Thanks Micks, that is part of MA I did not know about - a good resource
Some Googling:
Strength (and hardness) is inversely proportional to ductility; this means the greater the tensile strength (and hardness) the less ductile and more brittle is your material. Since it is already established that Ti alloys (like, Ti-Al-V) have high values of strength, it follows that they would be hard but brittle, which means while it would require greater stresses to fracture a Ti alloy than say Aluminum, once that stress is reached, a Ti alloy will instantaneously fracture and fail, unlike an Al alloy that will plastically deform significantly before failing.
What is peculiar about titanium and its alloys is that even very limited amounts of other elements, in quantities that in steel would be acceptable as impurities or would be beneficial, particularly oxygen and nitrogen are detrimental for toughness. Titanium is a sensible fella. He embrittles very rapidly if you allow him to oxidise or pick up other elements (he tends to pick them all up and by the way that’s why he's so difficult to machine, he has a tendency to allow other elements to diffuse into itself very easily).
