Metal Alloys.

1. Alloying a metal is done by combining it with one or more other metals or non-metals that often improve its properties.

2. The term alloy is used to describe a mixture of atoms in which the primary constituent is a metal.

3. The primary metal is called the base.

Reasons for Alloying.

• To improve its properties such as Toughness. Example: Iron is relatively soft, Carbon is brittle but Steel is very tough.

• To reduce the melting point. Example: Lead melts at 327 °c, Tin melts at 232 °c but Solder melts at approx. 183 °c.

• To prevent rusting. Example: Stainless steel is produced by alloying iron with chromium and carbon.The chromium forms an impenetrable oxide layer that adheres strongly to the iron surface preventing the formation of rust.

Aluminium Alloys.

However, pure aluminium is soft and lacks strength, but alloyed with small amounts of copper, magnesium, silicon, manganese, and other elements gives it very useful properties.

Aluminium is very abundant and is used in pure form for a lot of different things, like kitchen foil, mirrors, coins and industrial components.

Substitutional Alloys are where the metals combining have atoms of the same size. Brass is an example.

Interstitial Alloys are where one of the alloying elements combining has atoms of smaller size. Steel is an example. Carbon atoms are much smaller and fit into the gaps in the Iron atoms.

Classification of Alloys.

Substitutional Alloy.

Interstitial Alloy.

Solid Solution Alloys.Solid Solution Alloys:When metals combine they sometimes become completely soluble in each other. Metals which combine in this way are said to form solid solutions. When this type of alloy solidifies, only one type of crystal is formed. Under a microscope the crystalline structure of a solid solution alloy looks very like a pure metal. Solid solution alloys have similar properties to pure metals but have greater strength. They also have poorer electrical/thermal conductivity, greater hardness but not as elastic as pure metals.

How are Metal Alloys made.

Lead Tin

+ =

Solder

The metals forming the alloy are melted in a furnace in a crucible. They are then poured into blocks called ingots and allowed to solidify. When a molten metal is mixed with another molten metal and allowed to solidify an alloy is formed. The new solid metal will have gone through ‘Phase’ changes to become an alloy.  

Metallic Phases.

HeatMore Heat

CoolingMore Cooling

What is Thermal Equilibrium.

Left click with the mouse on the link above to play the video.

https://www.youtube.com/watch?v=vcgYQ-u7qfc

What are Thermal Equilibrium Diagrams.

Thermal equilibrium diagrams are special diagrams which contain information about changes that take place in alloys. The temperature at which a particular alloy changes from liquid to solid is an example of the type of thermal information contained in a thermal equilibrium diagram.

Cooling Curve of a Pure Metal.Cooling Curve of a Solid Solution (An Alloy).

Set of Cooling Curves for various Solid Solutions.

Developing a Thermal Equilibrium Diagram.

How a Thermal Equilibrium Diagram is Constructed.

Label the Thermal Equilibrium Diagram below.

• Solidus Line – All points below this line are in solid form.

• Liquidus Line – All points above this line are in liquid form.

• Pasty Region - Between the solidus and liquidus lines, all points are in a mixture of liquid and solid (pasty) form.

• Liquid Region – The alloy is completely liquid in this region.

• Solid Region – The alloy is completely solid in this region.

Parts of the Thermal Equilibrium Diagram.

Effects of Alloying Elements on Steel Properties:

Carbon: Raising carbon content increases hardness slightly and wear resistance considerably. Manganese: Small amounts of Manganese reduce brittleness and improve forgeability. Larger amounts of manganese improve hardenability, permit oil quenching, and reduce quenching deformation. Silicon: Improves strength, toughness, and shock resistance. Tungsten: Improves "hot hardness" - used in high-speed tool steel. Vanadium: Refines carbide structure and improves forgeability, also improving hardness and wear resistance. Molybdenum: Improves deep hardening, toughness, and in larger amounts, "hot hardness". Used in high speed tool steel because it's cheaper than tungsten. Chromium: Improves hardenability, wear resistance and toughness. Nickel: Improves toughness and wear resistance to a lesser degree. Including these elements in varying combinations can act synergistically, increasing the effects of using them alone.

(a) Draw a thermal equilibrium diagram from the table listed below.

Classwork /Homework:

(b) Label the diagram and describe its main features.

(c) For the alloy 50% B, determine from the diagram the ratio of the phases at 400 °C

Homework Answer:

(a)

(b)

Liquidus line:  for the alloy system this line represents the boundary between the fully liquid state and the beginning of solidification.Solidus line:  the boundary line that determines the end of solidification. Below this line, the alloy is completely solid.Liquid region:  the two metals are in a liquid state.Solid region:  the two metals are in a solid state.Pasty region:  between the liquidus and solidus lines, the alloy is in a partly liquid and a partly solid state. 

(c)

a) Explain any two of the following: (i) Allotropy; (ii) Interstitial solid solution; (iii) The difference between amorphous structures and crystalline structures;

(iv) The degree of brittleness in body-centered cubic (bcc) structures and the degree of brittleness in face-centered cubic (fcc) structures.

Homework/Question.

BCC FCC

(i) Allotropy: Allotropy is the ability of a material to exist in different forms.

(ii) Interstitial solid solution: An atom from another element moves into the space between the atoms of the parent metal lattice. This causes compression of the surrounding atoms and will strengthen the material as it takes a higher stress to cause deformation.

(iii) Amorphous structures do not have a pattern in the arrangement of their atoms but are a more random structure. Pitch, glass and some plastics have this type of structure. Crystalline structures have atoms that are bonded together in a pattern that is repeated. Metals with bcc and fcc unit cells are examples of a crystalline structure.

(iv) Brittleness in bcc and fcc structures: In the bcc structure, the structure is arranged with an atom at the corner of a cube and an atom in the centre of the cube. This structure is associated with brittleness.

In the fcc structure atoms are at the corners of a cube and a single atom in the centre of each face of the cube. Atoms are more tightly packed which allows metals to be more ductile.

Homework/Answer.

The table shows the solidification temperatures for the various alloys of metal A and metal B. The melting point of metal A is 1083 ºC and the melting point of metal B is 1453 ºC. Using the graph paper supplied: (i) Draw the thermal equilibrium diagram according to the given data; (ii) Label the main features of the diagram; (iii) Determine, from the diagram, the ratio of the phases at 1250ºC for the

alloy of 50% metal B.

Homework/Question.

% of metal B in alloy

10 20 30 40 50 60 70 80 90

Start of solidification (°C)

1160 1220 1270 1320 1350 1380 1400 1430 1440

End of solidification (°C)

1080 1090 1110 1140 1170 1220 1270 1330 1380

Homework/Answer.

(ii) Label the following features:Liquidus line, Solidus line, Liquid region, Solid region and Pasty region

(iii) Ratio of phases at 1250 °C for 50% metal B:

Mass of solid is 50-26 = 24 = 3Mass of liquid 66-50 16 2

1083 °C

1453 °C

1250 °CLiquid Region.

Solid Region.

Liquidus Line.

Solidus Line.

Pasty Region.