Microstructure, Mechanical Properties, and Solidification Behavior of Metals and Alloys (2nd Edition)

1. Introduction and Scope

The study of solidification behavior, microstructural evolution, and mechanical properties remains a fundamental area of research in materials science [1]. Understanding how solidification parameters influence phase formation, segregation, and mechanical behavior is crucial for designing advanced alloys and optimizing manufacturing techniques. The relationship between solidification kinetics and the final properties is particularly relevant in applications where the mechanical performance, corrosion resistance, and structural integrity are key considerations. This Special Issue of Metals presents a selection of studies exploring these interrelations, covering theoretical, experimental, and computational approaches that contribute to advancing metallic materials science. The featured articles focus on phase formation in aluminum and copper-based alloys, the influence of cooling rates on mechanical performance, the application of deep learning in metallurgical analysis, and the interfacial behavior of lead-free solder alloys. These contributions provide valuable insights into the correlation between processing, microstructure and properties, offering knowledge that is relevant to academic research and industrial innovation.

2. Contributions

This Special Issue compiles research addressing different metallic systems, each study highlighting the importance of microstructural control through solidification. One of the emerging trends explored in this collection is the use of artificial intelligence in microstructural characterization. Zhao et al. (contribution 1) employed deep learning techniques to recognize metallurgical images using a MobileNetV2 model trained on a cast iron and aluminum alloy dataset. Their study achieved 94.44% accuracy in fine classification, with different metallographic features being distinguished based on the composition, microscope type, magnification, and etching conditions. This work exemplifies the growing role of artificial intelligence in materials science, demonstrating its potential ability to automate microstructure analysis and reduce the subjectivity of image interpretation.

Two studies investigated the solidification behavior of aluminum and bronze alloys under controlled thermal gradients. Nascimento et al. (contribution 2) analyzed the CuAl₆Si₂ silicon–aluminum bronze alloy, demonstrating that variations in cooling rate significantly influence the formation of β and α phases, as well as the presence of intermetallic compounds such as Fe₃Si₂ and Fe₅Si₃. Higher cooling rates promoted refined β-phase structures, increasing the hardness up to 143 HB, while slower solidification favored α-phase formation and reduced the hardness. Similarly, Lobo et al. (contribution 3) studied the Cu-24Zn-6Al-4Mn-3Fe manganese bronze alloy, correlating the solidification conditions with microstructural changes and mechanical behavior. Their results confirmed that the β-phase remained dominant across all cooling conditions, while the fraction of the κ-phase (Fe₃Al) increased as the cooling rates decreased, contributing to an improved hardness. Interestingly, the microhardness decreased as the distance from the cooling surface increased; meanwhile, the microhardness exhibited the opposite trend, emphasizing the complex interplay between thermal gradients and phase evolution in these alloys.

Leal et al. (contribution 4) provided a significant contribution to the field of solder metallurgy and investigated the solidification and interfacial behavior of Sn-Bi and Sn-Bi-In solder alloys on Cu and Ni substrates. Their findings revealed that adding 10 wt.% increased the solidification range, producing a coarser microstructure and enhancing the wetting behavior. The Sn-40Bi-10In alloy exhibited a reduced contact angle of 24° on Cu and 26° on Ni, indicating superior wettability compared to binary Sn-Bi alloys. Additionally, the presence of In slowed the growth of the Cu₆Sn₅ intermetallic layer, improving the reliability of joints and reducing the risk of brittle failure in electronic applications. This study highlights the importance of composition control in optimizing solder performance, particularly in applications where the thickness of the intermetallic layer is a critical factor.

Another study on Fe-containing Al-Ni alloys (contribution 5) explored the relationship between cooling rates, microstructural evolution, and mechanical performance. The authors demonstrated that rapid cooling refined intermetallic distributions and increased the hardness by up to 36%, reinforcing that solidification kinetics play a decisive role in determining the properties of alloys. These findings provide valuable guidelines for designing alloys with tailored microstructures for specific engineering applications.

3. Conclusions and Outlook

The contributions presented in this Special Issue reinforce the essential role of solidification control in tailoring the microstructure and mechanical performance of metallic materials. The studies provide compelling insights into phase formation, interfacial stability, and microstructural refinement across diverse metallic systems. This knowledge is essential for advancing our understanding and application of material science. From fundamental research on dendritic growth and solidification kinetics to applied studies on welding, additive manufacturing, and lead-free soldering, these works provide valuable insights for metallurgists and materials engineers seeking to optimize processing routes and enhance material performance. The integration of computational tools such as artificial intelligence into microstructure characterization further expands opportunities for the real-time analysis and predictive modeling of microstructural evolution.

Future research directions should focus on developing advanced solidification models that integrate machine learning and experimental validation to refine predictive capabilities. The continued investigation of additive manufacturing processes and their solidification dynamics remains a promising avenue, particularly in the design of new alloys that are optimized for high-performance applications. Additionally, exploring multi-element alloying strategies and their impact on phase stability, mechanical behavior, and corrosion resistance will further contribute to the development of next-generation metallic materials.

This Special Issue would not have been possible without the valuable contributions of the authors, whose high-quality research has enriched the field of solidification and microstructural studies. The dedication of the reviewers, who provided rigorous assessments and constructive feedback, played a crucial role in ensuring the scientific integrity of these publications. I want to express my sincere gratitude to the editorial team at Metals for their unwavering support in making this collection a reality. Their remarkable contributions lay a vital foundation for advancing our understanding and control of solidification phenomena, which have significant implications for fundamental science and industrial applications.