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

This study discusses the effect of the direct current heating (DCH) on the creep behaviour of Ni-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) 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. Waspaloy, Alloy713C, and Alloy246 are used in different geometrical configurations, thus enabling us to establish the difference in creep behaviours caused by altering the sample sizes and the heating methods. The creep 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 coarsening rate of the fine γ′ precipitates and on the thickness of the oxide layer in Waspaloy during creep test at 732 °C. Moreover, the presence of an axial temperature gradient did not influence the creep 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 Waspaloy.