Comparison of fatigue crack propagation behaviour in two gas turbine disc alloys under creep–fatigue conditions: Evaluating microstructure, environment and temperature effects

Abstract

Gas turbine disc materials should possess excellent fatigue and creep performance due to the severe in service conditions experienced. In this study, a comparison of fatigue crack propagation behaviour in two turbine disc alloys, i.e. N18 and low solvus high refractory (LSHR) superalloy, has been made in terms of the propagation rate and fractography observed under equivalent testing conditions. Temperatures of 650 and 725°C are compared for a trapezoidal dwell fatigue cycle (1–20–1–1) in both air and vacuum at an R ratio of 0·1. It is found that coarse grained LSHR superalloy has better fatigue crack propagation resistance than fine grained N18 in vacuum, which is ascribed to its better creep performance. Oxidation causes significant degradation of fatigue performance of these two alloys, especially in the LSHR superalloy at higher temperature (725°C), resulting in its inferior fatigue performance compared with N18. In the LSHR superalloy, it seems that oxidation is the principal contributor to the deterioration of fatigue resistance. This is supported by observations of transgranular fracture in vacuum and intergranular fracture in air. In contrast, creep is a greater contributor to the deterioration in fatigue performance of N18 (as indicated by the intergranular failure modes observed in vacuum). An apparent activation energy analysis is able to provide further insight into the underlying mechanisms of fatigue crack propagation under creep–oxidation–fatigue conditions in these two alloys.

Influence of oxidation on fatigue crack initiation and propagation in turbine disc alloy N18

“…12, and the activation energies of the related thermally activated processes reported in the literature are shown in Table 6 [8,35,36]. It seems that any additional oxidation brought about by increased dwell time and/or temperature makes little contribution to crack growth rates under the investigated conditions, this differs from our findings for LSHR alloy, a more creep-resistant but more oxidation-sensitive disc alloy [4]. …”

“…Powder metallurgy (PM) Ni-based superalloys have been widely used for aeroengine turbine disc application due to their exceptional combined mechanical properties at elevated temperatures in combination with good oxidation/corrosion resistance [1][2][3]. However, oxidation accelerated fatigue failure (shorter fatigue life or faster crack growth rate) is usually observed when assessing the fatigue performance of disc alloys at elevated temperatures, especially when a dwell period is applied at the peak load [4][5][6][7][8][9][10]. Such a phenomenon is usually associated with intergranular fracture resulting from the interaction between GB oxidation/embrittlement effects and mechanical fatigue processes [8,[11][12][13][14][15][16].…”

“…The varying extent of intergranular fracture features observed is the consequence of the competitive effects of oxidation and cyclic fatigue processes in the advancing crack. Generally, intergranular features are dominant on the fracture surface when oxidation makes a significant contribution to the crack tip failure process, whereas transgranular fracture features dominate when the effect of oxidation is absent or is weak [4,17,18]. In some cases, a transition from intergranular features to transgranular features can be observed on the fracture surface as the stress intensity factor range (ΔK) increases, indicating the point where the mechanically-driven crack propagation process outstrips the crack tip oxidation process [18].…”

“…It is relatively complex to quantitatively evaluate unambiguously the simple effect of alloy composition on crack growth. This is due to the varying microstructure, grain boundary character and mechanical properties caused by not only the varying composition, but differing mechanical processing and heat treatment approaches, although some efforts have been made [4,22,24].…”

Parsons Turbine Conference

“…Everitt et al 5 examine microstructural and environmental effects on fatigue crack propagation in two nickel base gas turbine disc alloys, N18 and LSHR. The coarse grained LSHR superalloy has better fatigue crack propagation resistance than that of fine grained N18 in vacuum, but oxidation causes significant degradation of fatigue performance, especially at higher temperature (725uC).…”