Nano oxide dispersion strengthened (n-ODS) steels are being considered for high temperature applications owing to their remarkable high temperature strength and creep resistance. The potential applications of these materials are cladding tubes in generation IV fast breeder reactors and turbine blades in super-critical steam power plants. In the present work, high temperature plastic deformation behavior of n-ODS-18Cr ferritic steel is investigated over a wide range of temperatures (RT to 1573 K) and strain rates (10-10 to 10 s-1).
Hot compression tests were carried out at a range of temperatures from 1273 to 1573 K and strain rates from 10-2 to 10 s-1 on upset forged billets to determine the optimum conditions for thermo-mechanical processing. Compression tests were carried out on hot extruded material over a wide range of temperatures from RT to 1173 K and a range of strain rates from 10-4 to 10-2 s-1. Also, creep tests were carried out on the hot extruded material at a range of temperatures from 923 to 1023 K and stresses from 175 to 400 MPa. Detailed microstructural characterization was done using Electron backscatter diffraction, Transmission electron microscope and Small-angle X-ray scattering.
Three different regimes were noted based on the variation of flow stress (normalized with shear modulus) with temperature. Deformation mechanisms operating at all the three regimes were found out. Interestingly, it was noted that the Orowan strengthening mechanism is the only dominant mechanism operating in regime IV, i.e. 1273 1573 K.
From the creep curves, steady state creep rate was estimated and it was used to calculate the stress exponent and activation energy according to Mukherjee-Bird-Dorn equation. The calculated apparent stress exponent (> 17) and apparent activation energy ~ 690 kJ/mol are higher than the generally observed values in metals and alloys. This can be attributed to the origin of the threshold stress owing to the attractive interaction between nanoprecipitates and dislocations. Finally, the experimental results were rationalized based on the threshold stress model, dislocation detachment model and mechanical threshold stress model.