Chapter 3: The Structure of Crystalline Solids Theoretical Density, r Density = r = r = where Mass of Atoms in Unit Cell Total Volume of Unit Cell nA VC NA n = number of atoms/unit cell A = atomic weight VC = Volume of unit cell = a3 for cubic NA = Avogadro’s number = 6.022 x 1023 atoms/mol Chapter 3 - 1 Theoretical Density, r • Ex: Cr (BCC) A = 52.00 g/mol R = 0.125 nm n = 2 atoms/unit cell Adapted from Fig. 3.2(a), Callister & Rethwisch 8e. atoms unit cell r= volume unit cell R a 2 52.00 a3 6.022 x 1023 a = 4R/ 3 = 0.2887 nm g mol rtheoretical = 7.18 g/cm3 ractual atoms mol = 7.19 g/cm3 Chapter 3 - 2 Densities of Material Classes In general rmetals > rceramics > rpolymers 30 Why? Metals have... Ceramics have... • less dense packing • often lighter elements Polymers have... r (g/cm3 ) • close-packing (metallic bonding) • often large atomic masses • low packing density (often amorphous) • lighter elements (C,H,O) Composites have... • intermediate values Metals/ Alloys 20 Platinum Gold, W Tantalum 10 Silver, Mo Cu,Ni Steels Tin, Zinc 5 4 3 2 1 0.5 0.4 0.3 Titanium Aluminum Magnesium Graphite/ Ceramics/ Semicond 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). Zirconia Al oxide Diamond Si nitride Glass -soda Concrete Silicon Graphite PTFE Silicone PVC PET PC HDPE, PS PP, LDPE Glass fibers GFRE* Carbon fibers CFRE* Aramid fibers AFRE* Wood Data from Table B.1, Callister & Rethwisch, 8e. Chapter 3 - 3 Crystals as Building Blocks • Some engineering applications require single crystals: -- diamond single crystals for abrasives (Courtesy Martin Deakins, GE Superabrasives, Worthington, OH. Used with permission.) -- turbine blades Fig. 8.33(c), Callister & Rethwisch 8e. (Fig. 8.33(c) courtesy of Pratt and Whitney). • Properties of crystalline materials often related to crystal structure. -- Ex: Quartz fractures more easily along some crystal planes than others. (Courtesy P.M. Anderson) Chapter 3 - 4 Polycrystals • Most engineering materials are polycrystals. Anisotropic Adapted from Fig. K, color inset pages of Callister 5e. (Fig. K is courtesy of Paul E. Danielson, Teledyne Wah Chang Albany) 1 mm • Nb-Hf-W plate with an electron beam weld. • Each "grain" is a single crystal. • If grains are randomly oriented, Isotropic overall component properties are not directional. • Grain sizes typically range from 1 nm to 2 cm (i.e., from a few to millions of atomic layers). Chapter 3 - 5 Single vs Polycrystals • Single Crystals E (diagonal) = 273 GPa Data from Table 3.3, Callister & Rethwisch 8e. (Source of data is R.W. Hertzberg, Deformation and Fracture Mechanics of Engineering Materials, 3rd ed., John Wiley and Sons, 1989.) -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. (Epoly iron = 210 GPa) -If grains are textured, anisotropic. E (edge) = 125 GPa 200 mm Adapted from Fig. 4.14(b), Callister & Rethwisch 8e. (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].) Chapter 3 - 6 Polymorphism • Two or more distinct crystal structures for the same material. • Allotropy-when found in elemental solids. titanium , -Ti carbon diamond, graphite iron system liquid BCC 1538ºC -Fe FCC 1394ºC -Fe 912ºC BCC -Fe Chapter 3 - 7 Tin (Its Allotropic Transformation) White (or beta) tin, having a body-centered tetragonal crystal structure at room temperature, transforms, at 13.2°C (55.8°F), to gray (or alpha) tin, which has a crystal structure similar to diamond. The rate at which this change takes place is extremely slow; however, the lower the temperature (below 13.2°C) the faster the rate. This transformation results in: Increase in volume (27 %), and, a decrease in density (from 7.30 g/cm3 to 5.77 g/cm3). Consequently, this volume expansion results in the disintegration of the white tin metal into a coarse powder of the gray allotrope. This produced some rather dramatic results in 1850 in Russia. The winter that year was particularly cold, and record low temperatures persisted for extended periods of time. The uniforms of some Russian soldiers had tin buttons, many of which crumbled because of these extreme cold conditions, as did also many of the tin church organ Chapter 3 - 8 pipes. This problem came to be known as the “tin disease.” 3.7 Iron has a BCC crystal structure, an atomic radius of 0.124 nm, and an atomic weight of 55.85 g/mol. Compute and compare its theoretical density with the experimental value found inside the front cover. 3.8 Calculate the radius of an iridium atom, given that Ir has an FCC crystal structure, a density of 22.4 g/cm3, and an atomic weight of 192.2 g/mol. Chapter 3 - 9 Crystal Systems Unit cell: smallest repetitive volume which contains the complete lattice pattern of a crystal. Fig. 3.4, Callister & Rethwisch 8e. a, b, and c are the lattice constant Chapter 3 - 10 Crystal Structure and Crystal System Q. What is the difference between crystal structure and crystal system? A: A crystal structure is described by both the geometry of, and atomic arrangements within, the unit cell, whereas a crystal system is described only in terms of the unit cell geometry. For example, face-centered cubic and bodycentered cubic are crystal structures that belong to the cubic crystal system. Chapter 3 - 11 Point Coordinates z Point coordinates for unit cell center are 111 c a a/2, b/2, c/2 000 y b ½½½ Point coordinates for unit cell corner are 111 x Chapter 3 - Point coordinates for all atom positions for a BCC unit cell Chapter 3 - 13 Crystallographic Directions z Algorithm 1. Vector repositioned (if necessary) to pass through origin. 2. Read off projections in terms of unit cell dimensions a, b, and c y 3. Adjust to smallest integer values 4. Enclose in square brackets, no commas [uvw] x ex: 1, 0, ½ => 2, 0, 1 => [ 201 ] -1, 1, 1 => [ 111 ] where overbar represents a negative index families of directions <uvw> Chapter 3 - 14 HCP Crystallographic Directions z Algorithm a2 - a3 a1 1. Vector repositioned (if necessary) to pass through origin. 2. Read off projections in terms of unit cell dimensions a1, a2, a3, or c 3. Adjust to smallest integer values 4. Enclose in square brackets, no commas [uvtw] a 2 Adapted from Fig. 3.8(a), Callister & Rethwisch 8e. ex: ½, ½, -1, 0 -a3 a2 2 => [ 1120 ] a3 dashed red lines indicate projections onto a1 and a2 axes a1 2 a1 Chapter 3 - 15 Problem 3.32 3.32 Determine the indices for the directions shown in the following cubic unit cell: Chapter 3 - 16