EFFECTS OF GRAIN REFINEMENT ON CREEP PROPERTIES OF K417G SUPERALLOY

Grain size is one of the most important parameters which affect the mechanical properties of cast polycrystalline superalloys. To study the effect of grain refinement on the creep behaviors of K417G superalloy, the creep behaviors of K417G superalloy with four grain sizes were investigated at 760 degrees C/645 MPa, 900 degrees C/315 MPa and 950 degrees C/235 MPa. The longitudinal section of the fracture surface, crack propagation path, dislocation structure and plastic deformation distribution in the vicinity of the cracks were investigated by using SEM, TEM and EBSD techniques, thus the deformation mechanism and effect of grain refinement on the creep properties of K417G superalloy were determined under different creep conditions. The results showed that the effects of grain refinement on the creep property of the alloy varied with the temperatures and stress. At 760 degrees C/645 MPa, grain refinement improved the creep life and reduced the steady-state deformation rate of the alloy. The creep deformation was dominated by intragranular deformation. At 900 degrees C/315 MPa, as grain size decreased, the creep life increased firstly and then decreased, while the steady-state deformation rate decreased firstly and then increased. The creep deformation showed a competitive effect of intragranular deformation and grain boundary sliding. At 950 degrees C/235 MPa, the creep life decreased and the steady-state deformation rate increased with the decrease of the grain size. Grain boundary sliding was the main deformation mode. At the same time, grain refinement could cause a refinement of the dendrite and carbide of the alloy, which would also affect the creep behavior of the alloy to a small extent. The TEM observation showed that at 760 degrees C/645 MPa, the dislocations interacted with gamma' particles through shearing mechanism and no dislocation network was found in the matrix. While at 900 degrees C/315 MPa and 950 degrees C/235 MPa, the dislocations crossed the gamma' particles through Orowan bypass mechanism, dislocation network formed in the matrix, and M23C6 precipitated in the interior of the grains, which had a [001](M23C6)//[001](Matrix) orientation relationship between the M23C6 precipitates and matrix