Effect of Microstructure on Creep Crack Growth Behavior of a Near-α Titanium Alloy IMI-834

In the present study, the effect of microstructure (i.e., alpha+beta and transformed beta) on creep crack growth (CCG) behavior of a near-alpha (IMI 834) titanium alloy has been explored at temperatures 550 degrees C and 600 degrees C. For characterizing the CCG behavior of the alloy, both stress intensity factor (K) and energy integral parameter (C-t) were used in the present investigation. The use of stress intensity factor (K) as crack-tip parameter is not appropriate in the present study as no unique correlation between crack growth rate and K could be obtained from the observed trend due to transients in the creep crack rate data. On the other hand, C-t parameter for both microstructural conditions consolidates CCG data into a single trend. The alloy with fully transformed b microstructure exhibits better CCG resistance as compared to bimodal (alpha+beta) microstructure. This is consistent with the fact that the transformed b structure offers superior creep resistance as compared to alpha+beta microstructure. Microstructural examination has revealed that CCG for both microstructural conditions is accompanied by formation of damage zone in the form of numerous environmental-assisted secondary surface cracks (perpendicular to the stress axis) ahead of the main crack tip. For alpha+beta microstructure of the alloy, the surface creep cracks were formed by growth and coalescence of microcracks nucleated by fracture of primary a particles. While in the interior of the specimens, CCG occurred by growth and coalescence of microvoids nucleated at primary alpha/transformed beta (matrix) interfaces. For b microstructure of the alloy, while the surface creep cracks formed by growth and coalescence of microvoids nucleated at titanium enriched surface oxide particles, in the interior CCG occurred by nucleation of intergranular cavities.