Mechanism of Crack Initiation Induced by Oxide-Layer Cracking in High-Temperature Fatigue of the Nickel Base Single Crystal Superalloy CMSX-2
Naoya TADA, Ryuichi OHTANI and Atsuhide WADA
Abstract:Fatigue tests were conducted at 1273K using smooth bar specimens of the first-generation nickel-base single crystal superalloy CMSX-2. Oxide-layer on the surface of the specimen and the base metal were carefully observed by means of scanning electron microscope after the tests. It was clarified by the observation that crack initiation in high-temperature fatigue of CMSX-2 was mainly brought about by cracking of the oxide-layer which was formed on the surface of the specimen. Comparing the results obtained from the tests with different stress and strain waveforms, cracking of the oxide-layer found to be easily occurred when the following two conditions are satisfied. The first one is sufficient time for the formation of oxide-layer since cracking occurred in thick oxide-layer and the crack continued to grow inside the base metal. The second condition is that there is tensile-going strain after the formation of oxide-layer. The brittle oxide-layer was easily cracked by the following tensile loading. Moreover, there observed change in ƒÁ/ƒÁ'-structure during the fatigue test and this structural change brought the decrease in cyclic deformation resistance. This might have quickened the oxide-layer cracking particularly in stress-controlled condition. Key Words:Single crystal, Nickel-base superalloy, CMSX-2, High-temperature fatigue, Oxide-layer, Crack