Study of SCC Mechanism by Direct Observation and Analysis of Dislocation Substructure in α Phase of Ti-8Al-1Mo-1V in Stress Corrosion Cracking

Ti-8Al-1Mo-1V has an extremely high stress corrosion cracking (SCC) susceptibility. Crystallographic texture and α₂ precipitates are influencing its SCC property, and their effects have been studied by comparing the following samples tested in 0.1M NaCl aqueous solution:

1)      Milled Ti-8Al-1Mo-1V bar, which had α₂ precipitates and crystallographic texture.

2)      Hot isostatic pressed (HIPped) Ti-8Al-1Mo-1V, which had α₂ precipitates but no texture.

3)      HIPped Ti-8Al-1Mo-1V was heat-treated to supress α₂ phase transformation, and it did not have any crystallographic texture.

Presence of crystallographic texture (sample 1) increases the SCC crack velocity by 1 magnitude and decreases the SCC stress intensity factor more than 50% compared to sample without (sample 2). There is no SCC crack propagation in sample 3 due to the absence of texture and α₂ precipitates.

In the SCC tests of sample 1 and 2, there was a fracture mode transition from ductile failure during pre-cracking where dimples are dominated on fracture surface to brittle failure during NaCl environment where facets are dominated. In order to understand the SCC fracture mechanisms, transmission electron backscatter diffraction (T-EBSD) and transmission electron microscope (TEM) analysis were applied on focus ion beam (FIB) lift-out lamellas cut from pre-cracking region and SCC fractured region to investigate the relationship between the fracture surface and crystallographic orientation of sub-grains, and nature of activated dislocation underneath the fracture surface.

It has been found that:

• Ti-8Al-1Mo-1V does not suffer SCC when crystallographic texture and α₂ precipitates are eliminated.
• In pre-cracking region where failure occurred in air,  slip was the dominate slip system and slip with  component was also activated.
• The SCC fracture surfaces were either parallel to or inclined 15^o away from basal planes of a. Dislocations containing  component are observed to contribute to such failure and surprisingly no   dislocations are observed.