An alloy is a metal blended with other elements so that its internal structure, and so its strength, hardness, corrosion resistance or casting behaviour, differs from the pure metal. Steel with at least 10.5% chromium becomes stainless, aluminium alloyed and heat treated rises from about 120 MPa to above 500 MPa, and copper with 10-12% tin becomes bronze. Each family works through a different structural mechanism, and in a settlement they are first met as scrap that must be identified before it is remelted.
Part of Materials Science
Advanced alloys are engineering metals beyond plain carbon steel: stainless steels, aluminium alloys and bronzes, each designed to give a property that the pure metal lacks.
Alloying Works By Changing The Crystal In One Of Four Ways
Pure metals are seldom the best choice. The Advanced Materials article notes that pure aluminium is soft and weak, and Bronze Making says pure copper is soft while pure tin is too brittle for tools. Alloying fixes this by one of four routes:
| Mechanism | What happens | Example in this section |
|---|---|---|
| Solid-solution strengthening | Foreign atoms of a different size jam the slip of atomic planes | Tin in copper, giving bronze |
| Precipitation hardening | Fine particles form inside the grains and block slip | Aluminium alloys |
| Phase change on cooling | The crystal structure switches, trapping carbon | Quenched steel, see Heat Treatment |
| Protective film | An alloying element forms a stable surface oxide | Chromium in stainless steel |
The thresholds quoted in the corpus show how different the routes are: steel needs at least 10.5% chromium to become stainless, aluminium alloyed and heat treated climbs from about 120 MPa to above 500 MPa, and copper reaches its best balance with 10-12% tin. The mechanism decides what you can do afterwards. A precipitation-hardened part loses its strength if you weld it. A stainless steel can fail if its film cannot re-form. A bronze is hardened by its composition and cannot be hardened by quenching at all.
Match The Family To The Need
| Need | Family | Why |
|---|---|---|
| Resist corrosion in food, medicine or chemicals | Stainless steel | The chromium oxide film repairs itself |
| Light and strong structure | Aluminium alloys | One third the density of steel |
| Bearings, bells, seawater fittings, castings | Bronze alloys | Casts well, resists seawater, runs against steel |
| Cutting edges and dies | Tool steel | High carbon hardens to a lasting edge |
Tool steel is covered in its own article in the industrial-production section and is not repeated here.
The Realistic Source Is Scrap, Not Ore
The alloys are expensive to make from raw materials. Stainless Steel Production puts ferrochrome at roughly 4,000 kWh per tonne, and Aluminum Smelting puts aluminium at about 15 kWh per kilogram. Both are large-power projects. A settlement is far more likely to meet these metals as sinks, cookware, fasteners, pistons, window frames and bearings, and to sort and remelt them.
Sorting uses simple field tests:
- Magnet. Steel and ferritic stainless are attracted. Austenitic stainless, aluminium and bronze are not.
- Weight. Aluminium feels strikingly light, and the Advanced Materials article gives its density as 2.7 g/cm³ against 7.8 for steel.
- Colour and ring. Bronze is yellow-brown or red-gold and rings; the metals are discussed in the articles above.
- Spark test. Heat Treatment describes the spark test for carbon steel; aluminium and bronze give no sparks.
Common Mistakes
- Melting mixed scrap together — the result is an unknown alloy. Keep each family separate and cast a test bar from each melt.
- Expecting heat treatment to survive remelting — a remelted aluminium alloy has to be heat treated again, and an unknown alloy may not respond.
- Using a family outside its limits — aluminium loses strength in a hot fire, and high-tin bronze cracks under a hammer.
What’s Next
- Learn next: Stainless Steel — the alloy where a surface film does the work
- Combine with: Phase Diagrams — reading what an alloy will do on melting and cooling