The first titanium alloys — including the famous Ti6Al4V, which now accounts for more than 50% of total titanium production — were developed in the USA in the late 40s. Shortly after their development, an assessment from Pitler and Hurlich noted that these new alloys showed promise at defeating small arms projectiles. Despite numerous subsequent investigations, experiments, and studies over the 1950s and 60s — which included the development of extremely strong titanium alloys — titanium body armor was never issued to US or NATO soldiers. There is just one exception: River boat crews in Vietnam were issued a light titanium-nylon flak jacket, which was not intended to stop high-velocity projectiles; it was merely a lightweight analog of WWII’s steel-nylon aircrew flak jacket.
Over the ten-year period from roughly 1996-2006, titanium enjoyed a small resurgence in the US as an armor material: Several private US companies examined monolithic titanium body armor plates on a prototype or experimental basis, the Army Research Labs investigated hot pressing titanium metal powders in 2005, and the now-defunct DragonSkin’s namesake armor vest made use of titanium-ceramic composite armor tiles. Ultimately, none of these efforts met with much success.
The Soviets, in contrast to the Americans, made extensive use of titanium as a body armor material. In 1979, shortly after the initiation of the Soviet-Afghan war, the 6B2 armor vest was issued to troops on the ground in Afghanistan. The 6B2 consisted of an array of titanium alloy plates, each just 1.25mm thick, bonded to 30 layers of twill-weave aramid. The shell was made of nylon, with Velcro fasteners — and it’s worth noting that Velcro was a very new material at the time, so its appearance on Soviet vests led to speculation as to the vest’s origins. Total system weight was 4.8kgs, including front and back portions. Protection from shrapnel and low-velocity rounds was deemed adequate, but the 6B2 was completely incapable of stopping high-velocity aimed projectiles. In fact, it reportedly could not stop 7.62x39mm rounds fired from distances of 300-500 meters. The 6B2 was therefore much like the Vietnam-era flak jacket in many respects, even in appearance, but markedly heavier.
The 6B2 was quickly replaced in service by the 6B3TM, a still heavier version, where the thickness of the titanium plates was increased to 6.5mm. This change increased the weight of the vest to 12kg. It was then changed again, in the 6B3TM01, to a version with front plates which were 6.5mm thick, and rear plates which were 1.25mm thick.  This final version was roughly 9kg in total weight. Having said all of that, it is exceedingly unlikely that 6.5mm-thick plates of titanium would stop 7.62x39mm or 5.56x45mm rounds at anywhere near muzzle velocity — but they would have done the job at engagement distances of 200-500+ meters.
In 1985, midway through the war, the 6B4 was introduced. This was an armor vest made of boron carbide and aramid, similar to those issued to certain soldiers in Vietnam, doubtless of much greater protective ability than the titanium armor vests which preceded it. Ceramic strike faces feature heavily in subsequent models of Soviet and Russian armor — though, until quite recently, body armor in Russia often comprised titanium and steel portions. This appears to have been eschewed entirely in recent years; the most advanced Russian model at present, the 6B46 “Granite 5a” armor plate, appears to be made entirely of silicon carbide over aramid.
Mechanical and ballistic properties of selected titanium alloys used in modern armor systems:
Ti6Al4V is an alpha-beta titanium alloy comprised of 90% Ti, 6% Al, and 4% V. It has a density of 4.43g/cm3, a hardness of 334HB, a yield strength of 880MPa, tensile strength of 950MPa, charpy impact at 17 J, and elongation at 14% in 2″. It has a shear strength of just 550MPa.
Russian “armor-grade” titanium is a beta-titanium alloy also comprised of 3% Al, 5.15% V, 3.65% Cr, with trace amounts of boron, zirconium, and molybdenum. It has a hardness of 387HB, and a density of 4.62 g/cm3. Its other properties are unknown, but it is not unreasonable to assume that it possesses greater yield strength and tensile strength than Ti6Al4V, an inferior charpy impact strength, and inferior elongation.
Adjusted for weight, the US Army Research labs have determined that the ballistic performance of Ti6Al4V is superior by roughly 7% to that of the aforementioned Russian alloy. Adjusted for weight, I repeat. As Ti6Al4V is about 5% lighter, what this means is that the two alloys perform nearly identically at equivalent thicknesses. The differences in mechanical properties are apparently not of great importance; the lighter alloy performs better largely on account of its lightness.
It needs to be said that the Russian alloy was tested by US Military research labs, who are not exactly a neutral and unbiased third-party in this matter, nor were the alloys tested against every projectile — they were tested only against steel FSPs. It could well be that Russian side-by-side experiments have their alloy outperforming Ti-6-4 when testing against a qualitatively different threat, e.g. lead-core ball projectiles.
In any case, it’s clear that both titanium alloys possess a great strength-to-density ratio, and both are comparable in hardness to rolled homogenous armor (RHA) steel. What is not made clear from a cursory glance at those mechanical properties is titanium’s propensity to fail via plugging, which markedly reduces its utility as a standalone armor material. When a titanium alloy plate is struck by a high velocity projectile, shear strain, strain rate, and temperature can rise to very high values over a very small surface area. Titanium’s relatively poor shear strength, combined with its very poor heat transfer properties, make it inherently susceptible to fail catastrophically in such situations via a phenomenon known as adiabatic shear plugging. This problem becomes almost insurmountable when titanium alloy armors are used at low thicknesses — thicknesses typical of body armor!
Almost insurmountable, but not totally insurmountable. See the second post on the topic of titanium armor, and the Mantis Titanium Armor Plate, from Adept Armor.
 This is reminiscent of earlier eras. As was noted in a previous post, Alexander the Great and the tacticians of the 16th century both recommended that little-to-no armor be worn on the back, “on the ground that it [is] unnecessary, and that its absence would discourage cavalry from turning its back to the enemy.” This is, to a certain extent, nonsensical in modern warfare: Epidemiological studies indicate that the back sustains just as many injuries as the front, from shrapnel in particular. However, the back doesn’t need as much protection from rifle fire — as it is also quite typical that 70-80% of battlefield casualties who have sustained bullet wounds were shot from the front; bullet entry wounds in the back are markedly less common.