Feature Papers in Crystallography and Applications of Metallic Materials (2nd Edition)

1. Introduction and Scope

This Special Issue, “Feature Papers in Crystallography and Applications of Metallic Materials (2nd Edition)”, focuses on the influence of microstructures on the development of crystalline metallic materials for technological applications. This requires knowledge of the crystallographic structures of metallic materials, because the main factor influencing their functional response is their microstructure. Therefore, the crystallographic characterization of metal alloys is of scientific and technological interest. One of the goals is to design the microstructures of advanced materials. This Special Issue is the second edition; the first edition provides a comprehensive view of metallic materials, with articles and reviews devoted to research work on metallic materials (steels, Co-based, Ni-based, Al-based, Ti-based, Cu-based, Fe-based, and so on) and the study of their microstructures and properties, based on searching for relationships between structure, properties, and applications. Despite the relatively low number of published articles, these constitute clear examples of quality scientific studies in the field of metallic materials.

Regarding applicability, metallic alloys and compounds are fundamental in automobile, biomedical, energy, and sustainability fields due to various mechanical, electrical, magnetic, and optical properties [1,2]. Additional functional properties are corrosion resistance [3] and irradiation (including nanocrystallization of an amorphous phase) [4]. In scientific literature, some manuscripts explore the influence of composition and/or processing, including thermal annealing, applied pressure, or electromagnetic fields. Moreover, in addition to fundamental studies, more complex experimental studies that take into account crystallization, anisotropy, texture, and/or crystallographic defects and theoretical studies (ab initio, modeling, simulation, etc.) are also of interest [5,6].

Among the families of metallic materials, there is a growing interest in lightweight alloys based on Al, Ti, or Mg [7,8,9]. They are characterized by their low density and high strength, making them ideal for reducing weight in transportation and improving energy efficiency. Aluminum alloys are used in the aerospace sector (wings, fuselages), construction, and automotive industry (engine blocks, chassis). Magnesium alloys are used where lightness is critical, such as in vehicle components, aircraft cargo equipment, and portable electronic products. Titanium alloys are used in jet engines, medical implants, and high-performance sports equipment. In this Special Issue, two articles are linked to Al-based alloys.

One of the most studied families of metallic materials is that of steels. The multiplicity of microstructures and crystallographic phases present great variety in terms of their mechanical properties and corrosion resistance. There are multiple aspects to consider: the controlled addition of minor elements, heat treatments, and the optimization of the microstructure (including crystallographic defects and grain boundaries) [10].

Regarding magnetic alloys, based on Fe, Co, or Ni, these usually exhibit soft magnetic behavior (for example, Fe-based alloys such as Finemet, Nanoperm or Hitperm with amorphous or nanocrystalline microstructure) [11], being candidates in high-frequency applications or as transformer cores, or hard magnetic alloys such as permanent magnets [12].

One of the families of alloys that has garnered increasing interest in recent years is that of shape entropy alloys, which are multicomponent systems with a minimum of five elements in percentages between 5 and 35%. Parameters such as valence electron concentration (VEC) have been established that allow prediction of the final crystal structure [13].

Regarding production and processing techniques, there is a growing interest in additive manufacturing [14]. Its current applications focus on the production of high-value-added parts and components or on prototyping. With respect to the selection of materials, compositions, and processes, artificial intelligence (including machine learning) allows for the prediction of microstructure and mechanical and functional response, making collaboration with experimental research groups highly recommended [15,16].

2. Overview of Contributions

In this Special Issue, two articles are linked to Al-rich alloys and compounds. The first article presents an efficient methodology to simulate the mechanical behavior of metallic microstructures composed of a matrix and particles (precipitates). A volume element (RVE) model called “2.5D” was applied to an AlSi₁₀Mg alloy manufactured by additive manufacturing, which involves laser selective melting (L-PBF) and subsequent friction-processing (FSP). It should be noted that there is a good correlation between experimental results and the 3D models in isotropic materials. However, it is an approximation limited to uniaxial loading or cases where isotropy allows simplifications.

A specific family is that of Al-rich Al-Mg-based alloys due to the combination of high tensile strength and low density. It is known that increasing the Mg favors solid solution strengthening and reduces the stacking fault energy. The crystallographic issues are easy recrystallization and grain refinement. The second article, which focuses on Al-based alloys, is focused on improving the mechanical response and the electrical conductivity of alloys of the Al-Mg-Zn and Al-Mg-Si systems. A new processing pathway is applied for tempering: solution heat treatment, cold work, then artificial aging. The controlled addition of Zn-Mg and Mg-Si as alloying elements in the aluminum matrix induced the formation of precipitates during the aging heat treatment. It is found that aging improves mechanical properties while maintaining electrical resistivity. The study shows comparable or better results if compared with alloys of similar composition that are also produced by tempering.

There are two articles on high-entropy metallic alloys. One of them contains Fe among its elements. In this case, the aim is to study the magnetic response of alloys produced by mechanical alloying. The main crystallographic issues detected by X-Ray diffractograms analysis are the formation of the BCC-Fe-based supersaturated solid solution, the Fe₂Ti intermetallic, and the crystalline size refinement (10 nm). As expected in milled alloys, the lattice microstrain increases up to 1.15%. Concerning the magnetic behavior, after 100 h of milling, the magnetization of saturation, M_S, has a value of 28 emu/g and the coercivity has a value of 25 Am⁻¹, both consistent with soft magnetic behavior. Furthermore, the low M_S value is due to the formation of a non-magnetic phase, intermetallic HCP-Fe₂Ti.

A second high-entropy alloy study is linked to the analysis of the processes of plastic deformation and acoustic relaxation in Al_0.5CoCrCuFeNi. The main crystallographic and microstructure analyses were associated with the dominant dislocation defects; types of barriers that prevent dislocation motion; and mechanisms of thermally activated movement of various dislocation-line elements through barriers (at room and low temperatures). The main results allow authors to determine the following: (a) barriers preventing the motion of dislocation lines, (b) adequate mechanisms of thermally activated movement of elements (under moderate and deep cooling), and (c) quantitative estimation of dislocations’ interaction with barriers.

The last article focuses on specific crystallographic analysis with simulation. In this study, the effect of the kinematic constraints during the deep drawing process on the rotation of the crystal lattice in cubic-oriented aluminum single crystals is considered; the relation between crystal orientation and the formation of anisotropic defects is made clear. The simulation is based on crystalline plasticity models (CPFEM). A clear relationship is found between crystal rotation kinematics and ear formation during deep drawing. The optimization of the simulation will in turn optimize the selection of the processing conditions, favor the desired microstructure, and enhance the mechanical and functional properties.