Austenite has a face-centered cubic structure, and the interstitial atoms such as carbon and nitrogen are located in the octahedral interstitial center of austenite cell, and the midpoint of the center and edge of the face-centered cubic cell. If each octahedron accommodates one carbon atom at its center, the large solubility of carbon should be 50% (mole fraction), which corresponds to a mass fraction of about 20%. Actually the solubility of carbon in austenite is 2.11% (mass fraction), this is because the ? - Fe octahedral gap of the radius was only 0.052 nm, smaller than the radius of 0.086 nm carbon atoms. The more carbon atoms are dissolved, the greater the distortion will be. The lattice will be unstable. Therefore, not all octahedral gap centers can be dissolved into a carbon atom, and the solubility is limited. When carbon atoms dissolve into austenite, the lattice of austenite expands uniformly and equally, and the lattice constant increases with the increase of carbon content. Most of the alloy elements such as Mn, Cr, Ni, Co., Si, etc., in ? - Fe replace the position of the Fe atoms to form a substitutional solid solution. The solubility of replacement atoms in austenite varies from infinite solubility to very little solubility. A few elements, such as boron, exist only at extraction defects, such as grain boundaries, dislocations, etc.