Most bike frames are constructed using tubes made from aluminum alloys. These are cheap, corrosion resistant, fairly lightweight and easy to form into frames. Numbers like 6061, 6063 and 7005 are a code for the additives (chiefly silicon and magnesium) that are mixed with the aluminum to form each blend.
Steel is much denser than aluminum and much stronger too. This means frame builders can use smaller-diameter, thinner-walled tubes and maintain the same strength and stiffness. Weight is higher, but the skinny tubes make it easier to design in a little vertical flex for a comfier ride. Steel is harder and more expensive to mass-manufacture than aluminum.
3. Carbon fiber
Carbon fiber is a composite of sheets of strong carbon threads bonded together with resin. The advantage for bike frames is that it’s significantly lighter than metal for a given stiffness. It’s pricey, and expensive molds make it uneconomic for brands to change their designs regularly.
The main advantage of titanium over aluminum or steel is its fatigue resistance. Most metals have a defined number of load cycles before they’re likely to fail. Titanium is far more resilient to repeated stresses and strains, and this means skilled frame fabricators can build frames lighter and with more compliance without risk of failure. For a price.