High temperature properties of wrought nickel base superalloy EI 698 VD in creep fatigue conditions

Abstract

Isothermal cyclic creep (ICC) and cyclic creep with additionalthermal-mechanical fatigue stress component (TMF) were performed on a wrought nickelbase superalloy EI 698 VD. Both ICC and TMF tests were investigated in term ofdeformation characteristics and dislocation substructures of specimens. Transmissionelectron microscopy was conducted to examine the deformation substructures and revealcorrelations with both ICC and TMF deformation behaviours. They also were compared topure creep and high temperature fatigue at same maximum loading and temperature. Alltests were tensile load controlling at operated temperature of 650 degree C. Theeffect of the individual tensile hold periods in both tests was studied. Theexperimental results exhibit that the introduction of tensile hold periods in ICCtests causes an increase in fracture life time proportional to the duration of theindividual tensile hold periods. In TMF tests which were produced by cooling atcyclic creep between individual tensile cycles showed scattered and unreliableresults which might be due to the heterogenity of MC coarse carbide affecting to theinitial strain of tested specimens.