To understand and use Titanium, you must first forget most of what you know about metals. A metal's physical properties are controlled by its crystal structure. Titanium has a "hexagonal close packed" structure, other structural metals, Iron, Aluminum and Magnesium, are "Cubic." If you are a real student of structures, you know that the ideal hexagonal and cubic structures are the same, just viewed from a different perspective. Titanium’s hex is almost, but not quite, ideal. This is a very important feature that gives Titanium some of its outstanding properties.
Even the old, very common Titanium alloy 6-4 (6% Al, 4% V) has a higher strength to weight ratio than any steel or Aluminum alloy. Add in outstanding resistance to corrosion, fatigue and creep and Titanium really shines. Corrosion resistance is a good example of how odd-ball Titanium can be. Gold and Platinum are corrosion resistant because they are inert, Titanium is corrosion resistant because it is highly reactive.
Scratch a Titanium surface and it instantly reacts with air to reform the TiO2 surface. Add acid and electric current to drive the reaction and the TiO2 just gets thick enough to stop the reaction, called anodizing. Even stranger, it works both ways. Plating shops use Titanium for anode baskets to hold the metal to be dissolved and cathode racks to hold the pieces to be plated.
Toughness is another property of materials that shows the odd nature of Titanium. A notch in a metal bar concentrates the stress at that point. The testing standard is a notch with a radius of 0.010 inches, which yields a 6.5 X stress concentration at the root of the notch. A steel or Aluminum notched bar tensile test will fail at a stress much lower than a smooth test bar. Titanium gets stronger. A 6-4 bar, heat treated to 165KSI smooth will yield over 200KSI notched bar. The same concentration takes place, but the Titanium work-hardens under the notch instead of cracking. The hexagonal structure has less available slip planes, but, again, it is almost ideal, so it resists cracking.
Welcome to the wonderful world of TITANIUM!!