9.1 - Alloy

What is an Alloy?

• An alloy is a mixture of several types of metals or a mixture of metal and non-metal in a certain percentage.
• Common alloys: steel, pewter, bronze, brass and duralumin.
• Alloying is the process of forming an alloy to improve the properties of a pure metal.
• The composition or percentage of each element can be changed to suit the alloy’s use.

Steel

• Composition: iron 99% + carbon 1%.
• Properties: hard and strong.
• Uses: construction materials for buildings and bridges; frames of vehicles and railway tracks.

Pewter

• Composition: tin 96% + copper 3% + antimony 1%.
• Properties: lustrous and resistant to corrosion.
• Use: decorative items such as photo frames.

Bronze

• Composition: copper 88% + tin 12%.
• Properties: hard, resistant to corrosion and attractive in colour.
• Uses: monuments, metal sculptures, coins and medals.

Brass

• Composition: copper 75% + zinc 25%.
• Properties: strong, shiny, malleable and gold in colour.
• Uses: keys, door knobs and musical instruments such as trumpets.

Duralumin

• Composition: aluminium 95% + copper 3% + magnesium 1% + manganese 1%.
• Properties: light, strong and resistant to corrosion.
• Use: frames of aircraft and aeroplanes.

Why are Alloys Produced?

• Most pure metals are soft and corrode easily.
• Pure-metal atoms are arranged regularly in layers.

• When a force is applied, the layers slide over one another easily; the metal changes shape.
• During alloying, atoms of another metal or non-metal are mixed with the pure metal.
• The foreign atoms have different sizes and disrupt the regular arrangement.

• The layers become difficult to slide over one another; the alloy is stronger and harder than its pure metal.

Pure metal	Alloy
Same-sized atoms arranged regularly in layers	Foreign atoms of different sizes disrupt the regular layers
Layers slide easily when force is applied	Layers are difficult to slide when force is applied
Usually softer	Stronger and harder

Particle-model description: draw equal-sized circles in straight layers for a pure metal. For an alloy, insert different-sized circles among the pure-metal atoms so that the layers are no longer regular.

Experiment 9.1: Hardness of an Alloy and a Pure Metal

• Aim: To study the hardness of alloys compared to pure metals.
• Problem statement: Is an alloy harder than a pure metal?
• Hypothesis: An alloy is harder than a pure metal.
• Manipulated variable: Type of metal block.
• Responding variable: Diameter of the dent.
• Constant variables: Diameter of steel ball, height of the weight and mass of the weight.
• Materials: Copper block, bronze block, cellophane tape and thread.
• Apparatus: Steel ball, 1 kg weight, retort stand and clamps, ruler and metre rule.

Procedure

1. Fix a steel ball on a copper block with cellophane tape beneath a 1 kg weight.
2. Hang the weight 50 cm above the copper block.
3. Release the weight so that it falls onto the steel ball.
4. Measure the diameter of the dent on the copper block.
5. Repeat twice on different parts of the block; calculate the average diameter.
6. Repeat using a bronze block.

Result and Inference

• Copper: larger average dent → softer.
• Bronze: smaller average dent → harder.
• Bronze contains tin atoms of a different size among copper atoms. The disrupted layers are more difficult to slide.
• Conclusion: Bronze, an alloy, is harder than copper, its pure metal. The hypothesis is accepted.

Experiment 9.2: Resistance to Corrosion

• Aim: To study the resistance to corrosion of alloys compared to pure metals.
• Problem statement: Is an alloy more resistant to corrosion than a pure metal?
• Hypothesis: An alloy is more resistant to corrosion than a pure metal.
• Manipulated variable: Type of nail.
• Responding variable: Presence of a brown layer on the nail.
• Constant variables: Size of nail, type of solution and duration of experiment.
• Materials: Iron nail, steel nail and water.
• Apparatus: Test tubes and test tube rack.

Procedure

1. Clean both nails with sandpaper. Label two test tubes P and Q.
2. Add 10 ml of water to each test tube.
3. Put an iron nail into P and a steel nail into Q.
4. Leave both test tubes in a rack for one week.
5. Record the presence of a brown layer on each nail.

Result and Inference

• Iron nail: brown layer forms; the pure metal rusts.
• Steel nail: no brown layer, or less brown layer, forms; the alloy is more resistant to corrosion.
• Conclusion: Steel is more resistant to corrosion than iron. The hypothesis is accepted.

Uses of Alloys in Daily Life

• Steel → skyscrapers: hard and resistant to corrosion.
• Duralumin → aircraft bodies: light and strong.
• Brass → musical instruments: shiny and resistant to corrosion.

Superconductor Alloys

• Some metals and non-metals show superconducting properties at low temperatures and are mixed to form superconductor alloys.
• A superconductor conducts electric current with high efficiency without resistance.
• It can repel a magnetic field and levitate above a magnet.
• Maglev trains: superconductor alloys in high-powered railway tracks allow the train to levitate, eliminate friction between the track and train body, and move very fast.
• Magnetic resonance imaging (MRI): niobium-titanium and niobium-tin alloys are used in MRI machines for medical imaging.