Alloys: why mixing metals makes them harder sim
Test sixteen real samples: press a hard ball in to measure hardness, bend strips until they snap, find density by displacement and leave steels in salt water. Then drag the layers of atoms and see why different-sized atoms make it much harder for the layers to slide.
Metals and alloys: why alloys are harder, their compositions, properties and uses · covers the GCSE, IGCSE, O-Level topic
What the sim shows
An alloy is a mixture of a metal with another element. In a pure metal the atoms are all the same size and sit in neat layers, which slide over each other when a force is applied, so the metal is soft and bends easily. In an alloy, atoms of a different size (zinc in brass, tin in bronze, chromium and nickel in stainless steel, copper and silver in carat gold, or small carbon atoms in the gaps in steel) distort the layers, so it is harder for them to slide: the alloy is harder and stronger. Test it: press a 5 mm tungsten carbide ball into each sample with a 250 kgf load; the harder the metal, the smaller the dent (Brinell hardness HB = 2F/(πD(D − √(D² − d²)))). Bend strips until they snap: hardened high-carbon steel is hard but brittle. Find density from mass and the rise in water level. Leave samples in salt water: plain steel rusts, stainless steel does not. Pure gold is 24 carat; 18 carat is 18 parts in 24 gold, which is 75 %. Hardness and density values are rounded typical data-book values (ASM handbooks, the Copper Development Association, Johnson Matthey); in this range Brinell and Vickers numbers are about the same. Model limits: one load is used for every sample (real tests use bigger loads for steels and smaller ones for soft gold); the number of bends to snap and the salt-water results are illustrative; a week of salt water is shown one day per click.
- Sample tray: sixteen metals and alloys
- Hardness tester: a 5 mm carbide ball at 250 kgf
- Vice, balance and cylinder, salt-water dish
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