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MSE 2034 > Chapter 3 > Flashcards

Chapter 3 Flashcards

1
Q

What are two types of packing?

A
  • Non dense, random packing
  • Dense, ordered packing
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2
Q

Why are ordered structures more stable?

A

They tend to be nearer to the minimum bonding energy

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3
Q

What are the two types of materials/arrangements?

A
  • Crystalline
  • Noncrystalline/amorphous
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4
Q

What are some characteristics of crystalline materials?

A

They have atoms arranged in periodic, 3D arrays, and are typical of metals, many ceramics, and some polymers

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5
Q

What are some characteristics of noncrystalline/amorphous materials?

A

They have no periodic arrangement, are found in complex structures, and are often caused by rapid cooling

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6
Q

What are the characteristics of metallic crystal structures?

A
  • Dense atom packing
  • Simpler structures compared to ceramics and polymers
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7
Q

Why do metallic crystal structures have dense packing?

A
  • Bonds between atoms are nondirectional
  • Nearest neighbor distances are small which lowers bond energy
  • High degree of shielding of ion cores due to the free electron cloud
  • Typically only one element is present so all atomic radii are the same
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8
Q

What is a unit cell?

A

The smallest repetitive volume that contains the complete lattice pattern of a crystal

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9
Q

What are the unit cell geometries for the seven crystal systems?

A
  • Cubic
  • Hexagonal
  • Tetragonal
  • Rhombohedral (Trigonal)
  • Orthorhombic
  • Monoclinic
  • Triclinic
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10
Q

Sides/angles of cubic unit cells

A

All sides equal, all angles equal 90

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11
Q

Sides/angles of hexagonal unit cells

A

Two sides equal with 90 deg angles, other has 120

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12
Q

Sides/angles of tetragonal unit cells

A

Two sides equal, all angles equal 90

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13
Q

Sides/angles of rhombohedral unit cells

A

All sides equal, all angles do not equal 90

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14
Q

Sides/angles of orthorhombic unit cells

A

No sides equal, all angles equal 90

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15
Q

Sides/angles of monoclinic unit cells

A

No sides equal, two angles equal 90

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16
Q

Sides/angles of triclinic unit cells

A

No sides equal, all angles do not equal 90

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17
Q

What are simple cubic (sc) crystal structures?

A

Cubic, with the centers of atoms located at each of the eight corners. They are rare because they have low packing density (only Po has it). Closed-packed directions are cube edges, the coordination number is 6, have 1 atom/unit cell, and the APF = 0.52.

18
Q

What is a coordination number?

A

The number of nearest-neighbor/touching atoms

19
Q

What is the atomic packing factor?

A

The volume of atoms in a unit cell divided by the volume of the cell

20
Q

What are body-centered cubic (bcc) structures?

A

Cubic, centers of atoms at each corner and a single atom in the center. They have 2 atoms/unit cell, have a coordination number of 8, and an APF of 0.68. Often in metals that have covalent bonding. Ex: Cr, W, Fe, Ta, Mo

21
Q

What are face-centered cubic (fcc) structures?

A

Cubic, centers of atoms at each corner and six atoms at the center of each face. They have 4 atoms/unit cell, a coordination number of 12, an APF of 0.74 (which is the maximum possible APF), and an ABC stacking sequence. Ex: Al, Cu, Au, Pb, Ni, Pt, Ag

22
Q

What are hexagonal close-packed (hcp) structures?

A

Hexagonal, centers of atoms on each corner and center for A sites, and 3 atoms for B sites. They have 6 atoms/unit cell, a coordination number of 12, an APF of 0.74, ABA stacking sequence, and an ideal height/side length ratio of 1.633. Ex: Cd, Mg, Ti, Zn

23
Q

What is the theoretical density of metals?

A

Mass of atoms in the unit cell/total volume of the unit cell

24
Q

What is the order of densities for materials?

A

Metals > ceramics > polymers

Metals are close-packing and have large atomic masses
Ceramics often have lighter elements
Polymers have low packing density and lighter elements
Composites have moderate to low densities

25
Q

What are single crystals?

A

When the periodic arrangement of atoms (crystal structure) extends without interruption throughout the entire specimen (atoms are all aligned in one direction).

26
Q

What do the properties of crystalline materials depend on?

A

Crystal structure

27
Q

What are polycrystalline materials?

A

Materials composed of many small single crystals (grains)

28
Q

What is anisotropy?

A

Exhibiting different values of a property in different crystallographic directions, observed in single crystals. When grains of a polycrystal are textured, properties are anisotropic.

29
Q

What is isotropy?

A

Having identical values of a property in all directions, is observed in polycrystals when grains are randomly oriented

30
Q

What is polymorphism?

A

The ability of a solid material to exist in more than one form or crystal structure.

31
Q

What is allotropy?

A

The possibility of the existence of two or more different crystal structures for a substance (generally an elemental solid).

32
Q

What are point coordinates and how are they calculated?

A

A point coordinate is a lattice position in a unit cell determined as fractional multiples of a, b, and c unit cell edge lengths.

33
Q

What is a family of directions?

A

All directions that are crystallographically equivalent (have the same atomic spacing) – indicated by indices in angle brackets (< >).

34
Q

What is the linear density (ld) of atoms?

A

The number of atoms centered on a direction vector/length of the direction vector.

35
Q

What are Miller Indices?

A

A set of three integers (four for hexagonal) that designate crystallographic planes, as determined from reciprocals of fractional axial intercepts.

36
Q

What is a grain boundary?

A

The interface separating two adjoining grains having different crystallographic orientations.

37
Q

What are the steps to finding the Miller Indices of a plane?

A

1.If plane passes through selected origin, establish a new origin in another unit cell
2. Read off values of intercepts of plane (designated A, B, C) with x, y, and z axes in terms of a, b, c
3. Take reciprocals of intercepts
4. Normalize reciprocals of intercepts by multiplying by lattice parameters a, b, and c
5.Reduce to smallest integer values
6.Enclose resulting Miller Indices in parentheses, no
commas i.e., (hkl)

38
Q

What are the steps to finding the Miller Indices of a plane in a hexagonal unit cell?

A
  1. Relocate origin
  2. Intercepts
  3. Reciprocals
  4. Normalize
  5. Reduce
  6. Determine index: i = -(h + k)
  7. MBI (hkil)
39
Q

What is planar density?

A

The number of atoms centered on a plane/area of the plane

40
Q

What is interplanar spacing?

A

The distance between parallel planes of atoms

41
Q

How is light diffracted?

A

The diffraction grating spacing must be comparable to the light wavelength

MSE 2034 (15 decks)

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