Lecture7 - Exam is this Thursday at 4:30 in 210 Harrelson...

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Chapter 3 - 1 • Exam is this Thursday at 4:30 in 210 Harrelson
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Chapter 3 - 2 • Exam this Thursday at 4:30 in 210 Harrelson
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Chapter 3 - 3 • Comparison among crystal structures: FCC: many close-packed planes/directions; HCP: only one plane, 3 directions; BCC: none • Specimens that were tensile tested. Mg (HCP) Al (FCC) tensile direction • HCP materials have fewer slip planes => more brittle. • BCC - no close-packed planes, but close-packed directions (slip is thermally activated).
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Chapter 3 - 4 Plastic deformation Real materials contain large number of dislocations The movement of these dislocations through the material under applied stress is called slip and causes plastic deformation. The ease with which dislocations can move depends on: the crystal structure of the material the temperature of the material the presence of any barriers to slip (grain boundaries, point defects, impurities To strengthening materials - introduce slip barriers.
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Chapter 3 - 5 16 Grain boundaries : are boundaries between crystals. are produced by the solidification process, for example. have a change in crystal orientation across them. impede dislocation motion. grain boundaries heat flow Schematic Adapted from Fig. 4.7, Callister 6e. Adapted from Fig. 4.10, Callister 6e. (Fig. 4.10 is from Metals Handbook , Vol. 9, 9th edition, Metallography and Microstructures , Am. Society for Metals, Metals Park, OH, 1985.) ~ 8cm Metal Ingot AREA DEFECTS: GRAIN BOUNDARIES
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Chapter 3 - 6 Polycrystalline Materials Grain Boundaries regions between crystals transition from lattice of one region to that of the other slightly disordered low density in grain boundaries – high mobility – high diffusivity – high chemical reactivity Adapted from Fig. 4.7, Callister 7e.
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Chapter 3 - 7 Solidification Columnar in area with less undercooling Shell of equiaxed grains due to rapid cooling (greater T ) near wall Grain Refiner - added to make smaller, more uniform, equiaxed grains. heat flow Grains can be - equiaxed (roughly same size in all directions) - columnar (elongated grains) Adapted from Fig. 4.12, Callister 7e. ~ 8 cm
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Chapter 3 - 8 Planar Defects in Solids One case is a twin boundary (plane) – Essentially a reflection of atom positions across the twin plane . Stacking faults – For FCC metals an error in ABCABC packing sequence – Ex: ABCABABC Adapted from Fig. 4.9, Callister 7e.
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Chapter 3 - 9 Microscopic Examination • Crystallites (grains) and grain boundaries. Vary considerably in size. Can be quite large – ex: Large single crystal of quartz or diamond or Si – ex: Aluminum light post or garbage can - see the individual grains • Crystallites (grains) can be quite small (mm or less) – necessary to observe with a microscope.
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Chapter 3 - 10 Volume Defects • Inclusions → oxides, sulfides • Size >> atoms • Size can be >> grains • Typically bad for mechanical properties
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This note was uploaded on 11/25/2008 for the course MATSCI 104 taught by Professor Lan during the Spring '08 term at UCLA.

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Lecture7 - Exam is this Thursday at 4:30 in 210 Harrelson...

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