Metallic Functional Materials: Development and Applications

1. Introduction and Scope

Metallic functional materials have become a strategic focus in contemporary material research, driven by the rising demand for intelligent, adaptive, and high-performance systems across energy conversion, aerospace actuation, biomedical devices, and next-generation flexible or micro-electromechanical technologies. Beyond their structural roles, these materials—ranging from shape memory alloys to magnetic, thermoelectric, high-entropy, and metastable metallic systems—integrate tunable physical responses with mechanical reliability, bridging fundamental metallurgy and device-level innovation.

This Special Issue of Metals, brings together contributions that exemplify the rapid expansion of this field. The selected papers highlight how integrated computational approaches, materials-genome strategies, and machine-learning-assisted design accelerate the exploration of complex compositional spaces; how advanced processing routes such as additive manufacturing, rapid solidification, and severe plastic deformation stabilize novel metastable phases; and how operando characterization techniques deepen our understanding of phase transitions, defect kinetics, and multi-field coupling phenomena. Equally important are studies designed to propel functional metals toward real-world applications, including high-temperature actuators, precision damping elements, biomedical micro-components, and energy-harvesting or thermal management devices designed for stringent operational environments.

Several scientific trends clearly emerge across this Special Issue. Microstructure has become an active design variable: hierarchical interfaces, controlled disorder, gradient architectures, and nanoscale phase coexistence are now systematically engineered to achieve targeted functionalities. Predictive theory and data-driven modeling are reshaping alloy discovery, replacing traditional trial-and-error strategies with mechanism-guided, high-throughput exploration. Meanwhile, application-driven constraints are forcing a deeper examination of reliability, cyclic stability, fatigue resistance, and environmental interactions—issues critical in long-term deployment.

2. Contributions

This Special Issue brings together ten cutting-edge studies in the field of metal research, covering shape memory alloys, magnetic functional materials, energy storage materials, and biomedical structural applications.

2.1. Shape Memory Alloys and Heusler Alloys

Shape memory alloys (SMAs) and their functional properties represent a primary focus of this Special Issue. Chen et al. (contribution 1) significantly enhanced the fatigue resistance of superelastic NiTi wires by combining mechanical training with nanocrystallization. Their study revealed that stress-controlled training induces a preferred grain orientation, which effectively reduces local stress concentrations and prolongs structural fatigue life. Timofeeva et al. (contribution 2) investigated the cyclic stability of superelasticity in quenched and aged NiFeGaCo single crystals. They highlighted the role of Ω-phase precipitates in cyclic stability, pointing out that degradation mechanisms involve the inhomogeneous distribution of dislocations around particles and the formation of residual martensite. Daróczi et al. (contribution 3) utilized acoustic emission (AE) techniques to provide significant insights into the microscopic mechanisms of rubber-like deformation in NiFeGaCo single crystals. They found that the asymmetry in AE activity observed during unloading is associated with the difficulty of renucleating stress-induced martensite variants.

Regarding magnetic Heusler alloys, Sakon et al. (contribution 4) performed magnetostriction measurements on the ferromagnetic Heusler alloy Ni₂MnGaCu. The study demonstrated that the alloy exhibits a large magnetostriction of up to 1300 ppm near the martensitic transformation temperature, confirming magnetic-field-induced strain during the transition from paramagnetic austenite to ferromagnetic martensite. Kamantsev et al. (contribution 5) focused on the Inverse Magnetocaloric Effect in NiMnSnCu Heusler alloys at low temperatures. Through direct measurements and theoretical calculations, they observed a significant adiabatic temperature change near the magnetic structural transition at 117 K under a 10 T field, offering new insights for cryogenic magnetic refrigeration. Furthermore, Liu et al. (contribution 6) innovatively designed a terahertz metamaterial absorber using Ni-Mn-Sn ferromagnetic shape memory alloy films. The device allows for dynamic switching between broadband and narrowband absorption via external magnetic fields, showcasing the potential of smart materials in terahertz applications.

2.2. Magnetocaloric Materials

Beyond Heusler alloys, rare-earth-based intermetallic compounds remain a key subject for magnetic refrigeration. Kuchin et al. (contribution 7) reported that the GdRuSi compound exhibits a giant magnetic entropy change and refrigerant capacity near the nitrogen liquefaction temperature (~77 K). Theoretical calculations confirmed that the magnetic moment primarily originates from Gd atoms, identifying GdRuSi as an ideal candidate material for nitrogen liquefaction magnetic refrigerators.

2.3. Energy and Electronic Materials

In the fields of energy storage and electronic packaging, this Special Issue features two studies with high practical value. Kwak et al. (contribution 8) synthesized MgH₂-NaAlH₄ composite hydrogen storage materials via mechanical ball milling. They found that the addition of 30 wt% NaAlH₄ significantly reduced the decomposition temperature of MgH₂ and increased the effective hydrogen storage capacity (up to 7.42 wt%), thereby improving the hydrogenation/dehydrogenation kinetics. Jung et al. (contribution 9) addressed the fabrication of electrodes for temperature-sensitive electronic devices, such as perovskite solar cells, by proposing a silver paste formulation based on azeotrope solvent application. This method achieved rapid sintering of Ag films at a low temperature of 140 °C, resulting in extremely low electrical resistivity and resolving thermal damage issues in flexible electronic manufacturing.

2.4. Structural Materials for Medical Applications

Addressing the issue of “stress shielding” in orthopedic implants, Mironovs et al. (contribution 10) explored methods to regulate the elastic modulus of 316L stainless steel plates through perforation design. Experimental and finite element simulation results showed that increasing the perforation area effectively lowers the material’s Young modulus to more closely match that of human bone, thereby improving biomechanical compatibility.

3. Conclusions

We hope that this Special Issue of Metals not only reflects current scientific progress but also stimulates further collaboration across metallurgy, solid-state physics, mechanical design, and device engineering. We extend our sincere appreciation to all authors, reviewers, and editorial staff for their dedicated contributions. The advances showcased here reaffirm the central role of metallic functional materials in shaping future-adaptive, efficient, and high-performance technologies.