FIM Materials (1)

Atom is considered

a basic structural unit of matter

What does an atom consist of?

positively charged nucleus surrounded by a sufficient number of negatively charged electrons and uncharged neutrons

What is responsible for the atomic number of the atom?

number of electrons

Describe an ionic bond

atoms of one element dives up their outer electrons to the atoms of another element

What is required for an ionic bond?

a large difference in electronegativity

Describe the features of an ionic bond

high strength due to electrostatic forces

low electrical conductivity

poor ductility

Ionic bonding is predominant in

ceramics (oxides, nitrides, borides, carbides)

Describe a covalent bond

electrons are shared between atoms in their outermost shellf

What is required for an ionic bond?

small difference in electronegativity

Describe features of a covalent bond

high hardness

low electrical conductivity

Describe a metallic bond

outer shell electrons are shared by all atoms to form a general electron cloud which provides attractive forces to hold the atoms together and form a strong and rigid structure

Describe feature of a metallic bond

good electrical conductivity

good thermal conductivity

good ductility

Metallic bonding is a bonding mechanism

pure metals and alloys

Describe packing in crystalline materials

atoms pack in periodic, 3D arrays

Name typical crystalline materials

metals

ceramics

polymers

Desribe noncrystalline materials

atoms with no periodic packing that occurs in complex structures or when a material is rapidly cooled

What is a synonym for noncrystalline?

amorphous

Describe atomic hard-sphere model

atoms are assumed to be solid spheres with well-defined diameters

What is a lattice?

3D array of points coinciding with atom positions

What is a unit cell?

smallest repetitive volume which contains the complete lattice pattern of a crystal

SC crystal structure

simple cubic

close-packed directions are cube edges

CN = 6

rare due to low packing density (only Po)

BCC crystal structure

body centered cubic

atoms touch each other along cube diagonals

CN = 8

Cr, W, alpha-Fe, Mo

FCC crystal structure

face centered cubic

atoms touch each other along face diagonals

CN = 12

Al, Cu, Au, Pb, Ni, Pt, Ag

Grain boundaries

transition from lattice of one region to lattice of another region

What does low density in grain boundaries indicate?

high mobility

high diffusivity

high chemical reactivity

In single crystals and polycrystals with textured grains

properties vary with direction (anisotropic)

If grains are randomly oriented in polycrystals

properties do not vary with direction (isotropic)

How can grain size be determined?

linear intercept method

Describe linear intercept method

straight lines of the same length are drawn though the grain structure on the photomicrograph

the number of intersected grains are counted

average grain diameters equals the line length divied by the average grain number divided by magnification

Name common types of stress

simple tension

torsion (type of shear)

simple compression

Define elastic deformation

the stress and the strain are proportional

Describe plastic deformation

permanent deformation defined by yield strength and tensile strength

Define yield strength

stress at which noticeable plastic deformation occurs

Define tensile strength

maximum stress on engineering stress-strain curve

How is TS point identified in metals?

noticeable necking starts

How is TS point identified in polymers?

polymer backbone chains are aligned and about to break

Define ductility

plastic tensile strain at failure (percent elongation or percent reduction in area)

At lower temperatures

material experiences higher stress with low strain (low ductility, brittleness)

At higher temperatures

material is more ductile but has lower strength and stiffness (longer plastic region)

Define toughness

energy to break a unit volume of material

How is toughness approximated?

Area under the stress-strain curve

Ceramics have

low toughness

Metals have

large toughness

How can metals be strengthened?

grain refinement

solid solution

precipitation strengthening

cold work (strain hardening)

Explain grain refinement

grain boundaries are barriers to slip

barrier strength increases with increasing angle of misorientation

if the grains are small, there are more barriers, therefore refinement is done to achieve smaller grain size and higher yield strength (Hall-Petch equation)

Explain solid solution method

an alloying element is dissolved into the crystal lattice of the base metal creating localized lattice strains that can act as barriers for dislocation motion, therefore required greater stress to be applied to cause plastic deformation

Example of solid solution hardening

strengthening of Cu with Ni

Explain precipitation strengthening

a uniform distribution of a precipitate in the metal structure is achieved

the precipitates act as physical blocks that prevent the dislocation from advancing

Example of precipitation strengthening

ceramics in metals (e.g. SiC in Fe or Al)

Explain cold work (strain hardening)

Metals are deformed at room temperature

as cold work is increased, yield and tensile strengths increase but ductility decreases

e.g. forging, rolling, drawing, extrusion

What is a solubility limit?

max concentration for which only a solution occurs

What is the relationship between solubility limit and temperature?

Solubility limit increases with temperature

Define components

elements or compounds which are initially mixed

Define phases

physically and chemically distinct regions in the material that are the results of component mixing

What are the independent variable of a phase diagram of a binary system?

temperature

initial concentration

What can be found if the temperature and the initial concentration of components in a binary system are known?

number and types of phases present

composition of each phase

wt% of each phase relative to the mixture

What is an isomorphous system?

one component is completely soluble in another

What is a binary-eutectic system?

a two component system that has a special composition with a minimum melting temperature