凝固金属学.pptVIP

* * * * * * * * * * * * Competitive growth of the grains in the chill zone results in only those grains with favorable orientations developing into columnar grains * 优 * * * 优 优 * * * * Controlling grain structure in turbine blades: (a) conventional equiaxed grains, (b) directionally solidified columnar grains, and (C) single crystal. Solidification Processes Directional Solidification (定向凝固) * Silicon single crystal silicon wafer (c) Bridgman technique. Bridgman processes - A process to grow semiconductor and other single crystals. Directional Solidification (2) * 《第六章 金属的凝固》 金属凝固的基本规律 液态金属的结构、凝固的热力学条件、过冷现象 晶核的形成 均质形核：晶核形成时的能量变化，形核率，临界晶核半径，临界形核功 非均质形核 晶核的生长 液－固界面微观结构、液－固界面附近温度分布、晶体生长形态 铸件的组织及其控制 铸件三晶区及形成条件、影响铸件组织的因素、铸件晶粒尺寸的控制 作业 * 为什么非均匀形核比均匀形核容易进行? 从液态金属结构出发阐述合金凝固进行时需满足的几个条件？ 为什么金属结晶时一定要有过冷度？影响过冷度的因素是什么？ * * * f10_03_pg316.jpg * f10_04_pg316.jpg * f10_05_pg319.jpg * f10_05_pg319.jpg * f10_06_pg320.jpg * f10_07_pg321.jpg * * 稳态凝固阶段。 * * 试 * * * * * * * * * 凝固微观组织 强制性生长：晶体以和散热方式相反的方向生长，此时等温线的前进速度限定了晶体的生长速率。凝固时先在模壁上形核，生长取向是任意的，但长大速率最大的晶体学方向和传热方向平行的晶核会尽快长大并伸入液相，成为柱状晶。 * 自由生长：若液相普遍存在过冷，各处均可以独立形核和生长，最终得到等轴晶粒。 强制性生长和自由生长模式下只要界面前沿存在动力学过冷，均可生长成树枝晶。 * * * * * * * * 密 * * * * * * * * * * * Left: cast Al without grain refiner (inoculant) Right: with TiB2 inoculant （孕育剂） * Example: 1) Calculate the critical radius (in centimeters) of a homogeneous nucleus that forms when pure liquid copper solidifies. Assuming △T=0.2Tm; * continued Calculate the number of atoms in the critical-sized nucleus at this undercooling vol. of critical-sized nucleus: vol. of unit cell of Cu: Since there are four atoms per FCC unit cell of Cu, thus, the number of atoms per homogeneous critical nucleus is: atoms * Nucleation (形核)- The physical process by which a new phase is produced in a material. Critical radius (临界半径) - The minimum size that must be formed by atoms clustering together in the liquid before the solid particle is stabl