Titanium has a lower density than steel. Its stiffness is less than steel but significantly greater than aluminum. These properties make it ideal for many applications. High performance bicycle frames and golf clubs are made of titanium. Titanium with 4% aluminum is popular for surgical implants such as artificial joints since it is well tolerated by the body's tissues, resists corrosion, is strong and not too stiff. Another special property is that it can handle very high temperatures. The SR-71 "Blackbird" spy plane, built out of titanium alloy, flies so fast that its surface temperatures can reach nearly 1000°F. This actually succeeds in annealing the metal.
Although titanium is a very common metal, alloy development didn't begin until the late 1940's. The common ore, TiO2, could not simply be reduced with carbon because carbides were produced. An indirect chemical route was discovered. First titanium oxide reacts with chlorine gas to produce titanium chloride, with carbon carrying away the oxygen. Then the titanium chloride is reduced with magnesium. High temperatures are required for these reactions.
Even after the metal was available, there were still obstacles to its application. Molten titanium metal will react with atmospheric oxygen or nitrogen, so new processing methods in inert atmospheres were needed to replace existing forging and machining techniques.
Large amounts of oxygen make titanium strip brittle, more like the ore, but titanium with 0.3% oxygen is strong, harder to bend than pure titanium. Bicycle frames have used that alloy, as well as one that contains 3% aluminum and 2.5% vanadium. Slightly higher amounts of those metals result in a hard metal suitable for jet engines.
In 1959 the Soviet Union decided to build a titanium submarine. Compared to steel it had higher strength, lower weight, resisted corrosion. For identical hull weights, a titanium hull could withstand higher pressures, allowing deeper dives. Since titanium was non-magnetic it could also avoid magnetic mines and detection devices.