1. Material Properties

• Ideally materials are a microscopic matrix of small balls that form a larger solid. In reality the atoms that make of solids fall into local pockets of well organized matrices. It is very rare to find a solid that is made up of a single structure.

• If solids were made of single well organized molecules they would be significantly stronger. But, small deformations and cracks weaken materials to the values we are more accustomed to.

• Material properties are a function of multiple factors. Primarily chemistry determines what atoms are available to make up the structure. Also, the atoms are dispersed in a non-homogenous mix.

• Solids typically fail because cracks form, and then quickly propagate through solids. It is the chemistry and non-homogenous structure that can slow or stop these cracks. The composition of the solid also determines how stiff it is.

1.1 Terminology

• A basic list of terms commonly used are,

Brittleness: the tendency of a material to break before it undergoes plastic deformation

Ductility: the ability of certain materials to be plastically deformed without fracture (pulling).

Elasticity: The ability to deform and return to the undeformed shape. This follows Hooke’s law.

Hardness: the resistance to deformation and forced penetration

Malleability: the ability of a material to take a new shape when hammered or rolled.

Tensile Strength: the maximum tensile load that can be applied before a material fractures

Toughness: The ability to withstand cracking, as opposed to brittleness

Yield Strength: The load at which the material stops elastically deforming, and starts permanently deforming.

1.2 Microstructures

• To consider materials properly we must start with the basic atomic structure and then look at the more macroscopic aspects, and how they are related to the microscopic components.

• In an atom there are some fundamental ratios,

Each atom is understood to have a basic structure with a nucleus and orbiting electrons.

The nucleus is a combination of neutrons and protons.

The number of protons and neutrons in an atom are equivalent and these determine the atomic number. If there are additional neutrons in the nucleus this is called an isotope.

The mass of the atom is determined by the sum of the neutrons and protons (the electron mass is much smaller).

In a mole of material there are 6.023*10**23 atoms.

• How these components fit together is described in models,

Bohr model

- electrons have quantized energy levels

- electrons are discrete and orbit the nucleus

- a free electron has a negative energy level

Wave-mechanic model

- electron waves can behave like particles or waves

- an electron is described as an electron cloud

- electrons have energy levels including ground levels

- valence electrons are the outermost and most likely to be removed first

• The valences of electrons are determined with the ’spdf’ numbers.

• The basic atomic elements are listed in the periodic table. This is in sequence of the atomic masses, as well as proton counts. It can also be used to determine similarities in properties by proximity in the table.

 

• In the periodic table the metals are in the left hand side. They have 1 to 3 valence electrons. They tend to give up electrons when bonding.

• In the upper right hand of the periodic table are the non-metals. They typically are 1 to 3 valence electrons short of a full valence level. As a result they tend to consume electrons when bonding. These are,

He, N, O, F, Ne, P, S, Cl, Ar, Br, Kr, I, Xe, At, Rn

• There is a band of semimetals: including semiconductors. These often consume and give up electrons when bonding. These are

B, O, Si, Ge, As, Se, Te.

1.2.1.1 - Crystal Structures

• Understanding crystal structures can help understanding of crystalline materials such as metals.

• Think of dropping balls into a box. it can fall randomly, but often it will fall into patterns. This is like atoms in a solid.

• If all of the balls fall into a single organized pattern then we can say there is a single crystal.

• Three of the basic structure types to consider are,

bcc: body centered cubic

fcc: face centered cubic

hcp: hexagonal close packed

 

• In a common solid there will be many regions in the crystal, but there will also be boundaries where the crystal properties change. These are known as boundaries.

• A common effect that can occur is slippage along one of the planes of the crystal. An example is pictured below,

 

• Different crystal structures will result in different possible slip planes.

bcc has 48 possible slip planes

fcc has 12 possible

hcp has 3 possible

• Other slip structures are also possible

1.3 Irons and Steels

• Irons and steels are the most popular metals in use today. The production of iron was at one time a subject of mystic awe.

• Any engineer involved with modern engineering should have at least a passing knowledge of steels to understand many of the processes.

• Various steel alloys are commonly identified with the SAE-AISI numbers,

 

• Typical applications for plain steels (based on the SAE-AISI numbers) are,

 

1.3.1.1 - Alloying Elements

• A Short list of elements is given below,

 

• Typical elements that are left over from the manufacturing processes leave behind detrimental elements,

antimony

arsenic

hydrogen

nitrogen

oxygen

tin

• The basic process is,

1. Iron ore is mined, and crushed. At this point in contains iron, carbon, oxygen, and a variety of other minerals.

2. The ore is heated in a blast furnace with coke. This removes some of the elements, notably oxygen. Pig iron remains, and has high levels of carbon.

3. A refining furnace is then used to burn off the excess carbon, leaving a good quality steel.

1.3.2.1 - The Ores

• The ores come in a number of forms,

taconite:

hermatite: iron oxide mineral

limonite: iron oxide and water

• The ores are crushed to ease handling, and speed melting.

• After crushing, iron rich ore can be separated using magnets.

• the resulting ore is formed into pellets of about 65% iron.

1.3.2.2 - Coke

• The classic recipe for Coke begins with bituminous coal. It is then heated to 2100°F and then cooled with water.

• Coke will

generate higher level of heat during steel making

generate carbon monoxide which reacts with oxygen in iron oxide and leaves iron

1.3.2.3 - Flux, Slag

• Some materials are used as a flux, and to create slag,

limestone

dolomite

• By adding a flux material, it will react with impurities, causing them to flow.

• After the flux dissolves the impurities, it reacts with them to form a solid called slag. This floats to the top of the melt, where it is removed.

• These furnaces are large heated vessels, and they are lined with bricks of refractory materials.

• Iron pellets, limestone, and coke are mixed together and dumped into the top of the furnace.

• Air is preheated to 2000°F and this is used to ‘blast’ the mixture into the furnace. The coke ignites, and elevates the temperature of the mixture to 3000°F. This results in a reduction of the iron oxides, and separation of the slags.

• After some period of time (a few hours) the furnace is tapped, and the iron is drawn off to large ladles. This ‘pig’ iron typically has a impurity contents of 4% C, 1.5% Si, 1% Mn, 0.04% S, 0.4% P.

• Making steel is a process of reducing the following impurities in pig iron,

manganese

silicon

carbon

phosphorous

sulphur

etc

• This operation is commonly done in one of three furnaces,

open hearth: flames are directly applied to the metal, and can be seen from the open hearth.

electric

basic oxygen: a blast of pure oxygen reacts with impurities

• The basic procedures with all of these furnaces is,

1. Charge (pour in) scrap iron

2. Pour in molten (pig) iron

3. Add lime

4. Run the furnace

5. Tap the furnace to remove the steel: care must be used not to pour the slag on the surface

6. Pour off the slag off

• Any oxygen left in the steel when solidified will combined with carbon. The result is small voids that are actually pockets of carbon monoxide gas. A killed steel will have all oxygen removed.

1.3.4.1 - Electric Furnaces

• There are two basic types,

induction

arcing

• In induction furnaces large coils are wound around the crucible. AC current is applied, and this induces heat in the metal inside. Vacuum can be applied to the melt to increase purities of the final steel.

• Arcing furnaces use carbon electrodes at high potentials to create arcs. These act to heat the metal.

• The furnaces reach temperatures up to 3500°F.

• There are options after the steel has been processed,

ingots: the steel is poured into ingots, and stored to be formed later

continuous casting: the steel is poured, and immediately formed to bars, rolls, etc.

• Continuous casting uses a slow pour that when running smoothly,

1. Is liquid at the top where it is being poured.

2. It solidifies, still at forming temperatures, and typically moving at 5 fpm. A pulling action keeps a continuous rate.

3. It is rolled, bent, formed, and cut.

• The result of continuous casting is a single process that produces final steel sections without any of the intermediate problems that result from remelting ingots.

• These steels use a high Chromium content(10 to 12%) to form a protective layer of chromium oxide over the surface of the work that is resistant to many forms of corrosion

• General families of stainless steels include,

Austenitic (2xx, 3xx):

Ferritic (4xx):

Martensitic (4xx and 5xx):

Precipitation Hardened (PH):

Duplex:

1.4 Nonferrous Metals and Alloys

• silver colored

• close packed hexagonal structure (alpha phase)

• above 885°C the material undergoes beta phase transition to body centered cubic arrangements

• four commercial grades ASTM 1-4

 

• melts at 1800°C

• resistance to corrosion

• twice steels strength to density

• high affinity for carbon

• soft and ductile when annealed

1.5 Problems

Problem 1.1 Are anisotropic properties important in a material? Can a manufacturing process change whether a material is anisotrpoic?

Problem 1.2 Some materials (such as lead and tin) will recrystalize near room temperature. How does this affect design and manufacturing considerations?

Problem 1.3 A bimetallic cable is made of two types of wire strands. The 1” dia. cable is 40% mild steel, 50% aluminum, and the remainder is empty air. What is the combined stiffness of the cable?

Problem 1.4 The amount of heat generated when working a metal is proportional to the plastic deformation. Show how the work can be found using a stress strain diagram.

Problem 1.5 Describe how the cooling rate of steel affects the phases of metal structures. Why does cooling slowly result in a larger grain structure? What do phase structures contain dissimilar types of material?

Problem 1.6 What components will increase the strength of aluminum?