[wterrell]

we must keep in mind the balance of the pistol and feel in our hand.
With lightweight pistols the forces are not resisted by the weight of the gun, but are transferred to the hand and wrist.
Also, the two prime considerations of frame construction would be shock and elasticity.
Titanium alloys are quite different than pure titanium. The range of characteristics of these alloys is so wide and divergent that very few characteristics can be attributed to the whole titanium genre.
(Lack of elasticity would result in fracturing of the titanium. There are equations presented in most metalurgy texts demonstrating the relationship of force, shock, and elasticity which will answer your questions quickly.)
Aluminum presents the same problems.