Low Cycle Fatigue of Single Crystal Nickel-Base Superalloy CMSX-4 Coated With a New Coating IC1

High strength nickel base superalloys have often been used in turbine blades because of their superior performances at high temperatures. One of them is CMSX-4, an ultra high strength, single crystal. CMSX-4 is a second generation rhenium-containing, nickel-base superalloy capable of high temperature and stress operations of at least 1150 °C [1]. The superalloy has limited oxidation and corrosion resistance at the high temperatures and to improve the oxidation and corrosion resistance, the base material is protected with coatings [2]. However, coatings exhibit a ductile-to-brittle transition temperature (DBTT) which causes early cracking of the coating and failure due to fatigue. The paper details low cycle fatigue (LCF) properties and degradation mechanisms of uncoated and IC1 coated single crystal CMSX-4. The tests were performed at two temperatures, 500 °C and 900 °C. Cylindrical solid specimens were cyclically deformed with fully reversed tension-compression loading with total strain amplitude control and at a constant strain rate of 10⁻⁴s⁻⁵ in air atmosphere without any dwell time. At 500 °C the coating has a detrimental effect on the fatigue life of CMSX-4 while at 900 °C IC1 does improve the fatigue life of the superalloy. The reduction of the fatigue life can be related to early cracking of the coating under its ductile to brittle transition temperature while the beneficial effect of the coating at 900 °C may be due to slower propagation of cracks caused by oxidation at the front of the crack tip.