清华大学-材料科学基础17.pptVIP

材料科学基础Fundamental of Materials Chapter Ⅴ Imperfections in Crystals §5.1 Introduction 1. Classification: Ideal or perfect crystal real or defect crystal According to dimension: point defect: vacancies, interstitial atoms line defect: dislocation planar defect: stacking fault volume defect: mosaic structure 2. Effects: physical properties (point defect) mechanical properties (dislocation) metallurgical properties (all of the defects) §5.2 Point defects Ⅰ.Classification： 1. Schottky defect: 2. Frenkel defect: only vacancies pairs of vacancy and interstitial atom. Ⅱ.Equilibrium concentration of point defects. Equilibrium concentration of point defects in metal Example Ⅲ.Methods for obtaining extra- equilibrium point defects Quenching Cool-working Bombardment by high energy particles Ⅳ. Experimental Determination of Examples and Discussions Exercise * * Prof: Tian Min Bo Tel: E-mail: tmb@mail.tsinghua.edu.cn Department of Material Science and Engineering Tsinghua University. Beijing 100084 Lesson seventeen Suppose we introduce n vacancies into a crystal containing N atom sites. Determine: —— Entropy of mixing —— Energy of formation of vacancy G G0 C (for FCC A≈1) or nv: the number of vacancies per cm3. Nv: the number of atoms per cm3. For ionic crystals: : 正离子空位 : 负离子空位 : 正离子间隙 : 负离子间隙 Schottky Frenkel 1. Determine the number of vacancies needed for a BCC iron crystal to have a density of 7.87g/cm3. The lattice parameter of the iron is 2.866×10-8cm. Solution: So, there should be 2.00－1.9971=0.0029 vacancies per unit cell. ? Calculate the concentration of vacancies in copper at room temperature (25℃). ? What temperature will be needed to heat treat copper such that the concentration of vacancies produced will be 1000 times more than the equilibriu