Stress state mechanism of thickness debit effect in creep performances of a Ni-based single crystal superalloy

Weight reduction and advanced cooling schemes to improve aircraft engine efficiency drive the design of turbine blades toward thin-walled structures. The decrease of wall thickness leads to a decrease of creep rupture life and/or the increase of creep rate, which is termed as the thickness debit effect. To capture the nature of this effect and improve the reliability of turbine blades, creep tests at 1100 degrees C/130 MPa are carried out on sheet samples of a Ni-based single crystal superalloy with thicknesses from 0.60 to 2.17 mm. The creep rupture life is found to decrease with the decrease of sample thickness linearly, exhibiting an obvious thickness debit effect. The analysis of the microstructure evolution demonstrates that the rafting mechanism of the gamma' phase of all samples remains the same (i.e. stress-induced, diffusion-controlled process) and is independent of the sample thickness. The fracture morphology reveals that the fracture mechanism of thin sample is a mixed fracture including micro voids and cleavage-like planes, while that of thick sample is micro voids-dominated. Environmental degradation suggests that surface damage due to oxidation and nitridation is not the primary causes for thickness debit effect, yet promotes the effect integrally. Finite element analyses and the observed dislocation configurations reveal that the thin sample is in a plane stress state, that activates six slip systems of the type {111}< 110 > and one slip system of the type {111}< 112 > during creep, while the thick sample is close to the ideal uniaxial stress state which activates all possible eight slip systems of the type {111}< 110 > and two slip systems of the type {111}< 112 >. Less slip systems activated in thin sample induce less work hardening, which results in heterogeneous deformation and shorter rupture life.