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

Abstract

Abstract:Fatigue crack initiation and propagation behaviour in subsolvus heat treated turbine disc alloy N18 has been assessed in air and vacuum at 650 and 725 o C under three-point loading.Fatigue crack initiation processes have been evaluated using single edge U-notch specimens under a 1-1-1-1 trapezoidal loading waveform along with interrupted tests at 650 o C to allow intermittent observations of the notch surface. The results show apparent grain boundary (GB) oxidation can occur under an oxygen partial pressure of 10 -2~1 0 -3 Pa. Cracks mainly initiate from grain boundaries or γ/γʹ interfaces due to the formation and subsequent cracking of Crrich and/or Co-rich oxides, and occasionally initiate from surface pores. Fatigue life in these tests appears to be dominated by this crack initiation process and is significantly reduced by increasing temperature and/or application of an oxidizing environment. Crack growth tests conducted under 1-1-1-1 and 1-20-1-1 loading waveforms indicate that oxidation significantly degrades the crack growth resistance of N18 and is associated with more intergranular fracture surface features. Additional oxidation effects on propagation caused by higher temperature or prolonging dwell time appear limited, whereas a prolonged dwell period seems to instead promote additional creep process, which further enhance crack growth, especially at higher temperature.

Long-Life Fatigue of Carburized 12Cr2Ni Alloy Steel: Evaluation of Failure Characteristic and Prediction of Fatigue Strength

“…Ek  (8) where E(k) is between 1 and π/2. Thus, ΔKSD and ΔKSSA can be rewritten as follows: Figure 10.…”

“…High carbon [3], Ti-6Al-4V [4], and high-strength alloy steel [5] have been shown to transition between surface failure in short-life region and interior failure in long life region and form typical double S-N curves. In the long-life regime, although some factors relating to the fatigue properties of materials have been previously studied, such as load type [6,7], environment [8], defect size [6,9], surface conditions [10,11], and frequency [12], the development of a fatigue strength prediction model has still not been intensively studied.…”

Effect of microstructural characteristics on the low cycle fatigue behaviors of cast Ni-base superalloys

“…increase of temperature. Oxidation behaviors of Ni-base superalloys have been widely studied [17,18], and oxidation on alloy surface plays an important role in crack initiation and propagation [8,[19][20][21]. Fig.…”

Failure of Nickel-based super alloy (ME3) in aerospace gas turbine engines

“…These unique characteristics (Gao et al, 2005) are: exceptional elevated temperature strength, high resistance to creep, oxidation, corrosion (Jiang et al, 2015), and good fracture toughness.…”