Influences of microstructure and oxidation processes on fatigue failure mechanisms in advanced turbine disc Ni-based superalloy

Abstract

Most in-service Ni-based turbine disc alloys have a bi-modal or tri-modal γʹ size distribution, where primary γʹ size (or presence) is linked to the duration and temperature of the solution heat treatment (e.g. super or sub-solvus) whilst secondary γʹ and tertiary γʹ size are linked to the ageing temperature and the cooling rates. The effect and relative balance of these different γʹ size populations on slip character is a matter of continuing debate. Therefore it is necessary to establish the role of γʹ size on slip character for unimodal distributions to allow a deeper understanding of deformation behaviour. The accumulation of damage is intrinsically affected by the degree of slip heterogeneity at both low and high temperatures and this can affect both crack initiation and growth processes. In this work, a non-commercial heat treatment has been used on RR1000 alloy to produce two different unimodal γʹ size distributions. The heat treatments also caused a varying carbide distribution on grain boundary, with more continuous grain boundary carbides observed in the coarse γʹ variant. These carbides on grain boundaries also play a key role in strengthening and weakening damage mechanisms over a range of temperatures. Short fatigue crack tests (uninterrupted and interrupted tests) have been conducted on polished plain bend bars of both fine and coarse γʹ variants under three point bending, with a sine waveform (20Hz) and trapezoidal waveform (90s dwell time) loading applied at room temperature and 650^oC in air, respectively. At room temperature, in terms of deformation and slip character, it is seen that the fine γʹ variant exhibits more γʹ shearing and heterogeneous slip character (relatively higher planar slip and intense slip bands), which is more linked to less resistance to crack initiation while the coarse γʹ variant is apt to show more Orowan looping and homogeneous slip character. In both γʹ variants fatigue cracks mainly initiated at pores at the surface or sub-surface, slip bands or along twin boundary presenting a crystallographic faceting. High strain levels and more diffuse strain localisation take place in the vicinity of crack initiation areas such as pore and slip bands. Crack initiation along the twin boundary occurs with relatively high Schmid factor (SF) and further propagation is inclined to follow high SFs of adjacent grains with the activated slip systems. Short fatigue crack growth rates in both γʹ variants show a high extent of scatter at low ΔK level as a distinctive short crack behaviour, which is significantly linked to the complexity of local microstructure and intrinsic shielding effects. Under the testing conditions in this study, the lifetimes appear to be strongly connected to the number of crack initiation sites (partly linked to initiation resistance), which seems more associated with the defect distribution or grain misorientation than γʹ alone. More crack initiation sites generally lead to the multi-site coalescence events that contribute to enhanced fatigue crack growth rates and reduced lifetime. Three dimensional (3D) evaluation (sub-surface) of early crack growth processes by X-ray CT scanning has also been conducted on micro-tensile samples of the coarse γʹ variant at room temperature. Internal microstructure has been investigated from a series of tomographically reconstructed two dimensional (2D) slices assessing porosity distribution and complex morphology of the early crack, showing the crack propagated along slip bands slanted at around 45^o. Overall, the lifetimes at 650 ^oC are significantly shortened by the effect of dwell time and consequent oxidation damage. At elevated temperature, fatigue crack initiation mechanisms are influenced by synergistic effects of the microstructural features and oxidation processes. Cross slip and homogenous slip character are promoted in both γʹ variants due to the effects of elevated temperature. On applying a 90s dwell time, a large amount of grain boundary cracking at bulging Co-rich oxides can be seen on the surface in both γʹ variants. Such plentiful intergranular cracking promotes frequent crack coalescences and results in significantly accelerated fatigue crack growth rates, leading to shortened lifetime. The bulged oxides at particular grain boundaries are associated with enhanced mismatch strain and stress concentration, resulting in oxide cracking. Cracks also initiate at the interface between oxidised carbides and grain boundaries due to volume expansion and concomitant strain localisation. The complex influences of microstructural variation, loading frequency (linked to diffusion time), temperature and oxidation on long fatigue crack growth behaviour have also been assessed. Long fatigue crack tests on single edge notched bend samples of both γʹ variants have been carried out under three point bend and a trapezoidal waveform loading with dwell times of 1s and 90s at peak load at 650 ^oC and 725 ^oC in air. Overall, in both γʹ variants, intergranular failure modes are dominant and even more marked with longer dwell time and higher temperature. The fine γʹ variant shows a better fatigue resistance at 650 ^oC and 725 ^oC for both dwell times, and this is more marked with longer dwell time and higher temperature. The coarse γʹ variant exhibits more time dependent crack growth and distinctly more intergranular fracture modes, which can ascribed to the more continuous carbide distribution on grain boundaries. Oxidation damage zones ahead of the crack tip in both γʹ variants were investigated by switching between low and high loading frequencies at 650 ^oC, showing that the coarse γʹ variant exhibited the most significant oxidation damage zone in the transition. Such damage zones are linked to embrittlement cracking and accelerated crack growth rates. 3D analysis by X-ray CT scanning as well as 2D observation showed that the crack path at low frequency (longer diffusion time per cycle) is relatively more tortuous and contains greater secondary crack formation, with a discontinuous crack path and uncracked ligaments seen due to significant oxidation damage and embrittled zones formed which is more marked in the coarse γʹ variant.

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ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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