The stress-corrosion cracking in Ti-8Al-1Mo-1V

2017-03-29T05:58:35Z (GMT) by Sheng Cao
Compared to the widely used Ti-6Al-4V, Ti-8Al-1Mo-1V (Ti-811) is characterized by a lower density and a higher elastic modulus. However, it suffers from stress-corrosion cracking (SCC). Crystallographic texture and ordered a2 precipitates are two known factors influencing the SCC properties of Ti-811. However, previous studies were conducted on materials with an intrinsic texture, and the discussion of texture effects was on a macroscale. In this work, electron backscatter diffraction (EBSD) was applied to investigate the effect of microtexture in materials with an intrinsic texture, and designed hot isostatic pressing (HIPping) and post heat treatment scheme were conducted to study the effect of a2 precipitates without the complicating effects of texture. The result shows that the presence of both microtexture and a2 precipitates will increase the SCC susceptibility. EBSD investigation illustrates that the SCC crack propagation direction was aligned with the microtextured regions in wrought Ti-811, and a grains were favourably orientated for basal < a > slip along the SCC crack. In the absence of both crystallographic microtexture and a2 precipitates, there was no SCC crack propagation. Therefore, it is shown for the first time that SCC susceptibility in Ti-811 can be eliminated entirely by implementing the HIPping process and post heat treatment used in this work. 
A fracture mode transition from dimples to facets was presented at the starting point of aqueous NaCl SCC tests in both milled Ti-8Al-1Mo-1V bar and powder hot isostatic pressed (HIPped) Ti-8Al-1Mo-1V materials. An understanding of the transition between the pre-crack region (fractured in air) and SCC crack region (failed in 0.1M NaCl aqueous solutions) is needed to reveal the SCC mechanism. Transmission electron backscatter diffraction (T-EBSD) and transmission electron microscope analysis on focused ion beam (FIB) lift-out lamellae were used to study the dislocation nature and density underneath the fracture surface. It was found that the fracture mode transition accompanied with changes in activated dislocation type from basal < a > to basal < a > and < c + a >, and increased dislocation density, those changes are believed to be related to the absorbed and diffused hydrogen in SCC. The possible SCC mechanism might be a combination of absorption induced dislocation emission (AIDE) and hydrogen enhanced localized plasticity (HELP) mechanisms for Ti-8Al-1Mo-1V.