Crystallography and Structure ENGR 2110 R. R. Lindeke

Overview: Crystal Structure – matter assumes a periodic shape  Non-Crystalline or Amorphous “structures” no long range periodic shapes  FCC, BCC and HCP – common for metals  Xtal Systems – not structures but potentials Point, Direction and Planer ID’ing in Xtals X-Ray Diffraction and Xtal Structure

Non dense, random packing Dense, ordered packing Dense, ordered packed structures tend to have lower energies. Energy and Packing Energy r typical neighbor bond length typical neighbor bond energy Energy r typical neighbor bond length typical neighbor bond energy

Crystal Structure Means: PERIODIC ARRANGEMENT OF ATOMS/IONS OVER LARGE ATOMIC DISTANCES  Leads to structure displaying LONG-RANGE ORDER that is Measurable and Quantifiable All metals, many ceramics, some polymers exhibit this “High Bond Energy” – More Closely Packed Structure CRYSTAL STRUCTURES

Amorphous Materials These less densely packed lower bond energy “structures” can be found in Metal are observed in Ceramic GLASS and many “plastics” Materials Lacking Long range order

Crystal Systems – Some Definitional information 7 crystal systems of varying symmetry are known These systems are built by changing the lattice parameters: a, b, and c are the edge lengths , , and  are interaxial angles Fig. 3.4, Callister 7e. Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal.

Crystal Systems Crystal structures are divided into groups according to unit cell geometry (symmetry).

Tend to be densely packed. Reasons for dense packing: - Typically, only one element is present, so all atomic radii are the same. - Metallic bonding is not directional. - Nearest neighbor distances tend to be small in order to lower bond energy. - Electron cloud shields cores from each other Have the simplest crystal structures. We will examine three such structures (those of engineering importance) called: FCC, BCC and HCP – with a nod to Simple Cubic Metallic Crystal Structures

Crystal Structure of Metals – of engineering interest

CRYSTAL STRUCTURES – SIMPLE vs. Complex Typically we model them using the: ATOMIC HARD SPHERE MODEL That is: When describing crystalline structures, atoms (or ions) are thought of as being solid spheres having well-defined diameters. The nearest-neighbor atoms touch one another.

Metallic Crystal Structures How can we stack metal atoms to minimize empty space? First in 2-dimensions: vs. Now stack these 2-D layers to make 3-D structures – and note: the “Hard Spheres” touch as suggested above

Rare due to low packing density (only Po – Polonium -- has this structure) Close-packed directions are cube edges. Coordination No. = 6 (# nearest neighbors) for each atom as seen (Courtesy P.M. Anderson) Simple Cubic Structure (SC)

APF for a simple cubic structure = 0.52 APF = a  (0.5a) 3 1 atoms unit cell atom volume unit cell volume Atomic Packing Factor (APF) APF = Volume of atoms in unit cell* Volume of unit cell *assume hard spheres Adapted from Fig. 3.23, Callister 7e. close-packed directions a R=0.5a contains (8 x 1/8) = 1atom/unit cell Here: a = R at *2 Where R at is the ‘handbook’ atomic radius

Coordination # = 8 Adapted from Fig. 3.2, Callister 7e. (Courtesy P.M. Anderson) Atoms touch each other along cube diagonals. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. Body Centered Cubic Structure (BCC) ex: Cr, W, Fe (  ), Tantalum, Molybdenum 2 atoms/unit cell: (1 center) + (8 corners x 1/8)

Atomic Packing Factor: BCC a APF = 4 3  (3a/4) 3 2 atoms unit cell atom volume a 3 unit cell volume length = 4R = Close-packed directions: 3 a APF for a body-centered cubic structure = 0.68 a R Adapted from Fig. 3.2(a), Callister 7e. a 2 a 3

Coordination # = 12 Adapted from Fig. 3.1, Callister 7e. (Courtesy P.M. Anderson) Atoms touch each other along face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. Face Centered Cubic Structure (FCC) ex: Al, Cu, Au, Pb, Ni, Pt, Ag 4 atoms/unit cell: (6 face x ½) + (8 corners x 1/8)

APF for a face-centered cubic structure = 0.74 Atomic Packing Factor: FCC The maximum achievable APF! APF = 4 3  (2a/4) 3 4 atoms unit cell atom volume a 3 unit cell volume Close-packed directions: length = 4R = 2 a Unit cell contains: 6 x 1/2 + 8 x 1/8 =4 atoms/unit cell a 2 a Adapted from Fig. 3.1(a), Callister 7e. (a = 2  2*R)

A sites B B B B B BB C sites C C C A B B ABCABC... Stacking Sequence 2D Projection FCC Unit Cell FCC Stacking Sequence B B B B B BB B sites C C C A C C C A

Coordination # = 12 ABAB... Stacking Sequence APF = D Projection 2D Projection Adapted from Fig. 3.3(a), Callister 7e. Hexagonal Close-Packed Structure (HCP) 6 atoms/unit cell ex: Cd, Mg, Ti, Zn c/a = (ideal) c a A sites B sites A sites Bottom layer Middle layer Top layer

Theoretical Density,  where n = number of atoms/unit cell A = atomic weight V C = Volume of unit cell = a 3 for cubic N A = Avogadro’s number = x atoms/mol Density =  = VC NAVC NA n An A  = Cell Unit of VolumeTotal Cell Unit in Atomsof Mass

Ex: Cr (BCC) A = g/mol R = nm n = 2  a = 4R/  3 = nm a R  = a atoms unit cell mol g unit cell volume atoms mol x Theoretical Density,   theoretical  actual = 7.18 g/cm 3 = 7.19 g/cm 3

Densities of Material Classes  metals >  ceramics >  polymers Why? Data from Table B1, Callister 7e.  (g/cm ) 3 Graphite/ Ceramics/ Semicond Metals/ Alloys Composites/ fibers Polymers Based on data in Table B1, Callister *GFRE, CFRE, & AFRE are Glass, Carbon, & Aramid Fiber-Reinforced Epoxy composites (values based on 60% volume fraction of aligned fibers in an epoxy matrix) Magnesium Aluminum Steels Titanium Cu,Ni Tin, Zinc Silver, Mo Tantalum Gold, W Platinum Graphite Silicon Glass-soda Concrete Si nitride Diamond Al oxide Zirconia HDPE, PS PP, LDPE PC PTFE PET PVC Silicone Wood AFRE* CFRE* GFRE* Glass fibers Carbonfibers Aramid fibers Metals have... close-packing (metallic bonding) often large atomic masses Ceramics have... less dense packing often lighter elements Polymers have... low packing density (often amorphous) lighter elements (C,H,O) Composites have... intermediate values In general

Some engineering applications require single crystals: Properties of crystalline materials often related to crystal structure. (Courtesy P.M. Anderson) --Ex: Quartz fractures more easily along some crystal planes than others. --diamond single crystals for abrasives --turbine blades Fig. 8.33(c), Callister 7e. (Fig. 8.33(c) courtesy of Pratt and Whitney). (Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.) Crystals as Building Blocks

Most engineering materials are polycrystals. Nb-Hf-W plate with an electron beam weld. Each "grain" is a single crystal. If grains are randomly oriented, overall component properties are not directional. Grain sizes typ. range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers). Adapted from Fig. K, color inset pages of Callister 5e. (Fig. K is courtesy of Paul E. Danielson, Teledyne Wah Chang Albany) 1 mm Polycrystals Isotropic Anisotropic

Single Crystals -Properties vary with direction: anisotropic. -Example: the modulus of elasticity (E) in BCC iron: Polycrystals -Properties may/may not vary with direction. -If grains are randomly oriented: isotropic. (E poly iron = 210 GPa) -If grains are textured, anisotropic. 200  m Data from Table 3.3, Callister 7e. (Source of data is R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 3rd ed., John Wiley and Sons, 1989.) Adapted from Fig. 4.14(b), Callister 7e. (Fig. 4.14(b) is courtesy of L.C. Smith and C. Brady, the National Bureau of Standards, Washington, DC [now the National Institute of Standards and Technology, Gaithersburg, MD].) Single vs Polycrystals E (diagonal) = 273 GPa E (edge) = 125 GPa

Effects of Anisotropy:

Section 3.6 – Polymorphism Two or more distinct crystal structures for the same material (allotropy/polymorphism) titanium  (HCP),  (BCC)-Ti carbon: diamond, graphite BCC FCC BCC 1538ºC 1394ºC 912ºC  - Fe  - Fe  - Fe liquid iron system:

Locations in Lattices: Point Coordinates Point coordinates for unit cell center are a/2, b/2, c/2 ½ ½ ½ Point coordinates for unit cell corner are 111 Translation: integer multiple of lattice constants  identical position in another unit cell z x y a b c y z 2c2c b b

Crystallographic Directions 1. Vector repositioned (if necessary) to pass through the Unit Cell origin. 2. Read off line projections (to principal axes of UC) in terms of unit cell dimensions a, b, and c 3. Adjust to smallest integer values 4. Enclose in square brackets, no commas [uvw] ex: 1, 0, ½=> 2, 0, 1=> [ 201 ] -1, 1, 1 families of directions z x Algorithm where ‘overbar’ represents a negative index [ 111 ] => y

What is this Direction ????? Projections: Projections in terms of a,b and c: Reduction: Enclosure [brackets] x y z a/2b 0c 1/ [120]

ex: linear density of Al in [110] direction a = nm Linear Density – considers equivalance and is important in Slip Linear Density of Atoms  LD = a [110] Unit length of direction vector Number of atoms # atoms length nm a2 2 LD   # atoms centered on the direction of interest! Length is of the direction of interest within the U. Cell

HCP Crystallographic Directions 1. Vector repositioned (if necessary) to pass through origin. 2. Read off projections in terms of unit cell dimensions a 1, a 2, a 3, or c 3. Adjust to smallest integer values 4. Enclose in square brackets, no commas [uvtw] [ 1120 ] ex: ½, ½, -1, 0 => Adapted from Fig. 3.8(a), Callister 7e. dashed red lines indicate projections onto a 1 and a 2 axes a1a1 a2a2 a3a3 -a3-a3 2 a 2 2 a 1 - a3a3 a1a1 a2a2 z Algorithm

HCP Crystallographic Directions Hexagonal Crystals  4 parameter Miller-Bravais lattice coordinates are related to the direction indices (i.e., u'v'w') as follows.    'ww t v u )vu( +- )'u'v2( )'v'u2(  ]uvtw[]'w'v'u[  Fig. 3.8(a), Callister 7e. - a3a3 a1a1 a2a2 z

Computing HCP Miller- Bravais Directional Indices (an alternative way): Fig. 3.8(a), Callister 7e. - a3a3 a1a1 a2a2 z We confine ourselves to the parallelopiped in the hexagon a 1 -a 2 -Z and determine: (u’,v’w’) Here: [1 1 0] - so now apply the models to create M-B Indices

Defining Crystallographic Planes Miller Indices: Reciprocals of the (three) axial intercepts for a plane, cleared of fractions & common multiples. All parallel planes have same Miller indices. Algorithm (cubic lattices is direct) 1. Read off intercepts of plane with axes in terms of a, b, c 2. Take reciprocals of intercepts 3. Reduce to smallest integer values 4. Enclose in parentheses, no commas i.e., (hkl)  families {hkl}

Crystallographic Planes -- families Adapted from Fig. 3.9, Callister 7e.

Crystallographic Planes z x y a b c 4. Miller Indices (110) examplea b c z x y a b c 4. Miller Indices (100) 1. Intercepts 1 1  2. Reciprocals 1/1 1/1 1/  Reduction Intercepts 1/2   2. Reciprocals 1/½ 1/  1/  Reduction examplea b c

Crystallographic Planes z x y a b c 4. Miller Indices (634) example 1. Intercepts 1/2 1 3/4 a b c 2. Reciprocals 1/½ 1/1 1/¾ 21 4/3 3. Reduction 63 4 (001)(010), Family of Planes {hkl} (100),(010),(001),Ex: {100} = (100),

x y z Intercepts Intercept in terms of lattice parameters Reciprocals Reductions Enclosure  a -b c/2  -1 1/ N/A (012) Determine the Miller indices for the plane shown in the accompanying sketch (a)

Crystallographic Planes (HCP) In hexagonal unit cells the same idea is used example a 1 a 2 a 3 c 4. Miller-Bravais Indices(1011) 1. Intercepts 1  1 2. Reciprocals 1 1/  Reduction1 0 1 a2a2 a3a3 a1a1 z Adapted from Fig. 3.8(a), Callister 7e.

Crystallographic Planes We want to examine the atomic packing of crystallographic planes – those with the same packing are equivalent and part of families Iron foil can be used as a catalyst. The atomic packing of the exposed planes is important. a) Draw (100) and (111) crystallographic planes for Fe. b) Calculate the planar density for each of these planes.

Planar Density of (100) Iron Solution: At T < 912  C iron has the BCC structure. (100) Radius of iron R = nm R 3 34 a  Adapted from Fig. 3.2(c), Callister 7e. 2D repeat unit = Planar Density = a 2 1 atoms 2D repeat unit = nm 2 atoms 12.1 m2m2 atoms = 1.2 x R 3 34 area 2D repeat unit Atoms: wholly contained and centered in/on plane within U.C., area of plane in U.C.

Planar Density of (111) Iron Solution (cont): (111) plane 1/2 atom centered on plane/ unit cell atoms in plane atoms above plane atoms below plane ah 2 3  a 2 2D repeat unit 3*1/6 = = nm 2 atoms 7.0 m2m2 atoms 0.70 x Planar Density = atoms 2D repeat unit area 2D repeat unit Area 2D Unit: ½ hb = ½*[(  3/2)a][(  2)a]=1/2(  3)a 2 =8R 2 /(  3)

Section X-Ray Diffraction Diffraction gratings must have spacings comparable to the wavelength of diffracted radiation. Can’t resolve spacings  Spacing is the distance between parallel planes of atoms.

X-Rays to Determine Crystal Structure X-ray intensity (from detector)   c d d  n 2 sin  c Measurement of critical angle,  c, allows computation of planar spacing, d. Incoming X-rays diffract from crystal planes. Adapted from Fig. 3.19, Callister 7e. reflections must be in phase for a detectable signal! spacing between planes d incoming X-rays outgoing X-rays detector   extra distance traveled by wave “2” “1” “2” “1” “2” For Cubic Crystals: h, k, l are Miller Indices

X-Ray Diffraction Pattern Adapted from Fig. 3.20, Callister 5e. (110) (200) (211) z x y a b c Diffraction angle 2  Diffraction pattern for polycrystalline  -iron (BCC) Intensity (relative) z x y a b c z x y a b c

Diffraction in Cubic Crystals: With BCC materials: only planes where the sum of the Miller Indices (h+k+l) is “EVEN” will present signatures! For FCC Metals: h, k, & l must all be EVEN or all be ODD to present diffraction signatures! In diffraction patterns (as seen in the text or references) the angle given is twice the actual diffraction pattern of the crystal – a function of the way samples are taken in a “diffractometer”

Atoms may assemble into crystalline or amorphous structures. We can predict the density of a material, provided we know the atomic weight, atomic radius, and crystal geometry (e.g., FCC, BCC, HCP). SUMMARY Common metallic crystal structures are FCC, BCC, and HCP. Coordination number and atomic packing factor are the same for both FCC and HCP crystal structures. Crystallographic points, directions and planes are specified in terms of indexing schemes. Crystallographic directions and planes are related to atomic linear densities and planar densities.

Some materials can have more than one crystal structure. This is referred to as polymorphism (or allotropy). SUMMARY Materials can be single crystals or polycrystalline. Material properties generally vary with single crystal orientation (i.e., they are anisotropic), but are generally non-directional (i.e., they are isotropic) in polycrystals with randomly oriented grains. X-ray diffraction is used for crystal structure and interplanar spacing determinations.