Analysis of high angle boundaries in directionally solidified turbine blade made of CMSX-4 (registered) superalloy

High angle boundary (HAB) and low angle boundary (LAB) are casting defects of single crystal (SX) and directionally solidified (DS) turbine blades and decrease the lifetime of these blades during service. During directional solidification, primary dendrite arms grow in the opposite direction of the thermal gradient direction and perpendicular to the mushy zone interface. When this interface is not flat, primary dendrite arms growing in various areas of the mushy zone are characterized by different growth directions. Then, after the primary dendrite tips contact each other, LABs or HABs form (depending on the angle between the directions of the primary dendrite arms). This paper presents characterization studies of HABs in a DS turbine blade made of CMSX-4 (registered) superalloy. The blade was characterized by three columnar grains with HABs. Qualitative and quantitative analyses of the HABs using electron backscatter diffraction in a scanning electron microscope (SEM-EBSD) were carried out. The EBSD technique helped to determine the crystallographic orientation of the grains near the HAB, misorientation angles between grains (and inside each grain), and the angle of deviation between the [001] direction and the blade axis.