The Effect of Direct Current Heating on the Creep Behaviour of Polycrystalline Ni-Based Superalloys

This study discusses the effect of the direct current heatingDirect current heating (DCH) on the creepCreep behaviour of Ni-based superalloysNi-based superalloys, in an attempt to converge on best practice and also reliability. Particular features are the use of an electro-thermal mechanical testing (ETMT)Electro-Thermal Mechanical Testing (ETMT) system which has allowed for a direct comparison of the DCH heating method with the more conventional resistance furnace heating (FH) approach. In addition, particular attention is paid to a testing protocol to confirm the accuracy of temperature measurement during DCH. WaspaloyWaspaloy, Alloy713CAlloy713C, and Alloy246Alloy246 are used in different geometrical configurations, thus enabling us to establish the difference in creepCreep behaviours caused by altering the sample sizes and the heating methods. The creepCreep rupture lives of the materials tested by the DCH method were consistently shorter than that tested by the FH method. Microstructural investigation revealed that there is no influence of the heating method on the coarseningCoarsening rate of the fine γ′ precipitatesPrecipitates and on the thickness of the oxide layer in WaspaloyWaspaloy during creepCreep test at 732 °C. Moreover, the presence of an axial temperature gradient did not influence the creepCreep life unduly. An analytical model calculation indicated that the radial temperature gradient is not sufficient for the short creep life. It is hypothesised that the diffusivity is increased by the direct current, and the dislocation climb is facilitated in the DCH testing for WaspaloyWaspaloy.