Chapter 1. The Structure of Metals. Body Centered Cubic (BCC) Structures

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1 Chapter 1 The Structure of Metals Body Centered Cubic (BCC) Structures Figure 1. The body-centered cubic (bcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, Examples of metals with this structure Alpha Iron Chromium Tungsten 1

2 Face Centered Cubic (FCC) Structures Figure 1.3 The face-centered cubic (fcc) crystal structure: (a) hard-ball model; (b) unit cell; and (c) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, Examples of metals with this structure Gamma Iron Aluminum Gold Hexagonal Close Packed (HCP) Structures Figure 1.4 The hexagonal closepacked (hcp) crystal structure: (a) unit cell; and (b) single crystal with many unit cells. Source: W. G. Moffatt, et al., The Structure and Properties of Materials, Vol. 1, John Wiley & Sons, Examples of metals with this structure Cobalt Magnesium Alpha

3 Dislocations Figure 1.1 Movement of an edge dislocation across the crystal lattice under a shear stress. Dislocations help explain why the actual strength of metals in much lower than that predicted by theory. Dislocations Lowers the theoretical strength of a perfect material Slip planes containing a dislocation require less shear stress to allow slip Examples Caterpillar Moving a rug Solidification Figure 1.11 Schematic illustration of the stages during solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other. Source: W. Rosenhain. 3

4 Grain Size Grain size Large grain Low strength Low hardness Small grains More grain boundaries Disruption of dislocation motion Higher strength TABLE 1.1 ASTM No. Grains/mm Grains/mm ,4,48 4,96 8, 16,4 3, ,,9 8, 3, 65, 185, 5, 1,5, 4,, Recovery, Recrystallization, and Grain Growth (Heat Treatments) Figure 1.14 Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains. Note the formation of small new grains during recrystallization. Source: G. Sachs. 4

5 Recovery, Recrystallization, and Grain Growth (Heat Treatments) Recovery T < T recrystallization Stresses in highly deformed regions relieved Recrystallization T recrystallization is between.3 and.5 T m Decreases dislocation density Lowers strength Raises the ductility Grain Growth Larger grains Increased ductility Chapter Mechanical Behavior, Testing, and Manufacturing Properties of Materials 5

6 Strength, Hardness, Toughness, and Stiffness Comparisons TABLE.1 Strength Hardness Toughness Stiffness Strength/Density Glass fibers Graphite fibers Kevlar fibers Carbides Molybdenum Steels Tantalum Reinforced Reinforced Lead Diamond Cubic boron nitride Carbides Hardened steels Cast irons Magnesium thermosets thermoplastics Lead Rubbers Ductile metals Reinforced plastics Wood Ceramics Glass Ceramics Reinforced Tin Diamond Carbides Tungsten Steel Aluminum Tantalum plastics Wood Reinforced plastics Steel Aluminum Magnesium Beryllium Tensile Testing (b) Figure.1 (a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths. (b) A typical tensile-testing machine. 6

7 Stress Strain Diagram Figure. A typical stressstrain curve obtained from a tension test, showing various features. Mechanical Properties of Materials TABLE. Mechanical Properties of Various Materials at Room Temperature Metals (Wrought) E (GPa) Y (MPa) UTS (MPa) Aluminum and its alloys and its alloys Lead and its alloys Magnesium and its alloys Molybdenum and its alloys Nickel and its alloys Steels and its alloys Tungsten and its alloys Nonmetallic materials Ceramics Diamond Glass and porcelain Rubbers, reinforced Boron fibers Carbon fibers Glass fibers Kevlar fibers Elongation in 5 mm (%) Note: In the upper table the lowest values for E, Y, and UTS and the highest values for elongation are for pure metals. Multiply gigapascals (GPa) by 145, to obtain pounds per square in. (psi), megapascals (MPa) by 145 to obtain psi