Stress-assisted versus strain-induced martensites formed by cryogenic ultrasonic shot peening in austenitic stainless steels
Résumé
Introduction Severe plastic deformation (SPD) is used to create nanocrystalline metallic materials resulting in high strength but associated, generally and unfortunately, with a reduced ductility [1]. On one side, the cryogenic temperature that improves the grain refinement by preventing dynamic recrystallization or self-annealing, has been used during SPD processes such as equal channel angular extrusion (ECAE) or high pressure torsion (HPT), effectively producing significant extra grain refinement down to the nanometer scale [2-4]. On the other side, numerous research works have been done to improve the low ductility by creating multi-length scale structures [5] or grain size gradients [6]. In steels, other mechanisms can be active and lead to a significant improvement of the strength/ductility balance such as TRIP (Transformation Induce Plasticity) [7] or the TWIP (TWinning Induced Plasticity) [8] effects. In the case of the metastable austenitic stainless steel, the TRIP effect is produced through the martensitic phase transformation. The martensitic transformation requires an activation energy to be triggered which can be produced either thermally or by a mechanical loading. Two temperatures, the Ms and Md30, are used to evaluate the occurrence of the martensitic transformation. The Ms temperature represents the temperature at which the martensitic phase transformation can be triggered spontaneously without an external loading. By applying a loading, the transformation can take place at higher temperatures than Ms and the stress or strain required to activate the process will vary with the temperature [9]. The Md30 temperature, higher than the Ms, reflects the temperature at which a martensitic fraction of 50% can be formed under a true strain of 30 %. When the martensitic phase transformation is triggered slightly higher than the material Ms temperature, elastic stresses in the microstructure are enough to activate the transformation and the elastic energy induced in the material is enough to compensate the missing chemical driving force at this temperature [11]. On the other hand, when the deformation is applied close to the material Md30 temperature, the transformation will be mainly controlled by plastic deformation and the role of deformation defects will control the transformation process [10]. The so-formed martensites can then be considered as different and called Stress-Assisted Martensite (SAM) and Strain-Induced Martensite (SIM), respectively. On the other hand, TWIP can happen when Stacking Fault Energies (SFE) is in the range 18-45 m.Jm-2 for austenitic structures. Deformation twinning is especially promoted by high strain rate. The ' martensite can be produced at the intersection of mechanical twins as this volume is double-sheared, resulting in the nucleation of the phase: (twins) '. In the case of lower SFE (<18 mJm-2), martensitic transformation can involve the formation of a transient phase named -martensite. The formation of the -martensite is driven by the insertion of Shockley partial dislocations in every two successive {111} plans [13]. The face-centered cubic austenite is consequently transformed in the hexagonal close-packed -martensite as they share their same atomic packing factor. Thus, under increasing loading, the -martensite will act as a transient phase to produce the more stable ' martensite as follows: '.
Domaines
Matériaux
Origine : Fichiers produits par l'(les) auteur(s)
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