics owing to its compatibility with the semiconductor industry. The microstructure determines physical and chemical characteristics of a material; however, it is difficult to observe the microstructural evolution using traditional experimental methods. In this study, the melting process of Si nanoparticles is studied in depth by classical molecular dynamics (MD) simulation. Using the entropy of the largest standard cluster and the pair distribution function, the microstructural evolution characteristics can be determined. The transformation of Si-like structures was examined by largest standard cluster analysis (LaSCA). The results show that the melting process begins at the surface and rapidly penetrates into the nanoparticles at a high heating rate, indicating a liquid nucleation and growth (LNG) mode. At a higher heating rate, the temperature range of the solid and liquid coexistence will be larger. This study provides an understanding of the melting behavior of Si nanoparticles at the atomic scale and a useful reference for the preparation and processing of nanodevices.

Microstructural evolution and kinetic characteristics during metadynamic recrystallization in a Ni-Cr-Mo alloy

The metadynamic recrystallization (MDRX) characteristics of a Ni-Cr-Mo alloy are investigated using the double-pass thermal compression tests. The impacts of double-pass compression parameters on the kinetic features and microstructural changes are clarified. Results signify that the MDRX softening behaviors are remarkably accelerated with increasing of compression temperature, first-pass strain or strain rate. The bulging of the MDRX grain boundary and the annihilation/interaction of substructures become weaken at low compression temperature or small first-pass strain, enhancing the grain refining during MDRX. However, the MDRX grains coarsen rapidly with decrease of strain rate. Based on the comprehensive impact of double-pass compression parameters on softening fraction and grain size, the MDRX kinetic model and MDRX grain size model are developed. The predicted results are well matched the experimental data, which suggests that the established models can precisely characterize the changes of softening characteristics and grain size of the researched alloy during MDRX.