Strain Rate Dependent Environment Assisted Cracking of α/β-Ti Alloys in Chloride Solution

Abstract

Environment assisted cracking (EAC) occurs in annealed-α/β Ti-6A1-4V (ELI) and Ti-6Al-2Zr-2Sn-2Cr-2Mo in aqueous chloride solution, as evidenced by rising-displacement threshold stress intensity factors well below KIC and a mode transition from ductile fracture to transgranular cleavage of α. The acicular-α/α2 microstructure of Ti-6-2222 is susceptible to severe cracking in 3.5% NaCl compared to the equiaxed-α/α2 structure of Ti-6-4. Thresholds and growth rates depend on loading rate, and appear most environment sensitive at intermediate crack tip strain rates. EAC is sustained to high loading rates (dK/dt = 0.3 MPa√m/s for Ti-6-4 and above 10 MPa√m/s for Ti-6-2222) and crack growth rates are rapid (10 μm/s). The hydrogen environment embrittlement mechanism is consistent with such rapid cracking kinetics, but only if the process zone is within 0.001 to 1 μm of the crack tip surface. A near-tip process zone may be promoted by high H produced electrochemically on active areas of the crack surface, or by a sharp crack tip.