Structural metals are rated for their resistance to failure under stress (fracture toughness) by a factor called K1C. We start with a pre-cracked specimen and measure the stress needed to cause the crack to grow. In air test, Titanium and its alloys show very high K values when compared with other metals. When a corrosive environment (usually standard sea salt) is added to the test, we can rate metals with a factor called KISCC, for the onset of Stress Corrosion Cracking. The failure mode is quasi-cleavage, no ductile deformation on the crack surface. The inter-granular (along grain boundaries, not through grains) has a definitive "Rock Candy" appearance. All calculations are based on an infinite length crack, edge effects, ductility from the free surface, invalidates the test if over an arbitrary 10%.
Thin sheet specimens work well for other metals, but Titanium is so tough, thick plate is required for K1C and KISCC testing. Because of its outstanding resistance to corrosion, Titanium was thought to be immune from stress corrosion failure. Then in 1965 Brown, at the NRL, reported SCC in 6-4 Titanium! He had seen Rock Candy!!! Nothing in science is accepted until it is repeated by another independent lab. My boss called me in and appointed me as our "expert" on SCC of Titanium.
I found that SCC is defined as delayed cracking at a stress below the yield strength of the material. So long term testing of thick plate specimens was required. I built a huge rack with recording timers and other equipment. Because of the very high fatigue limit ( 70% for 6-4, 50% for steels) making the high cycle, low stress fatigue cracks in many one inch thick plate specimens was a chore! Normal production “in spec." materials showed nothing, so I had our melt lab produce "Off spec" ingots that were rolled down to the one inch thickness. Still nothing, so I tested unusual heat treatments. Finally, after six full months of work, I at last saw my rock candy surface!
I found that I must produce just the right starting fatigue crack in material that was over spec in both Aluminum and Oxygen content. I had to heat treat the material for long times at 1300F to precipitate the intermetallic compound Ti5Al3 in the microstructure. I had to start the salt water drip before applying any stress to avoid work hardening of the root of the crack. Then I did the math, no SCC. The root stress was well over the yield strength of the metal. I never saw true Stress Corrosion Cracking in 6-4 in sea water and I know of no one else finding it.
One interesting side note: we used full lab prep for our sea salt solutions. To see how important this was, we diluted the solution by 50%, then another 50%, then another with the same results. We tested pure distilled water, same results! We had always seen some variation in our baseline "air" testing that overlapped our sea salt tests. Humid days gave lower tests. Just the ad/absorbed film of water in the crack from the air can skew the test results. Aint Science wonderful????
