The effect of primary α content on creep and creep crack growth behaviour of a near α-Ti alloy has been investigated at 600°C. The alloy was heat treated at different temperatures so as to obtain different volume fractions of equiaxed primary α in the range from 5 to 40%. Constant load creep tests were carried out at 600°C in the stress range 250–400 MPa until rupture of the specimens. Creep crack growth tests were carried out at 600°C and at an initial stress intensity level of 25 MPa m1/2. Creep data reveal that minimum creep rate increases and time to rupture decreases with increase in primary α content indicating that higher primary α leads to creep weakening. On similar lines, maximum creep crack growth resistance is associated with the alloy with lowest primary α content (i.e. 5%). Microstructural and fractographic examination has revealed that creep fracture occurs by nucleation, growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces. On the other hand, creep crack growth occurs by surface cracks nucleated by fracture of primary α particles as well as by growth and coalescence of microvoids nucleated at primary α/transformed β (matrix) interfaces in the interior of the specimen ahead of the crack tip.
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.
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