An ultrafine grained (UFG) high alloy TRIP steel (16.6Cr-7.1Mn-6.4Ni-0.05C-0.02N) obtained by rotary swaging and subsequent reversion annealing was investigated in total strain controlled fatigue tests including strain amplitudes in the range of 0.3% ≤ Δεt/2 ≤ 1.2%. The UFG material exhibited grain sizes with a mean diameter of 0.7 μm and the fatigue behaviour is compared to results of its coarse grained (CG) counterpart which exhibited a mean grain diameter of 14 μm.
As expected, the cyclic stress amplitude is clearly increased for the UFG material due to the Hall-Petch effect. Furthermore, a fatigue induced martensitic phase transformation is observed down to a total strain amplitude of 0.4 % and down to 0.3 % for the CG state, respectively. Anyway, the cyclic hardening effect caused by the phase transformation is more pronounced for the CG steel, even for comparable α’-martensite portions. This is due to the already high strength of the austenitic phase in case of the UFG-steel, whose overall cyclic stress amplitudes still remain higher at any time. Regarding the fatigue life the Basquin-Manson-Coffin relationship predicts an increase for the whole range of tested strain amplitudes for the UFG state compared to the CG material. This is unusual, since most UFG-materials exhibit a superior fatigue life only at small strain amplitudes.
The microstructure after reversion annealing as well as in the cyclically deformed state was investigated by BSE (backscattered electrons) and EBSD (electron backscatter diffraction). The initial state appears to be fully austenitic with small fractions of retained deformed austenite. At small and medium strain amplitudes a lot of deformation bands are formed which are indexed as a hexagonal lattice structure in the EBSD when a certain amount of stacking faults is reached. With increasing strain amplitudes this so called ε-martensite acts as a nucleus for the phase transformation into α’-martensite, which leads to α’-martensite fractions of about 90 % at the highest strain amplitude.