Lecture7 - 1 Chapter 3 • Exam is this Thursday at 4:30 in...

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Unformatted text preview: 1 Chapter 3 - • Exam is this Thursday at 4:30 in 210 Harrelson 2 Chapter 3 - • Exam this Thursday at 4:30 in 210 Harrelson 3 Chapter 3 - Dislocations & Crystal Structures • 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). 4 Chapter 3 - 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. 5 Chapter 3 - 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 6 Chapter 3 - 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. 7 Chapter 3 - 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 8 Chapter 3 - 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. 9 Chapter 3 - 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. 10 Chapter 3 - Volume Defects • Inclusions → oxides, sulfides • Size >> atoms • Size can be >> grains • Typically bad for mechanical properties 11...
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Lecture7 - 1 Chapter 3 • Exam is this Thursday at 4:30 in...

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