Microstructural and strain rate effects on the environment-assisted cracking of alpha/beta-titanium alloys in aqueous chloride.

摘要

The objectives of this research are to determine the effect of microstructure and crack tip strain rate on environment assisted cracking (EAC) of Ti-6Al-2Sn-2Zr-2Mo-2Cr-Si, and to understand crack tip damage in terms of hydrogen/microstructure/plasticity interaction.; Beta-extrusion of Ti-6-22-22, in the α/β solution treated and aged (STA) condition, is susceptible to EAC under active loading in NaCl. K_JTH is as low as 33% of K_JIC and a fracture mode transition to transgranular (α-cleavage is affected by chloride exposure. Ti-6-4 (ELI) in chloride solution exhibits a minimum K_JTH of 65 MPa√m, corresponding to a mill-annealed microstructure with α₂ (K _JIC = 79 MPa√m), and higher EAC resistance for a Widmanstätten microstructure. EAC growth rates in Ti-6-4 (ELI) average 3μm/s and are 10-fold slower than those for Ti-6-22-22. Ti-6-22-22 was susceptible to EAC for active loading rates ranging between 10–4 MPa√m/s and 5 Mpa√m/s.; α₂, formed during slow cooling or isothermal aging, dominates the EAC susceptibility of Ti-6-22-22. α/β microstructures without intermetallics or α_s exhibit the highest EAC resistance (K_JTH = 50 MPa√m, K_JIC = 74 MPa√m). α ₂ promotes a large reduction in the EAC threshold; K_JTH = 34 MPa√m when α₂ and α_s are present (K_JIC = 69 MPa√m). For all microstructures, the fracture path is transgranular through α plates.; Based on hydrogen environment embrittlement, the increased EAC in STA Ti-6-22-22 vs STA Ti-6-4 is due to increased strain localization occurring in the Ti-6-22-22 which results in larger slip offsets at the surface and increased hydrogen uptake. α_s precipitates in Ti-6–22–22 may exacerbate strain localization; α_s precipitates were not observed in Ti-6-4.; A change in the operative dislocation mechanism to dislocation looping would have homogenized deformation and reduced EAC susceptibility; rather, severe EAC was observed in all aged Ti-6-22-22 microstructures. (Abstract shortened by UMI.)