Crack growth threshold under hold time conditions in DA Inconel 718 – A transition in the crack growth mechanism

Abstract

Aeroengine manufacturers have to demonstrate that critical components such as turbine disks, made of DA Inconel 718, meet the certification requirements in term of fatigue crack growth. In order to be more representative of the in service loading conditions, crack growth under hold time conditions is studied. Modelling crack growth under these conditions is challenging due to the combined effect of fatigue, creep and environment. Under these conditions, established models are often conservative but the degree of conservatism can be reduced by introducing the crack growth threshold in models. Here, the emphasis is laid on the characterization of crack growth rates in the low ΔK regime under hold time conditions and in particular, on the involved crack growth mechanism. Crack growth tests were carried out at high temperature (550 °C to 650 °C) under hold time conditions (up to 1200 s) in the low ΔK regime using a K-decreasing procedure. Scanning electron microscopy was used to identify the fracture mode involved in the low ΔK regime. EBSD analyses and BSE imaging were also carried out along the crack path for a more accurate identification of the fracture mode. A transition from intergranular to transgranular fracture was evidenced in the low ΔK regime and slip bands have also been observed at the tip of an arrested crack at low ΔK. Transgranular fracture and slip bands are usually observed under pure fatigue loading conditions. At low ΔK, hold time cycles are believed to act as equivalent pure fatigue cycles. This change in the crack growth mechanism under hold time conditions at low ΔK is discussed regarding results related to intergranular crack tip oxidation and its effect on the crack growth behaviour of Inconel 718 alloy. A concept based on an "effective oxygen partial pressure" at the crack tip is proposed to explain the transition from transgranular to intergranular fracture in the low ΔK regime.

Relation between crack growth behaviour and crack front morphology under hold-time conditions in DA Inconel 718

“…Reducing the loading frequency also leads to increased CGRs. At high temperature, under hold-time conditions, increased CGRs are accompanied with intergranular fracture while transgranular fracture is usually observed under pure fatigue loading, or under hold-time conditions in the near-threshold regime [11].…”

The Mechanism of Creep during Crack Propagation of a Superalloy under Fatigue–Creep–Environment Interactions

“…In general, the fatigue-crack growth rate of an alloy increases with the increase of hold-time, because of the effects of creep and oxidation. Consequently, the creep damage at the grain boundary is aggravated and the embrittlement phenomenon caused by oxidation occurs, that accelerate the expansion of cracks [ 26 , 27 ]. However, it was found that the acting time of creep had a significant effect on the fatigue-crack growth rate in 718Plus alloy ( Figure 4 ), especially in the low stress intensity factor range (Δ K ).…”