Formation conditions of mechanically modified superficial structures for two steels
Résumé
The aim of this work was to study the formation mechanisms of tribologically transformed structures (TTS). As these transformations need not be associated with phase modifications but usually with grain refinement or nanostructuring, the term mechanically modified superficial structures (M2S2) has been proposed to name any intermediary modification of material structures between the initial and nanostructured states. Severe plastic deformation processes allow producing transformations of material in volume and reproducing conditions that are close to those found under contact. Thus, a Bridgman Anvil apparatus which allows shearing samples in torsion under pressure is used to test two carbon steels (AISI 1005, AISI 1045). This experimental study is coupled with finite element analysis used to simulate the test and to study the specificity of the stress and strain cycles of the transformed layers obtained. For both materials, M2S2 layers are produced. The results show that the two materials follow the same trends. M2S2 formation can be associated with high hydrostatic pressure, high plastic strains, high strain rates and high strain gradients. This work is a first step for proposing mechanical criteria for the M2S2/TTS formation, but also for studying the influence of the initial alloy (microstructure, composition) on these transformations.