ENGR2000_crystallinesolids

ENGR2000_crystallinesolids - Introduction to Engineering...

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Introduction to Engineering Materials ENGR2000 Chapter 3: The Structure of Crystalline Solids Dr. Priya Thamburaj Goeser
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The structure of a material - arrangement of its internal components Subatomic structure Electrons & nuclei within the individual atoms •A t o m i c s t r u c t u r e Atoms & molecules within the material Microscopic structure Large groups of atoms aggregated together Macroscopic structure Elements that may be viewed with the naked eye
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3.2 Fundamental Concepts • Crystalline materials – Atoms are situated in a repeating or periodic array over large atomic distances – Long-range order • Non-crystalline materials (amorphous materials) – Do not have long-range order – May have short-range order
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3.2 Fundamental Concepts • Atomic hard sphere model – Atoms or ions are thought of as being solid spheres having well-defined diameters. – Spheres representing nearest-neighbor atoms touch one another.
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3.2 Fundamental Concepts • Lattice – A 3-D array of points coinciding with atomic positions
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3.3 Unit Cells • In describing crystal structures, it is often convenient to subdivide the structure into small repeat entities called unit cells.
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3.4 Metallic Crystal Structures • Atomic bonding is metallic • High number of nearest neighbors • High atomic packing density • Each sphere in the hard sphere model represents an ion core
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3.4 Metallic Crystal Structures • Crystal structures of common metals – Face-centered cubic crystal structure – Body-centered cubic crystal structures – Hexagonal close-packed crystal structure
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Face-Centered Cubic (FCC) Crystal Structure
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Face-Centered Cubic (FCC) Crystal Structure () 2 2 4 2 : parameter lattice The r FCC a r a = =
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Face-Centered Cubic (FCC) Crystal Structure • The total number of atoms per unit cell – each corner atom is shared by 8 cells – hence, only 1/8 of each corner atom is contained within each cell – each face is shared by 2 unit cells – hence, only 1/2 of each face atom is contained within each cell () ( ) 4 6 2 / 1 8 8 / 1 cell unit per atoms of number total = × + × =
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Face-Centered Cubic (FCC) Crystal Structure • The coordination number – Number of nearest-neighbor – or touching atoms () 12 = FCC CN
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Face-Centered Cubic (FCC) Crystal Structure () ( ) cell unit the of volume total atom an of volume cell unit in the atoms of number or cell unit the of volume total cell unit in the atoms of volume : (APF) Factor Packing Atomic × = = APF APF
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Example 3.1 • Calculate the volume of an FCC unit cell in terms of the atomic radius r.
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ENGR2000_crystallinesolids - Introduction to Engineering...

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