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

Abstract

In the present study, the effect of microstructure (i.e., a + b and transformed b) on creep crack growth (CCG) behavior of a near-alpha (IMI 834) titanium alloy has been explored at temperatures 550°C and 600°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 (a + b) microstructure. This is consistent with the fact that the transformed b structure offers superior creep resistance as compared to a + b 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 a + b 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 a/transformed b (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.

Effect of primary α content on creep and creep crack growth behaviour of near α-Ti alloy

“…A recent study 4 has revealed that a path independent energy line integral parameter like C* or C t is a more appropriate crack tip parameter than the stress intensity parameter K for characterising CCG behaviour in Timetal 834 Ti alloy. This is because the stress intensity parameter K could not uniquely describe the crack growth rate data over the entire range, as it does not take transient effects in crack growth rate into account.…”

“…Several investigators1 – 3 have reported that the Ti alloys with fully transformed β structure exhibit superior creep and stress rupture properties as compared to bimodal microstructure consisting of primary α and transformed β . Furthermore, a recent study by the authors4 has revealed that the Timetal 834 alloy with fully transformed β structure also exhibits better creep crack growth (CCG) resistance. However, Ti alloys with fully transformed β structure is well known to exhibit inferior fatigue properties 2,5 – 7.…”

Effect of microstructure on creep and creep crack growth behaviour of titanium alloy

“…Detailed fractographic and microstructural examination of damage zone has revealed that the creep crack growth in stable crack growth regime comprises two stages viz., (i) nucleation of microvoids / microcracks around crack tip and (ii) growth and coalescence of microvoids / microcracks, which subsequently joins the main crack. Microcracks have been found to form, by fracture of primary α particles at the specimen surface in bimodal microstructure (Fig.4b) [4]. This may be due to the well documented fact that α phase has got more affinity as well as solid solubility for oxygen.…”

“…A recent study [4] has revealed that a path independent energy line integral parameter like C* or C t is more appropriate crack tip parameter than the stress intensity parameter, K, for characterizing creep crack growth behaviour in IMI 834 titanium alloy. This is because the stress intensity parameter, K, could not uniquely describe the crack growth rate data over entire range as it does not take transient effects in crack growth rate into account.…”

“…Several investigators have reported that the titanium alloys with fully transformed β structure exhibit superior creep and stress rupture properties as compared to bimodal microstructure consisting of primary α and transformed β [1][2][3]. Further, a recent study has revealed that the alloy with fully transformed β structure also exhibits better creep crack growth resistance [4]. However, titanium alloys with fully transformed structure are known to exhibit inferior fatigue properties.…”