Thermodynamics is one of the indispensable components of materials science and engineering. Successful materials and process design require reliable thermodynamic data. In the past, the thermodynamic properties of materials were mainly obtained by experimental means such as differential thermal analysis, chemical analysis, X-ray diffraction and energy spectrum analysis. However, with the continuous advancement of science and technology, the number of components in materials is increasing, and it is increasingly difficult to experimentally determine thermodynamic data, and it is difficult to obtain sufficient data in a limited time. Thermodynamic calculations based on the CALPHAD method are the best way to solve this problem. It can calculate the thermodynamic properties of multi-component systems from the thermodynamic data of low-component material systems to save time and cost, or to speculate under extreme conditions (high temperature, high pressure, radioactivity, etc.) or experiments by experimentally easily and accurately determined experimental data. Thermodynamic data that is difficult to accurately measure.
The CALPHAD method is based on thermodynamic theory, based on the crystal structure of each constituent phase (including gas phase, liquid phase, solid solution and compound) to establish a thermodynamic model, through evaluation and screening of experimental and theoretical calculations of multi-material systems at a certain temperature and pressure (including first principles) Calculations, statistical methods and empirical, semi-empirical formulas) data, fitting optimization model parameters, determining the Gibbs free energy of each phase in the system, and finally establishing a thermodynamic database of multivariate multi-component material systems. Figure 1 is a schematic flow chart of the CALPHAD method. The CALPHAD method is currently the only thermodynamic calculation method that can calculate the thermodynamic properties of multi-component systems and meet the requirements of practical application. It is also the thermodynamic basis for the simulation of material dynamics and microstructure evolution. Therefore, the CALPHAD method is widely used in the design of new materials and the design of new processes.
Figure 1 is a schematic diagram of the CALPHAD method flow [1]
This article will introduce the CALPHAD method in the design of traditional alloy alloys, the development of high-entropy alloys, 3D printing, applications in lithium-ion batteries.
1. Traditional alloy alloy development
The intermetallic compound NiAl has great application prospects in the field of superalloys, but poor ductility greatly limits its application. Kainuma et al. [2] determined the compositional intervals of NiAl, Ni solid solution and Ni3Al phase by using the CALPHAD method and experimentally determined phase diagram of Ni-Al-Fe system (Fig. 2a). By optimizing the alloy component heat treatment process and alloy composition (Fig. 2b), three NiAl-based alloy materials composed of NiAl and Ni3Al phases but with distinct microstructures were obtained. The ductility of the material has been greatly improved compared with NiAl, reaching 10%, and the strength is as high as 750-1000 MPa.
Fig. 2 1) Ni-25Al-xFe vertical cross section, 2) Microstructure evolution of three different NiAl and Ni3Al binary structures, from top to bottom, Ni-25Al-(18,15,13)Fe
2. Development of high entropy alloy
High entropy alloy (HEA) represents a new concept of alloy design. HEA usually contains 5 or more elements. These elements have the same or approximate atomic fraction. Unlike traditional alloys, HEA usually forms a simple body-centered cubic (bcc) or face-centered cubic phase (fcc). Compared with traditional alloys, HEA has many excellent properties. Through reasonable formula design, it can obtain high strength, high hardness, high work hardening, high temperature softening resistance, high temperature oxidation resistance, corrosion resistance and high electrical resistivity. It has been widely concerned and has great application prospects.
Figure 3 Composition of 10626 components in the Co-Cr-Fe-Mn-Ni system calculated using the TCHEA1 database
Bracq et al. [3] used the TCHEA1 high-entropy alloy database to study the stability of the fcc phase in the Co-Cr-Fe-Mn-Ni system. Through the calculation of 10626 components, the stability interval of a single fcc phase was determined (Fig. 3), and the accuracy of the calculation was verified experimentally. The calculation further indicates that an increase in the Cr and Mn contents lowers the stable phase of the fcc phase and Ni and Co can improve the stability of the fcc phase. This result makes it possible to optimize the properties of the alloy by optimizing the chemical composition of the Co-Cr-Fe-Mn-Ni-based HEA.
3. Additive manufacturing (3D printing)
This year, due to the flexibility, controllability, and the ability to prepare complex structural parts that cannot be prepared by traditional manufacturing methods, it has received extensive attention. Multiphase diagrams are considered to be "maps" of 3D printed metal parts. In 3D printing to prepare gradient metal materials , Hofmann et al. [4] used the phase diagram obtained by thermodynamic calculation to determine the optimal component gradient path to avoid the formation of brittle phase. Using this concept, a series of gradient materials such as 304L/Inconel626, 304L/Invar36, Ti-6-4/Nb, Ti-6-4/V/420 stainless steel, and Ti-6-4/TiC were successfully prepared.
Figure 4 1) Possible component gradient paths between two alloys in a ternary phase diagram, 2) Ti-6-4/Nb gradient material prepared by 3D printing
4. Lithium Ion Battery
In the field of lithium ion batteries, alloy anode materials have high charge density and low chemical potential, so they are expected to replace carbonaceous anode materials, and the safety of batteries will also be improved. Sn-based alloys are one of the most important alloy anode candidates due to their high theoretical charge density (Li22Sn5, 994 mAh/g) and low chemical potential. Li et al. [5] used the CALPHAD method to establish a thermodynamic database of the Li-Sn system, and used the database to calculate the open circuit voltage of different compounds in the Li-Sn system, as shown in Fig. 5.
Figure 5 Calculated open circuit voltage of Li–Sn alloy relative to pure lithium
Copper oxides are expected to be used as electrode materials in next-generation lithium ion batteries. Lepple et al. [6] used the CALPHAD method to establish a thermodynamic database of Li-Cu-O system. Using this database, the relationship between lithium ion battery voltage and lithium content when CuO or Cu2O is used as the cathode material is calculated. Calculation shows
(1) When CuO is used as the cathode material, as the Li content increases, the battery voltage will appear in three plateau intervals and decrease as the lithium content increases. The battery voltage in the first two platform sections will decrease with increasing temperature as the temperature of the third platform section increases.
(2) When Cu2O is used as the cathode material, there are only two plateau intervals. The battery voltage in the first platform section increases with temperature and the voltage in the second platform section decreases as the temperature increases.
Figure 6 shows the relationship between lithium ion battery voltage and lithium content when (a) CuO and (b) Cu2O are used as cathode materials.
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