Short answers, pulled from the story.
An alloy is a mixture of chemical elements in which at least one is usually a metal, joined by metallic bonding rather than the covalent bonds of chemical compounds. Alloys often have properties different from the pure elements they are made from, such as increased strength, hardness, or corrosion resistance.
In a substitutional alloy, atoms of similar size swap places, with some atoms of the base metal replaced by atoms of the other element, as in bronze and brass. In an interstitial alloy, much smaller atoms slip into the gaps between the larger atoms of the crystal matrix, as carbon does in steel.
When steel cools slowly, carbon diffuses out of the iron and precipitates as iron carbide, producing a soft, heterogeneous mix of cementite and ferrite. When steel cools quickly, the carbon is trapped in a diffusionless martensite transformation, leaving the steel very hard but more brittle and less ductile.
Alfred Wilm discovered precipitation hardening alloys in 1906 while searching for a way to harden aluminium for machine-gun cartridge cases. His ternary alloy of aluminium, copper, and magnesium hardened overnight at room temperature, and the result, duralumin, became the primary building material for the first Zeppelins.
Meteoric iron, a naturally occurring alloy of nickel and iron, was the first alloy used by humans. It is the main constituent of iron meteorites and was used as it was, forged from a red heat or cold-hammered into tools, weapons, knives, and arrowheads.
304 grade stainless steel, sometimes called 18/8, is an alloy of roughly 74 percent iron, 18 percent chromium, and 8 percent nickel. The chromium and nickel add strength and hardness, but their main function is to make the steel resistant to rust and corrosion.
The Krupp Ironworks in Germany developed the first stainless steel in 1912 by adding 21 percent chromium and 7 percent nickel to produce a rust-resistant steel.