Metals combine with other metals primarily by forming alloys, which are mixtures of metallic elements held together by strong metallic bonds. This process typically involves melting the constituent metals, mixing them thoroughly, and then allowing them to solidify.
The Foundation: Metallic Bonds
Metallic bonds are the fundamental forces that hold metal atoms together. This unique type of chemical bond occurs among metal atoms and involves a "sea" of delocalized valence electrons shared across a lattice of positively charged metal ions. Unlike ionic bonds, which join metals to non-metals by electron transfer to form compounds, metallic bonding joins a bulk of metal atoms through this communal sharing of electrons. This "electron sea" model explains many characteristic metallic properties, such as their excellent electrical and thermal conductivity, malleability, and ductility.
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How Metals Form Alloys
When metals combine with other metals, they typically form alloys. An alloy is a homogeneous mixture of two or more metallic elements, or of a metallic element and a non-metallic element, that retains metallic properties. Alloys are created to achieve properties that are superior to those of the individual pure metals.
The primary methods for forming alloys include:
- Melting and Mixing: The most common method involves melting the constituent metals together, mixing them thoroughly in their liquid state, and then allowing the mixture to cool and solidify.
- Powder Metallurgy: Fine powders of different metals are mixed, pressed into a desired shape, and then heated (sintered) at a temperature below their melting point, causing the particles to bond.
- Electrodeposition: Metals are deposited simultaneously from an electrolyte solution to form an alloy coating.
More details on alloys can be found here.
Types of Alloys
Metals combine to form alloys in several ways, primarily categorized by how the atoms arrange themselves in the solid structure:
- Substitutional Alloys:
- In these alloys, atoms of one metal replace atoms of another metal within the crystal lattice.
- This typically occurs when the two metals have similar atomic radii (within about 15%) and the same crystal structure.
- Examples: Brass (copper and zinc), where zinc atoms substitute for some copper atoms; Bronze (copper and tin).
- Interstitial Alloys:
- These alloys form when smaller atoms (often non-metals like carbon, nitrogen, or boron) fit into the small spaces (interstices) between the larger metal atoms in the crystal lattice.
- The smaller atoms do not replace the host metal atoms but rather occupy the gaps, which significantly increases the hardness and strength of the material by impeding the movement of metal atoms.
- Example: Steel (iron and carbon), where tiny carbon atoms fit into the interstitial sites of the iron lattice.
- Intermetallic Compounds:
- Sometimes, metals can combine in fixed, precise ratios to form distinct chemical compounds with specific crystal structures, rather than just solid solutions. These are still held together by metallic bonds but exhibit characteristics of chemical compounds.
- Example: Some phases in aluminum-lithium alloys used in aerospace.
Why Form Alloys? Enhanced Properties
The primary reason metals are combined into alloys is to achieve a blend of properties that are superior to those of the individual pure metals. These enhancements often include:
- Increased Strength and Hardness: Alloying can make metals much stronger and more resistant to deformation.
- Improved Corrosion Resistance: Many alloys are designed to resist rust and other forms of corrosion more effectively than pure metals.
- Modified Melting Point: Alloys can have lower or higher melting points than their constituent metals, which is useful for applications like soldering or high-temperature resistance.
- Enhanced Electrical or Thermal Properties: Specific alloys can be engineered for improved conductivity or resistivity.
- Better Aesthetic Qualities: For example, sterling silver (silver and copper) is harder and more durable than pure silver for jewelry.
Common Examples of Metal-Metal Combinations (Alloys)
Here are some widely used alloys and their primary characteristics:
| Alloy | Primary Metals Combined | Key Property / Use |
|---|---|---|
| Steel | Iron + Carbon (non-metal) | High strength, durability, versatility (construction, tools) |
| Brass | Copper + Zinc | Malleability, corrosion resistance, attractive appearance (fittings, musical instruments) |
| Bronze | Copper + Tin | Hardness, wear resistance, excellent casting properties (sculptures, bearings, bells) |
| Solder | Tin + Lead (or Tin + Silver) | Low melting point for joining metals (electronics, plumbing) |
| Amalgam | Mercury + Silver, Tin, Copper | Sets quickly, durable (dental fillings) |
| Sterling Silver | Silver + Copper | Increased hardness and durability for jewelry and silverware |
| Pewter | Tin + Copper, Antimony, Bismuth | Low melting point, malleable, decorative items |
