Modul 4

Modul 5Cacat Kristal/Atomicimperfections in the Atomic and Ionic ArrangementsObyektif modul 5three basic types of imperfections: point defects, line defects (or dislocations), and surface defects

Cacat Titik(a) vacancy, (b) interstitial atom, (c) small substitutional atom, (d) large substitutional atom, (e) Frenkel defect, (f) Schottky defect

Point defects: (a) vacancy, (b) interstitial atom, (c) small substitutional atom, (d) large substitutional atom, (e) Frenkel defect, (f) Schottky defect. vacancyVacancy terbentuk selama terjadi proses pembekuan (solidification ) dan juga hasil dari getaran otomic yang menyebabkan terjadinya perpindahan atom dari kisi normalnya. Jika dalam keadaan setimbang jumlah vacancy dinyatakan dengan Nv, maka dengan naiknya temperature jumlah vacancy dinyatakan dalam pesamaan sebagai berikut

Pengaruh Temperatur

dimana;Nv = Jumlah lowongan N = Jumlah atomik Qv = Energi aktivasi (energi getaran yang diperlukan untuk membentuk lowongan)T = Temperatur obsolut ( kalvin ).K = Konstata boltzmanns 1,38. 10-23 J/atom K atau 8,62.10-5 eV/atom-KDari persamaan (5.1 ) terlihat bahwa jumlah vacancy dipengaruhi oleh ekponensial temperatur - ( , sedangkan pada temperatur di bawah melting pointnyaContohCalculate the concentration of vacancies in copper at room temperature (25oC). 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 vacancies in copper.The lattice parameter of FCC copper is 0.36151 nm. The basis is 1, therefore, the number of copper atoms, or lattice points, per cm3 is:

(Continued)

At room temperature, T = 25 + 273 = 298 K:We could do this by heating the copper to a temperature at which this number of vacancies forms:

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(a) The location of the , , 0 interstitial site in BCC metals, showing the arrangement of the normal atoms and the interstitial atom (b) , 0, 0 site in FCC metals, (for Example 4.3). (c) Edge centers and cube centers are some of the interstitial sites in the FCC structure (Example 4.3).

Schottky defectDislocation Dislocation - A line imperfection in a crystalline material.Screw dislocation - A dislocation produced by skewing a crystal so that one atomic plane produces a spiral ramp about the dislocation.Edge dislocation - A dislocation introduced into the crystal by adding an extra half plane of atoms.Mixed dislocation - A dislocation that contains partly edge components and partly screw components.Slip - Deformation of a metallic material by the movement of dislocations through the crystal12

Figure 4.8 (a) When a shear stress is applied to the dislocation in (a), the atoms are displaced, causing the dislocation to move one Burgers vector in the slip direction (b). Continued movement of the dislocation eventually creates a step (c), and the crystal is deformed. (Adapted from A.G. Guy, Essentials of Materials Science, McGraw-Hill, 1976.) (b) Motion of caterpillar is analogous to the motion of a dislocation.13Figure 4.6 A mixed dislocation. The screw dislocation at the front face of the crystal gradually changes to an edge dislocation at the side of the crystal. (Adapted from W.T. Read, Dislocations in Crystals. McGraw-Hill, 1953.)

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15Figure 4.13 Electron photomicrographs of dislocations in Ti3Al: (a) Dislocation pileups (x26,500). (b) Micrograph at x 100 showing slip lines and grain boundaries in AI. (c) Schematic of slip bands development.

(c) 2003 Brooks/Cole Publishing / Thomson Learning

(c) 2003 Brooks/Cole Publishing / Thomson Learning

(c)2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning is a trademark used herein under license.16 Surface DefectsSurface defects - Imperfections, such as grain boundaries, that form a two-dimensional plane within the crystal.Hall-Petch equation - The relationship between yield strength and grain size in a metallic materialthat is, ASTM grain size number (n) - A measure of the size of the grains in a crystalline material obtained by counting the number of grains per square inch a magnification 100.Small angle grain boundary - An array of dislocations causing a small misorientation of the crystal across the surface of the imperfection.

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Figure 4.16 (a) The atoms near the boundaries of the three grains do not have an equilibrium spacing or arrangement. (b) Grains and grain boundaries in a stainless steel sample. (Courtesy Dr. A. Deardo.)18Suppose we count 16 grains per square inch in a photomicrograph taken at magnification 250. What is the ASTM grain size number?SOLUTIONIf we count 16 grains per square inch at magnification 250, then at magnification 100 we must have:N = (250/100)2 (16) = 100 grains/in.2 = 2n-1Log 100 = (n 1) log 22 = (n 1)(0.301)n = 7.64 Calculation of ASTM Grain Size Number19

Figure 4.19 The small angle grain boundary is produced by an array of dislocations, causing an angular mismatch between lattices on either side of the boundary.20

Figure 4.20 Application of a stress to the perfect crystal (a) may cause a displacement of the atoms, (b) causing the formation of a twin. Note that the crystal has deformed as a result of twinning.21

Figure 4.20 (c) A micrograph of twins within a grain of brass (x250).22

Figure 4.22 If the dislocation at point A moves to the left, it is blocked by the point defect. If the dislocation moves to the right, it interacts with the disturbed lattice near the second dislocation at point B. If the dislocation moves farther to the right, it is blocked by a grain boundary.TERIMAKASIH