This article ismainly to introduce the principles and applications of alloy design based onthermodynamics. This report will include three parts, high-entropy alloydesign, genetic alloy design and alloy design ofhigh-temperature solder. The introductions, principles and applications also will beincluded in each part. In thepart of high-entropy alloy(HEA) design, the thermodynamics criteria for mixingelements to get HEA is introduced. In thesecond part, a theoretical guidance for the design of alloy design is presented.The aim of the approach based on the guidance is to optimize the desired performance,microstructure and the alloy compositions. Thethird part will introduce how thermodynamics help decide several specificcompositions and how the compositions are examined in terms of meltingbehavior, electrical resistivity, wetting angle and hardness.
These materialswere used in high-temperature solders. Part1.Application in high-entropy design High entropy alloys (HEA) represents anew concept of alloy design, a revolution for traditional alloy design method. Thetraditional design method of the alloy is usually based on one or at most twokey elements. Instead, HEAs has many elements. HEA usually forms a simple solidsolution structure, such as face-centered cubic (fcc), body-centered cubic(bcc) or a mixture of both.
This is because of the high entropy of mixing inthe solution phase. In thermodynamics, when the gibbs free energy of the systemachieves its global minimum under constant temperature and pressure, the systemwill be at equilibrium. The solution phase of the mixing Gibbs can be describedas follows1: ?Hmixmeans the enthalpy of mixing. ?Smixmeans the entropy of mixing. For a random mixing ofcomponents, the configurational entropyof mixing is calculated by: T is the temperature in Kelvin and R isthe gas constant. For the n-element solution phase, ?Smixreaches its maximum Rln (n) when using the equivalent mole fraction of eachelement (xi). The higher entropy of mixing leads to lower Gibbs energy atconstant temperature, which stabilizes the solution1. Recently, HEA becomes more and morepopular for the reason that it has many outstanding properties, such as highstrength, high thermal ability, high abrasion resistance and high antioxidant properties.
Even though the HEA consists of manyelements, it does not mean that HEAs can be produced by simply mixing a seriesof elements together with an equal atomic ratio. Here are some criteria1: a. High entropy of mixing (?Smix> 1.61R), which requires at least five principal components in the systemwith equal atomic ratio. b.
Small enthalpy of mixing (-15< ?mix<5 kJ/mol), which is due to the fact that a large positive enthalpy of mixingresults in the segregation of different elements, and a large negative enthalpyof mixing leads to the formation of compounds.c. Small atomic size difference (?<4.6), which favors the formation of solid-solution phase. `Part 2.Genetic alloy design based on thermodynamics and kinetics This essay is mainly about a novelcomputational approach to alloy design.
The method is based on the expectedmicrostructure, and the aim is to combine strength maximization with corrosionresistance. The required alloys are searched based on thermodynamiccalculations. So, I will primarily introduce this part. It defines multiple targets,microstructure and strict parameters that reflect different aspects. Thecomposition of alloy is the corresponding design. There are 13 elements areconsidered: C, Cr, Ni, Ti, Mo, Al, Cu, Co, Nb, N, V, Mn and Si.
Table 1 liststhe concentration ranges of each element in the optimization process. Theyexplain industrial and technical constraints related to the manufacture ofalloys. For each alloy element, the composition range is divided into 32 equalintervals. It is also important to note that multiple alloy elements may haveconflicting interactions due to multiple targets.
Therefore, the number of differentalloying elements must be balanced while Optimization the alloy composition2. Genetic algorithm is chosen as theoptimal solution in order to scan the wide range of solution space for 32candidate alloys and find the best compromise solution among different goals. Amajor FORTRAN program was developed, mainly through genetic optimizationalgorithm from program execution thermodynamics calculation and evaluation ofevolution standard (figure 1). For each candidate solution, use the followingalgorithm to evaluate the above criteria2.
a. Define the system in ThermoCalc andenter calculation conditions (composition and temperature). b.
The Ms temperature is calculated fromEquation (1) and a go/no-go criterion of Ms?200Co was imposed to ensurethe formation of lath martensite. Ms temperature is estimated byconsidering the energy variation of both the chemical and mechanicalcontributions of the alloying elements. c. Thermodynamic equilibrium calculation;Obtain the volume fraction and composition of the equilibrium phase. The totalintegral number of the phase that not desired is added, and the Crconcentration recorded in the matrix is retained. d. Set Fe-rich BCC phase as matrix, andcalculate the precipitation driving force at the fixed precipitation temperatureof 500 ? as well as the critical nucleation radius . e.
The strengthening factor is calculated.