Evolution of geometrically necessary dislocation at the gamma-gamma ' interface and its effect on tensile deformation behaviour of disk super alloy

Abstract

The evolution of geometrically necessary dislocation density (GND) at the gamma and primary gamma' interface during tensile deformation of turbine disk alloy was studied. Extensive electron microscopy analysis (i.e. EBSD and TEM) was carried out to explore the structure and GND evolution at the gamma-gamma' interfaces. It is found that GND value is highest for tensile test specimens at 800 degrees C, whereas the lowest for tensile specimen tested at 350 degrees C, and this is attributed to gamma/gamma' misfit (delta) value. At high-temperature tensile deformation (i.e. at 720 and 800 degrees C) specimen starts necking just after the yield point, compared to RT, 350 and 650 degrees C tensile tested specimens. It is evidenced from GND values and grain interior distortion that localized plastic deformation in gamma-matrix near to gamma-gamma' interfaces is the primary reason for necking just after yielding of material. Higher GND values and grain interior distortion are caused by the greater value of delta at high test temperature due to difference in thermal expansion between gamma and gamma'.