The following is about Titanium (really Titanium alloys) that were used to build some Russian submarines in the Cold War (ending in the early 1990s). Titanium alloys are metals that contain a mixture of Titanium (Ti) and other chemical elements. Such alloys have very high tensile or yield strength and toughness. They are light in weight compared to steel and are very corrosion (including seawater) resistant.
The cost and difficulty of working with Titanium alloy suggests Russia will not return to using it for submarine pressure hulls.