Creep Damage Assessment of Notched Material Made of a Solidification Control Ni-Base Superalloy Using the EBSD Method

Changes in misorientation with deformation were measured by various misorientation analysis methods using the electron backscattered diffraction (EBSD) method, and quantitative assessments were attempted to estimate the amount of strain or damage. Misorientations were correlated with macroscopic plastic or creep strains for comparative well-strained materials such as austenitic stainless steels. Ni-base superalloys used for components requiring high temperature strength such as gas turbine blades, have low ductility with precipitation of the γ’ phase in grains, therefore the change of crystal orientation was considered to be extremely suppressed in comparison with austenitic stainless steels. In addition, it was anticipated that the extremely large grains of Ni-base superalloys made it difficult to properly assess the damage as misorientation. However, with the current advances in the EBSD acquisition systems in conjunction with scanning electron microscopy, it has become possible to make unprecedented resolved measurements of the local crystal structure distribution at a millimeter scale. In particular, in order to assess the damage of gas turbine blades, the complex blade inner cooling system complicates the distribution of temperatures and stresses in the blades, which implies that it is required to assess the influence of geometry at stress concentrated regions in addition to the condition of temperatures, stresses and creep fatigue wave forms. To date, in the case of the conventional casting material or the same geometry notched specimen of the directionally solidified (DS) superalloy, the average misorientation which means the grain reference orientation deviation (GROD) within grains in a certain predetermined evaluation area including the notch increases linearly up to the initiation of creep cracks regardless of the testing temperatures, strain rates and the effect of fatigue under the creep dominant condition. However, the different notch geometry of the DS superalloy shows the different characteristics of the misorientation development. This paper focuses on a misorientation parameter which can assess the creep crack initiation life independent of the geometry at stress concentrated regions. In order to assess the creep crack initiation life at various stress concentrated areas of the DS superalloy, the development of a unified life assessment method independent of the individual notch geometries was discussed. As a result of this study, a parameter dividing the GROD by the initial notch opening value, φ0, was proposed and it was confirmed that the proposed parameter, GROD/φ0 shows similar characteristics with the relative notch opening displacement (RNOD) curves which correspond to the local strain energy and the initiation of creep crack at the notch tip independent of the geometry at a stress concentrated region.