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me_350_-_lect_2_-_ch_2___3.20090826.4a954ed356a901.47262290

me_350_-_lect_2_-_ch_2___3.20090826.4a954ed356a901.47262290...

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ME 350 – Lecture 2 – Chapter 2 & 3 Ch 2 – The Nature of Materials Crystal structure Defects Stress & Strain Crystalline vs noncrystalline - also called:
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Crystalline Structure Structure in which atoms are located at regular and recurring positions in three dimensions Basic geometric grouping of atoms that is repeated called: Pattern may be replicated millions of times within a given crystal Characteristic structure of virtually all metals, as well as many ceramics and some polymers
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Three Crystal Structures in Metals 1. Body-centered cubic (BCC) e.g. Chromium, Iron, Molybdenum, Tungsten 1. Face centered cubic (FCC) e.g. Aluminum, Copper, Gold, Lead, Silver, Nickel 1. Hexagonal close-packed (HCP) e.g. Magnesium, Titanium, Zinc How many atoms in each unit cell?
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Imperfections (Defects) in Crystals Imperfections often arise due to inability of solidifying material to continue replication of unit cell, e.g., grain boundaries in metals Imperfections can also be introduced purposely; e.g., addition of alloying ingredient in metal Types of defects: 1. Point defects 2. Line defects 3. Surface defects
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Point Defects Imperfections in crystal structure involving either a single atom or a few number of atoms Figure 2.9 Point defects: (a) vacancy, (b) ion‑pair vacancy, (c) interstitialcy, (d) displaced ion (also known as a Frenkel Defect).
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Line Defects Connected group of point defects that forms a line in the lattice structure. Examples: Extra plane of atoms: dislocation Spiral within the lattice: dislocation
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Surface Defects Imperfections that extend in two directions to form a boundary Examples: External: the surface of a crystalline object is an interruption in the lattice structure Internal surface interruptions:
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Elastic Strain When a crystal experiences a gradually increasing stress, it first deforms elastically If force is removed lattice structure its original shape
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Plastic Strain If stress is higher than forces holding atoms in their lattice positions, a shape change occurs
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Effect of Dislocations on Strain In the series of diagrams, the movement of the dislocation allows deformation to occur under a stress than in a perfect lattice Figure 2.12 Effect of dislocations in the lattice structure under stress
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Slip on a Macroscopic Scale Slip occurs many times over throughout the metal when subjected to a deforming load, thus causing it to exhibit its macroscopic behavior in the stress-strain relationship Dislocations are a good‑news‑bad‑news situation Good news in manufacturing – the metal is easier to form Bad news in design – the metal is not as strong as the designer would like HCP has the fewest slip directions (thus usually has ductility), then FCC, and BCC has the most.
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