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1、材料科学基础Fundamental of Materials*材料科学基础Chapter Imperfections in CrystalsLesson seventeen*材料科学基础5.1 Introduction1. Classification:Ideal or perfect crystalreal or defect crystalAccording to dimension:u point defect: vacancies, interstitial atomsu line defect: dislocationu planar defect: stacking faultu
2、volume defect: mosaic structureDateDate材料科学基础材料科学基础2. Effects:u physical properties (point defect)u mechanical properties (dislocation)u metallurgical properties (all of the defects)DateDate材料科学基础材料科学基础5.2 Point defectsu.Classification：1. Schottky defect: 2. Frenkel defect:only vacancies pairs of va
3、cancy and interstitial atom.DateDate材料科学基础材料科学基础u.Equilibrium concentration of point defects.Equilibrium concentration of point defects in metalSuppose we introduce Suppose we introduce n n vacancies into a crystal vacancies into a crystal containing containing N N atom sites. atom sites.Determine:
4、Determine: DateDate材料科学基础材料科学基础 Entropy of mixing Energy of formation of vacancyDateDate材料科学基础材料科学基础DateDate材料科学基础材料科学基础GG0C(for FCC A1)or nv: the number of vacancies per cm3.Nv: the number of atoms per cm3.DateDate材料科学基础材料科学基础For ionic crystals: 正离子空位: 负离子空位: 正离子间隙: 负离子间隙SchottkyFrenkelDateDate材料科学
5、基础材料科学基础Example1. 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 2.86610-8cm.Solution:Solution:So, there should be 2.001.9971=0.0029 vacancies per unit cell.DateDate材料科学基础材料科学基础2. Calculate the concentration of vac
6、ancies 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 equilibrium concentration of vacancies at room temperature? Assume that 20,000 cal are required to produce a mole of vaca
7、ncies in copper.Solution:Solution: The lattice parameter of FCC copper is 0.36151nm.DateDate材料科学基础材料科学基础DateDate材料科学基础材料科学基础3. In FCC iron, carbon atoms are located at octahedral sites at the center of each edge of the unit cell (1/2,0,0) and at the center of the unit cell (1/2, 1/2, 1/2). In BCC ir
8、on, carbon atoms enter tetrahedral sites, such as (1/4,1/2,0). The lattice parameter is 0.3571 nm for FCC iron and 0.2866 nm for BCC iron. Assume that carbon atoms have a radius of 0.071 nm. Would we expect a greater distortion of the crystal by an interstitial carbon atom in FCC or BCC iron? What w
9、ould be the atomic percentage of carbon in each type of iron if all the interstitial sites were filled?Solution:Solution: For BCC iron:DateDate材料科学基础材料科学基础DateDate材料科学基础材料科学基础For FCC iron:Carbon distorts the BCC crystal structure more than the FCC crystal. As a result, fewer carbon atoms are expecte
10、d to enter interstitial positions in BCC iron than in FCC iron.DateDate材料科学基础材料科学基础 For BCC iron:If all of the interstitial sites were filled:For FCC iron:If all of the octahedral interstitial sites were filled:DateDate材料科学基础材料科学基础u .Methods for obtaining extra- equilibrium point defectsQuenchingCool-workingBombardment by high energy particlesDateDate材料科学基础材料科学基础u . Experimental Determination ofDateDate材料科学基础材料科学基础Examples and DiscussionsDateDate材料科学基础材料科学基础ExerciseDateDate材料科学基础材料科学基础Thank you !1717 DateDate材料科学基础材料科学基础
