Abstract
The influence of microstructure on the dwell fatigue crack growth behaviour of an advanced nickel-based superalloy was investigated at a temperature of 700°C. Microstructural variations were induced by heat treatment variables: different cooling rates of quenching from super-solvus solution heat treatment, 0.7 and 1.8°Cs−1, and an addition of a high temperature stabilisation heat treatment (2h at 857°C) between the solution treatment and the final ageing treatment. With a one hour dwell introduced at the peak load of the fatigue cycle, such different microstructural conditions can lead to a difference of up to two orders of magnitude in crack growth rates in air, when compared to those obtained under baseline fatigue loading. By performing such dwell fatigue and baseline fatigue tests in vacuum, it is confirmed that such increases in crack growth rates under dwell fatigue loading in air are mainly environmentally related. Transmission electron microscopy (TEM) was utilised to analyse both crack tip oxides and associated deformation mechanisms in the matrix. A novel mechanism taking into account competing interactions of crack tip oxidation (leading to increases in crack growth rates) and stress relaxation (leading to decreases in crack growth rates) is outlined.
Highlights
The relentless drive for lighter and more efficient gas turbine aero-engines has resulted in the development of a new generation of superalloys for turbine rotor disc applications
These observations are consistent with other studies [17,18,19], which support the argument that formation of oxides ahead of the crack tip and their rupture is the primary cause of environmentally assisted crack growth, a process captured by the term Stress Assisted Grain Boundary Oxidation” (SAGBO) in the literature
The use of block cycles in terms of alternating frequencies/waveforms and long (1 h) dwell times, has revealed some interesting characteristics of dwell fatigue crack growth behaviour: (1) At a temperature of 700 °C in air, crack growth resistance under 1 h peak load dwell fatigue loading is extremely sensitive to the size and distribution of tertiary c0 precipitates
Summary
The relentless drive for lighter and more efficient gas turbine aero-engines has resulted in the development of a new generation of superalloys for turbine rotor disc applications. Creep damage by cavity nucleation and linkage at grain boundaries can lead to accelerated crack growth [12]; while creep strain accumulation (deformation) can give rise to stress relaxation ahead of the crack tip Such stress relaxation can potentially lower ‘effective’ mechanical driving forces and may play a beneficial role in reducing rates of dwell (fatigue) crack growth [13]. With advanced material characterisation and sample preparation techniques, such as atom probe tomography (APT), and focused ion beam for site-specific sample extraction prior to transmission electron microscopy (FIB-TEM), detailed chemical analysis at the crack tip is possible Recent observations address both oxidation and oxygen segregation at, and ahead of the crack tip [17,18,19]. Together with the aid of TEM characterisation of crack tip oxides and observations of deformation, a novel mechanism for environmentally assisted crack growth is proposed
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