🎯[재료과학] 금속상변태 결정구조 초간단 완벽정리 (SC, BCC, FCC, HCP, Crystal Structure 결정격자구조) 쉬운 이해 암기법

#crystal structure #metallic materials #materials research center #chemical engineering One of the reasons why metal materials are widely used is that they have a wide range of mechanical properties that they can have, and these properties can be easily implemented by various methods. Generally, in single-phase and two-phase alloys, changes in microstructure are accompanied by phase transformations - changes in the type or number of phases. Transformation rate Phase transformations do not occur instantaneously, but progress depending on time, that is, the transformation rate. Phase transformations Various types of phase transformations are important in material processing, especially in processing that involves changes in microstructure. For this discussion, transformations are classified into three types. The first is a transformation that occurs simply by diffusion without changing the number or composition of phases. This includes solidification of pure metals, isotopic transformations, recrystallization, and grain growth. Another type of transformation involving diffusion involves a change in the composition and number of phases. In this case, the final microstructure contains two phases. The eutectoid reaction falls into this category. The third is a diffusionless transformation, where a metastable phase is formed. The martensitic transformation of alloy steels falls into this category. Phase transformation kinetics Depending on the phase transformation, at least one new phase is formed, usually with different physical/chemical properties or structures from the parent phase. Also, most phase transformations are not instantaneous. The phase transformation begins with the formation of numerous small particles of the new phase(s), which increase in size until the transformation is complete. The phase transformation process can be roughly divided into two stages: nucleation and growth. In the nucleation stage, very small particles appear, and these nuclei of the new phase (consisting of only a few hundred atoms) can continue to grow. As the size of these nuclei increases in the growth stage, the original phase is completely or partially destroyed, and the transformation is completed when the new phase reaches its equilibrium fraction. Nucleation There are two types of nucleation: homogeneous nucleation and heterogeneous nucleation. The difference between the two types is the location where nucleation occurs. In homogeneous nucleation, the new phase nuclei are formed uniformly throughout the original phase, whereas in heterogeneous nucleation, the new phase nuclei are formed first at structurally uneven regions such as the container walls, insoluble impurities, grain boundaries, and dislocations. Homogeneous Nucleation The theory of nucleation involves a thermodynamic parameter, Gibbs free energy G. In short, free energy is a function of the internal energy of the system and a measure of the disorder (entropy) of the atoms or molecules. For simplicity, let us first consider the solidification of a pure material. Just as atoms are stacked one after another in a solid phase, atoms in a liquid form solid nuclei by clumping together, and each nucleus is assumed to be a sphere with a radius of r. The change in total free energy related to the solidification transformation can be divided into two. The first is the difference in free energy between the solid and liquid phases, i.e. the volume free energy. This value is negative when the temperature is below the equilibrium solidification temperature, and its amount is the product of the volume of the spherical nuclei. The second is the change in free energy due to the formation of the solid-liquid phase boundary during the solidification transformation. The criterion for dividing amorphous solids and crystalline solids is the presence or absence of a unit cell or unit lattice. Crystalline solids have a unit cell, which is a unit in which the structure is repeated regularly. Therefore, when the state changes to a liquid, the melting point is constant because the structure that is repeated regularly is broken. Since amorphous solids do not have a constant structure, the force between particles is not constant, so the melting point is not constant even though they are not a mixture. There are various types of crystalline solids, and therefore there are various types of crystal structures. In general chemistry, we only deal with simple cubic (sc), body- @plant_kim_CM