Transformation induced plasticity as well as plasticity induced phase transformation appear when the corresponding driving force from the parent (P) to the daughter phase (D) increases or/and when the kinetics of the transformation becomes favourable. In steels, this condition usually corresponds to an important temperature variation or/and to a high plastic deformation. The result is then likely to bring a dramatic refinement of the microstructure and often leads to an improvement of mechanical properties of the alloy. In the present work, the effect of cyclic plastic deformation on microstructural transformation and on the damage mode in a high purity austenitic stainless steel is investigated. The low cycle fatigue tests have been carried out on smooth samples, under plastic strain control, at temperatures between -50 and 120℃. Observations of different martensitic morphologies appearing at the surface, at the grain or twin boundaries, along cracks or inside the austenitic grains are presented. It is shown that although the fatigue deformation amplitude appears to be low, the cumulated deformation is high enough to promote martensitic transformation of the austenite. Moreover, local stress concentrations at the tip of short fatigue cracks can be high enough to produce local transformation even if the usual threshold conditions for a given temperature (M_d) corresponding to the overall applied stress or strain, in particular plastic strain amplitude and cumulative plastic strain, are not reached. In such conditions, the fatigue crack propagation can take place exclusively in the martensite which is continuously formed in front of the crack tip without any variation of the macroscopic behaviour of the material. Local phenomena, e.g. martensitic transformation favoured by stress/strain concentrations induced by the interaction between slip bands and grain boundaries, are also responsible for the relationship between the volume fraction of martensite formed at a given plastic strain amplitudeΔε_p and the grain size of the material. The threshold temperature (M_d) for the fatigue induced martensitic transformation appears therefore as dependent on the morphological parameters of the microstructure. The aim of the discussion is to draw some conclusions from the comparison of the specific case investigated in this work with the generic one in order to evaluate the respective roles of stress and of strain and to predict some possible application in the field of thermome-chanical treatments.
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