ES195 Materials for Engineering

Question 1

5 “families” of materials

Answer 1

Metals/alloys (Gold, Steel, Cu alloys)
Ceramics (Alumina, Silicon carbide)
Polymers (Polyester, phenolic)
Glasses (soda glass, borosilicate)
Rubbers (Silicones, EVA)

Hybrids combine 2 or more e.g. GFRP, composites

Question 2

Purpose of selection methodology

Answer 2

Material properties vary enormously

Multiple factors affect design choice
Function - loading, environment its in.
Commercial - availability of raw materials, required manufacturing volume
Environmental - impact of different materials e.g. embodied energy, pollutants

Question 3

Three way relationship

Answer 3

Between properties, processing and structure

In manufacturing, we deliberately change
compositions and thermo-mechanically
process materials to change their
structure and properties

Question 4

Material structures

Answer 4

Crystalline/polycrystallaline - long range order
Amorphous - short range order

Question 5

Typical values for elastic modulus

Answer 5

Steel ≈ 200GPa
Al alloys ≈ 70 GPa
Ti alloys ≈ 110 Gpa

Question 6

Ball and spring model of elasticity

Answer 6

Material in tension, bonds stretch and atoms move apart, vice versa for compression.
Also represents how spring gets narrower in the middle

εz = -νεy (poisons ratio)

Question 7

Primary vs secondary bonds

Answer 7

Primary
Ionic, covalent, metallic

Secondary
Van Der walls (dipolar attraction between uncharged atoms)
Hydrogen - dipole attraction between bonded hydrogen and neighbouring ions

Question 8

Structure of diff bonding types

Answer 8

Ionic - highly ordered, simple structure e.g. cubic. Stuff, strong and brittle

Covalent - highly ordered, often complex, stiff strong and brittle

Metallic - complex, alloying possible, variety of structures, moderate stiffness, variety of strength and ductility

Question 9

Why covalent bonds easier to deform

Answer 9

Materials deform when defects propagate through them. Defects move easier along a line/simple plane. Easier in a cubic (ionic crystal) rather than a complex structure (covalent)

Question 10

Directions denoted by

Answer 10

Square brackets

Question 11

Crystalline structure

Answer 11

Atoms situated in repeated, periodic array over large distances, 3d pattern, each atom bonded to nearest neighbour

For those that do not crystallize, this long-range atomic order is absent; these non-crystalline
or amorphous materials

Question 12

What type of materials from crystalline structure

Answer 12

All metals, many ceramics and certain polymers (under normal solidification conditions)

Question 13

Atomic hard sphere model

Answer 13

Spheres representing nearest neighbour atoms touching each other

Question 14

Lattice

Answer 14

“lattice” means a three-dimensional array of
points coinciding with atom positions (or
sphere centres).

Question 15

Unit cell

Answer 15

The unit cell is the basic structural unit of the crystal structure.

Chosen to represent the
symmetry of the crystal structure,
wherein all the atom positions in the
crystal may be generated by
translations of the unit cell integral
distances along each of its edges

Question 16

Atomic bonding in metallic crystal structures

Answer 16

Metallic and nondirectional, leads to relatively large numbers of nearest neighbours and dense atomic packings for most metallic crystal structures. Also in hard model, each sphere represents an ion

Question 17

Derive a in terms of r, apf and coordination number for FCC and BCC

Also effective number of atoms hcp

Answer 17

FCC - a=2√2r APF 0.74 cn 12
BCC - a = 4√3r/3 APF 0.68 cn 8

effective atoms FCC 4 BCC 2

6 for hcp

Question 18

Polymorphism and allotropy + examples

Answer 18

Polymorphism - metals/non metals having more than one crystal structure
Allotropy is when found in elemental solids

Carbon- graphite is stable polymorph, diamond under high pressure

Pure iron BCC at room temp, FCC at 912C

Question 19

Equivalent directions

Answer 19

Spacing along each atom is same along direction

Question 20

Grain boundary

Answer 20

Atomic mismatch within the region where two grains meet; this area, called a grain boundary

Grains have same lattice structure but may be oriented differently

Question 21

Anisotropy

Answer 21

Properties of single crystals depending on direction in which measurement is taken from

For polycrystalline materials, each crystal grain is anisotropic, but when a specimen is taken the aggregate behaves isotropically

Sometimes grains have preferential crystallographic orientation - material is said to have a texture (aka fibre)

Question 22

Crystalline defect

Answer 22

A lattice irregularity having one or more of its dimensions on the
order of an atomic diameter.