Crystals

Friction stir welding (FSW) is now widely acknowledged as an effective method for joining high-strength steels. Numerical simulation of FSW has become a vital tool for optimizing process parameters. In the present study, a 3-D numerical model based on the Coupled Eulerian–Lagrangian (CEL) formulation is developed to simulate the plunge, dwell, and travel phases during the FSW of 304 stainless steel under various rotational speeds, plunge rates, dwell times, and welding speeds. The model is validated using the experimental plunge force data across all three process phases, demonstrating good agreement. The numerical findings indicate that the tool rotational speed exerts a more pronounced influence on the peak temperature, plastic strain, stir zone (SZ) geometry and heat-affected zone (HAZ) profile compared to welding speed. A favorable balance between peak tool thermal levels and surface contact stress is achieved at 300 rpm and 1.25 mm/s welding speed. Furthermore, plastic deformation heat accounts for 18.8% and 43.5% of the total heat generation at 200 and 500 rpm, respectively. The developed model provides a robust baseline tool for FSW process optimization.

Perovskites provide a flexible structural platform for tailoring electronic, optical, ferroic, catalytic, and transport properties. Their performance depends on composition, octahedral connectivity, defects, microstructure, and interfaces, but the effects of these variables differ between oxide and halide systems. This review examines selected advances in these material families, with emphasis on the links between crystal chemistry and device behavior. It first introduces the ideal perovskite structure, the scope and limitations of geometric stability descriptors, and the distinction between compositional and structural classifications. It then discusses synthesis routes and representative applications in photovoltaics, light emission, radiation detection, gas sensing, electrocatalysis, and electrochemical energy storage. The review also discusses the trade-offs associated with compositional substitution, dimensional reduction, and defect control. Computational methods and machine learning are considered as complementary tools for exploring these relationships. Across the reviewed applications, improved performance in a laboratory test does not necessarily establish operational stability or scalability; meaningful comparisons require consistent measurement conditions and a clear distinction between materials, electrodes, and complete devices. The review identifies priorities for more reliable reporting and for integrating structural characterization, computational predictions, and experimental validation.

Spherical TiZrNbTa refractory high-entropy alloy powders with uniform composition and good flowability are essential feedstock for powder-based additive manufacturing; however, irregular powders produced by the hydrogenation-dehydrogenation route generally require further shaping treatment. Radio frequency (RF) plasma spheroidization of TiZrNbTa refractory high-entropy alloy powders prepared by hydrogenation-dehydrogenation (HDH) was investigated at a fixed power of 12 kW. Fine (10–40 μm) and coarse (40–70 μm) powders were compared at 5.0 g/min, followed by feed-rate tests (2.5–7.5 g/min) on the coarse fraction. At 5.0 g/min, the spheroidization degrees of the fine and coarse powders were 45.33% and 50.88%, respectively. The fine powder exhibited satellite particles and a broadened particle-size distribution, whereas the coarse powder retained some irregular particles. Decreasing the coarse-powder feed rate from 7.5 to 2.5 g/min increased the spheroidization degree from 45.71% to 94.55%, with median particle size (D₅₀) reaching 56.84 μm and a narrower particle-size distribution. The powders retained predominantly body-centered cubic (BCC) diffraction signatures, while dendritic substructures and local Ta/Zr segregation varied with processing conditions. Testing showed that untreated coarse HDH powder did not flow unaided through a Hall funnel, whereas the 5.0 g/min spheroidized product flowed in 18.7 s/50 g; after spheroidization, its loose bulk density increased from 4.68 to 5.47 g/cm³. These results clarify the effects of particle size and powder feed rate on spheroidization behavior within the investigated conditions.