TiCoCrFeMn (BCC + C14) High-Entropy Alloy Multiphase Structure Analysis Based on the Theory of Molecular Orbitals

Abstract

High-entropy alloys (HEA) are a group of modern, perspective materials that have been intensively developed in recent years due to their superior properties and potential applications in many fields. The complexity of their chemical composition and the further interactions of main elements significantly inhibit the prediction of phases that may form during material processing. Thus, at the design stage of HEA fabrication, the molecular orbitals theory was proposed. In this method, the connection of the average strength of covalent bonding between the alloying elements (Bo parameter) and the average energy level of the d-orbital (parameter Md) enables for a preliminary assessment of the phase structure and the type of lattice for individual components in the formed alloy. The designed TiCoCrFeMn alloy was produced by the powder metallurgy method, preceded by mechanical alloying of the initial elementary powders and at the temperature of 1050 °C for 60 s. An ultra-fine-grained structured alloy was homogenized at 1000 °C for 1000 h. The X-ray diffraction and scanning electron microscopy analysis confirmed the correctness of the methodology proposed as the assumed phase structure consisted of the body-centered cubic (BCC) solid solution and the C14 Laves phase was obtained.

Structural Stability of Titanium-Based High-Entropy Alloys Assessed Based on Changes in Grain Size and Hardness

“…In the two-phase area, much slower The grain size values estimated according to the above-described methodology are summarized in Table 2 and Figure 7. In the analysis of the obtained results, it can be noticed that as in the case of previous studies on other parameters, such as changes in chemical composition [27], the grain size was also a derivative of the structural rebuilding of the produced alloys that took place during annealing. During the initial annealing period, comprising up to approximately 2 h of the process, in the structure inherited from the mechanical alloying and sintering process, a local redistribution of alloy components took place based on the original grains formed after sintering, leading to the formation of high-entropic crystal lattices of individual phase components of the alloy.…”

“…The sintering process and long-term homogenization at a temperature of 1000 • C were applied for up to 1000 h, followed by the observation of the material with a scanning electron microscope SEM-BSE, which enabled us to derive the conclusion that the matrix structure of the analyzed TiCoCrFeMn sinter consisted of two phases, differing in grayscale (Figure 1). An analysis of changes in the chemical composition after different annealing times supported by a detailed XRD and EBSD phase investigation presented in [27] allowed us to state that the sinter matrix was composed of a BCC solid solution with a lattice parameter a ≈ 8.9 Å and a hexagonal lattice Laves C14 phase characterized by a c/a ratio of approximately 1.63. It should also be emphasized that between 10 and 100 h of annealing, intensive changes in the chemical composition were observed in the cited work, leading to the redistribution of alloy components and consequently to the formation of a phase-stable matrix.…”

“…The quantitative analysis of changes in the primary phases, BCC or HCP (C14 Laves phase), carried out by comparing the obtained XRD reflections (Figure 2a,b) to the external RIR (Reference Intensity Ratio) standard, showed that in the initial annealing period, 1-50 h, an increase in the share of the solid solution occurred (Figure 2c). Then, as a result of the rapid fluctuations in the chemical composition, as well as the non-monotonic changes in the chromium diffusion coefficient in the solution [27], an increase in the share of the hexagonal structure of the Laves accompanied by a decrease in the content of the solution in the time interval of 50-100 h was observed. Further homogenization caused a very slow increase in the solid solution content.…”

“…According to Figure 5, it should be noted that in the structure of the obtained sinter, there are local areas of the BCC solid solution with a noticeably larger grain size. However, these zones become defragmented and disappear with the extension of the annealing time and the redistribution of the alloying elements [27]. Therefore, only statistically significant areas of the fine-grained BCC solution and the ultra-fine-grained mixture of the BCC solution and the Laves C14 phase were analyzed for grain size.…”

“…To confirm the suggestions regarding the influence of the slowed diffusion effect in the tested high-entropy TiCoCrFeMn alloy on a noticeable reduction in the grain growth rate stabilized in terms of chemical composition and phase structure of the matrix, an analysis of changes in the diffusion coefficient of two basic elements, chromium and titanium, responsible for the formation of the BCC solid solution and the Laves phase, was carried out [27]. For this purpose, the areas of occurrence of both phases with a visible interphase boundary were isolated, on which linear analyses of the chemical composition were performed using the EDS method, focusing on the changes in the shares of chromium and titanium, taking into account the following: for titanium, the stage of transition from a solid solution to the intermetallic phase, and for chromium, diffusion from the Laves phase into the solid solution, which allowed for the analysis of the process of diffusion of the two elements into the phases stabilized by them.…”