An attempt to produce ex situ TTS to understand their mechanical formation conditions - The case of an ultra high purity iron
Résumé
High pressure torsion (HPT) tests can be used to transform the structure of materials. Several studies in the literature highlight that such tests are a means of investigating certain formation mechanisms of Tribological Transformed Structures (TTS) outside contacts, as they reproduce conditions that are close to those found inside them (quasi-hydrostatic pressures and shearing) under conditions that are easier to control than in a real contact. As there is no unique path leading to deformation with TTS in real contacts, it has been decided to simplify the history of deformation as the first step in investigating the occurrence of TTS. In this study, a Bridgman anvil apparatus, which allows shearing samples in torsion under pressure, is used to test ultra high purity iron (99.99999%) with an initial grain size of 100 μm, under a mean pressure of 0.5 GPa. The microstructural investigations led to establishing a map identifying the refined zones within the volume of the sample and linked to contact conditions. This work considers these zones with the finest microstructure (size grains of about 220 nm) as "TTS". The experimental study is coupled with finite element analysis to investigate the specificity of the stress and strain cycles of the microstructure produced. Although further work is needed to validate the suggested comparison to friction induced transformed layers, the results showed that "TTS" are produced in a specific sample volume linked to sliding/adhesion contact conditions. The coupling between experiments and modeling shows that "TTS" formation can be associated with high hydrostatic stress components, high local plastic strains and high strain gradients.