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The grain refinement mechanisms along the material flow path in pure and high-purity Al were examined, using the marker insert and tool stop action methods, during the rapid cooling friction stir welding using liquid CO2. In pure Al subjected to a low welding temperature of 0.56Tm (Tm: melting point), the resultant microstructure consisted of a mixture of equiaxed and elongated grains, including the subgrains. Discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX), and geometric dynamic recrystallization are the potential mechanisms of grain refinement. Increasing the welding temperature and Al purity encouraged dynamic recovery, including dislocation annihilation and rearrangement into subgrains, leading to the acceleration of CDRX and inhibition of DDRX. Both C- and B/B^-type shear textures were developed in microstructures consisting of equiaxed and elongated grains. In addition, DDRX via high-angle boundary bulging resulted in the development of the 45° rotated cube texture. The B/B^ shear texture was strengthened for the fine microstructure, where equiaxed recrystallized grains were fully developed through CDRX. In these cases, the texture is closely related to grain structure development.

The microstructural evolution and underlying mechanism of a new high strength, high toughness near β titanium alloy, β20C, during hot deformation, and heat treatment were studied qualitatively and quantitatively. It was found that dynamic recovery occurs mainly in β phase, while α phase undergoes both a dynamic recovery and continuous incomplete dynamic recrystallization with a fraction of high-angle grain boundaries (≥15°) of 21.1% under hot-rolling. Subsequently, α phase undergoes static recrystallization with an increasing fraction of high-angle grain boundaries (21.1%→60.7%) under annealing, while the grains are equiaxed with refined grain sizes of 1.63 µm observed from the rolling direction (RD) and 1.66 µm observed from the transverse direction (TD). Moreover, the average aspect ratio of the lamellar α phase was 2.44 observed from the RD and 3.12 observed from the TD after hot rolling, but decreased to 2.20 observed from the RD, and 2.53 observed from the TD after annealing. Furthermore, the strict Burgers’ relationship between α and β phases changed after hot-rolling and remains the distortion, even after the static recrystallization process of α phase during annealing.

In order to enhance dielectric properties of polymer-derived SiC ceramics, a novel single-source-precursor was synthesized by the reaction of an allylhydrido polycarbosilane (AHPCS) and divinyl benzene (DVB) to form carbon-rich SiC. As expected, the free carbon contents of resultant SiC ceramics annealed at 1600 °C are significantly enhanced from 6.62 wt% to 44.67 wt%. After annealing at 900–1600 °C, the obtained carbon-rich SiC ceramics undergo phase separation from amorphous to crystalline feature where superfine SiC nanocrystals and turbostratic carbon networks are dispersed in an amorphous SiC(O) matrix. The dielectric properties and electromagnetic (EM) absorption performance of as-synthesized carbon-rich SiC ceramics are significantly improved by increasing the structural order and content of free carbon. For the 1600 °C ceramics mixed with paraffin wax, the minimum reflection coefficient ( RC min ) reaches –56.8 dB at 15.2 GHz with the thickness of 1.51 mm and a relatively broad effective bandwidth (the bandwidth of RC values lower than –10 dB) of 4.43 GHz, indicating the excellent EM absorption performance. The carbon-rich SiC ceramics have to be considered as harsh environmental EM absorbers with excellent chemical stability, high temperature, and oxidation and corrosion resistance.

Microstructural phase evolution during melting and casting depends on the rate of cooling, the collective mobility of constituent elements, and binary constituent pairs. Parameters used in mechanical alloying and spark plasma sintering, the initial structure of binary alloy pairs, are some of the factors that influence phase evolution in powder-metallurgy-produced HEAs. Factors such as powder flowability, laser power, powder thickness and shape, scan spacing, and volumetric energy density (VED) all play important roles in determining the resulting microstructure in additive manufacturing technology. Large lattice distortion could hinder dislocation motion in HEAs, and this could influence the microstructure, especially at high temperatures, leading to improved mechanical properties in some HEAs. Mechanical properties of some HEAs can be influenced through solid solution hardening, precipitation hardening, grain boundary strengthening, and dislocation hardening. Despite the HEA system showing reliable potential engineering properties if commercialized, there is a need to examine the effects that processing routes have on the microstructure in relation to mechanical properties. This review discusses these effects as well as other factors involved.

This paper presents a comprehensive review of the current state of knowledge of second harmonic generation (SHG) measurements, a subset of nonlinear ultrasonic nondestructive evaluation techniques. These SHG techniques exploit the material nonlinearity of metals in order to measure the acoustic nonlinearity parameter, β . In these measurements, a second harmonic wave is generated from a propagating monochromatic elastic wave, due to the anharmonicity of the crystal lattice, as well as the presence of microstructural features such as dislocations and precipitates. This article provides a summary of models that relate the different microstructural contributions to β , and provides details of the different SHG measurement and analysis techniques available, focusing on longitudinal and Rayleigh wave methods. The main focus of this paper is a critical review of the literature that utilizes these SHG methods for the nondestructive evaluation of plasticity, fatigue, thermal aging, creep, and radiation damage in metals.

Sintering is a key technology for processing ceramic and metallic powders into solid objects of complex geometry, particularly in the burgeoning field of energy storage materials. The modeling of sintering processes, however, has not kept pace with applications. Conventional models, which assume ideal arrangements of constituent powders while ignoring their underlying crystallinity, achieve at best a qualitative description of the rearrangement, densification, and coarsening of powder compacts during thermal processing. Treating a semisolid Al–Cu alloy as a model system for late-stage sintering—during which densification plays a subordinate role to coarsening—we have used 3D X-ray diffraction microscopy to track the changes in sample microstructure induced by annealing. The results establish the occurrence of significant particle rotations, driven in part by the dependence of boundary energy on crystallographic misorientation. Evidently, a comprehensive model for sintering must incorporate crystallographic parameters into the thermodynamic driving forces governing microstructural evolution.

The AA7075 holds significant importance in the aerospace field. Understanding its microstructure evolution and constitutive relationships during warm deformation is crucial for optimizing forming processes. To this end, isothermal compression experiments were conducted at different temperatures and strain rates to analyze their flow stress behavior. The microstructure evolution was characterized using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Microstructural analysis confirmed that dynamic recovery constitutes the predominant softening mechanism under warm forming conditions. The results indicate that flow stress is highly sensitive to deformation parameters, decreasing with increasing temperature and rising with increasing strain rate. To accurately describe the flow behavior, two distinct constitutive models were formulated: (1) a phenomenological Hensel–Spittel–Garofalo (HSG) model; (2) a novel hybrid machine-learning model that innovatively integrates the Harris Hawks Optimization (HHO) algorithm with an LSTM model. Both constitutive models demonstrate reasonable predictive accuracy. In comparison, the HHO-LSTM model demonstrated a superior ability to capture complex nonlinear relationships, achieving highly precise predictions of flow stress across the full range of deformation conditions tested in this work. The hybrid machine-learning model proposed in this study provides a highly accurate method for describing and predicting the flow behavior of the AA7075 during warm forming, offering a powerful predictive tool for engineering applications.

The present study systematically investigated the hot deformation behavior of GH2787 superalloy within the temperature range of 1060–1120 °C and strain rates of 0.1–10 s−1. An Arrhenius-type constitutive equation was developed that accurately predicts the flow behavior, and the calculated thermal deformation activation energy Q is 364,401.19 J/mol. The hot working map was constructed based on the dynamic material model, which identified two preferred processing regions with power dissipation efficiency exceeding 0.3, and no flow instability was observed across the entire parameter range. Microstructural analysis reveals that the extent of dynamic recrystallization significantly increases with rising temperature and strain rate. Discontinuous dynamic recrystallization (via grain boundary bulging nucleation) serves as the dominant recrystallization mechanism in GH2787 superalloy during hot deformation, while continuous dynamic recrystallization (via subgrain rotation and coalescence) acts as a synergistic auxiliary mechanism, jointly driving microstructural evolution. This study provides important theoretical foundations for optimizing the hot working processes of GH2787 superalloy.

PurposeIt is widely acknowledged that the mechanical behavior of a soil is significantly influenced by the soil microstructure; and the microstructure can evolve as a result of any mechanical, hydraulic, chemical or thermal change taken place on soil sample. The present study aimed to investigate the microstructures of intact loess and remolded loess and to explore the evolution of the microstructure and PSD (pore-size distribution) due to consolidation for both intact and remolded loess.Materials and methodsA loess from the Loess Plateau of China was used as test material. A series of intact loess specimens were consolidated to various vertical pressures in oedometer cells. The same loess was remolded at the optimum water content state to various compaction degrees using static compaction method. It is equivalent to consolidate the remolded loess with the least compaction degree under various stress levels. The microstructures of intact loess specimens after consolidation tests and remolded loess specimens are characterized using the SEM (scanning electron microscope) and MIP (mercury intrusion porosimetry) techniques. The micrographs and PSDs under various consolidation pressures (or compaction energies) were compared to investigate the evolution of the microstructure and PSD induced by mechanical loading for each kind of soil.Results and discussionThe microstructure of intact loess is more homogeneous and is less dependent on consolidation stress than remolded loess. In both soils, the pores with entrance diameter smaller than 6 μm are almost not influenced by consolidation. In intact loess, inter-aggregate pores with entrance diameter greater than 6 μm are compressed randomly due to loading. However, in remolded loess, the pores are compressed until all larger pores have been compressed. The cumulative pore volume curve of remolded loess of any compaction degree can be divided into two segments, large-pore segment (6–50 μm) and small-pore segment (< 6 μm). The large-pore segment is simplified into a horizontal line and a straight line which slope is independent on the compactness.ConclusionsRemolded loess has very different microstructure and PSD from intact loess even though they may have the same GSD (grain-size distribution), mineralogical composition, and some other physical properties. The microstructural evolution induced by mechanical loading also varies in both kinds of soils. Based upon the measured PSDs, a method is proposed for predicting the PSD of remolded loess of any compaction degree using a reference PSD.

High-content cemented soils are critical for modern geotechnical technologies (e.g., pre-bored precast piles), yet their long-term durability remains underexplored. This study investigates the 28- to 365-day mechanical and microstructural evolution of high-content cemented silty clay under freshwater and seawater curing via UCS, SEM, MIP, and XRD. Under freshwater, cement content directly dictated strength, with the 8:2 mix reaching 24.31 MPa at 365 days. However, marginal efficiency analysis confirmed diminishing returns for excessive binder, establishing the 7:3 ratio as the optimal baseline. Seawater exposure induced a biphasic response: a 4.6% early strength gain at 28 days, followed by severe degradation (a 23.5% drop at 365 days). Concurrently, the failure mode shifted to macroscopic “pseudo-ductility,” with peak strain increasing from 2.37% to 3.04%. Crucially, a micro–macro inconsistency emerged: although seawater physically refined the pore structure (micropore proportion doubled to 30.2% at 90 days) via expansive salts filling mesopores, macroscopic strength declined. XRD confirmed this degradation coincides with severe long-term alkaline buffer (Ca(OH)2) depletion. Consequently, lifecycle durability assessments for high-binder marine systems must not rely solely on physical metrics like porosity, but adopt a coupled multi-factor framework prioritizing chemical stability.