Influence of pores and their arrangement with respect to loading direction on high temperature low cycle fatigue of single crystal Ni-base superalloys

Abstract

High temperature Low Cycle Fatigue (LCF) is one of the main load cases of turbine blades especially in aircraft engines. Rotating turbine blades in the first stages of the turbine have to withstand very high temperatures and complex mechanical stresses resulting from the superposition of the centrifugal force and thermal gradients [1]. At present, only Ni-base superalloys are applied to operate under those conditions. In high performance turbines single crystal (SX) alloys are used, which exhibit high strength and resistance against creep and fatigue at high temperatures which is provided by the well-known γ-/γ’- microstructure and solid solution strengthening. SX Ni-base superalloys are directionally solidified along the [001] crystallographic axis. The solidification is dendritic resulting in linear aligned pores that form in interdendritic regions. Even though this material class is used for turbine blades for decades of years the role and influence of microstructural features such as pores on LCF is not yet fully understood. In order to be able to selectively test and analyze microstructural features under LCF loading a cylindrical miniature specimen were developed with a diameter of 2 mm and a measurement length of 6 mm. For this work miniature specimens were prepared from one heat treated cast plate of a second generation SX Ni-base superalloy with the chemical composition close to CMSX-4® in [001] and [010] orientation parallel to the loading axis [2]. These orientations are crystallographic identical but differ in the arrangement of the interdendritic porosity. In [001] orientation the pores are aligned parallel and in [010] perpendicular to the loading direction. Specimens were tested load-controlled at 950°C which is close to the materials service temperature. The tests have been conducted at R-ratios from 0.62 – 0.65 with triangular signal shape and a frequency of 0.25 Hz until specimen fracture. The test results show that specimens in [010] orientation fail earlier than specimens in [001] orientation . Polished cross sections show crack initiation at pores over the whole measurement length for both orientations. Fractographic investigations reveal the alignment of the pores on the fracture surface in case of the [010] specimens. These observations lead to the conclusion that pores and their arrangement are limiting the high temperature LCF life.