The mechanical surface treatments confer better local mechanical properties against wear or fatigue service conditions. In the case of impact-based treatments, a local microstructure refinement in the near surface is produced by a severe plastic deformation of the material, leading to a progressive reduction of the grain size over a few tens of microns, and consequently an increase of the hardness and mechanical properties. These zones are commonly known as Tribologically Transformed Surfaces (TTS). In this project, the micro-structural transformation in the near surface is produced on pure α-iron samples using a repetitive impact-based procedure: Micro-percussion treatment. In this technique, every impact is effectuated on the same position with a rigid conical indenter (tungsten carbide), controlling the number, angle and velocity of impacts. The resulting imprint (figure 1) is characterized by a significant grain size refinement and consequently a graded strengthening as a function of distance to the impacted surface. Moreover, several in-situ micro-pillar compression tests are carried out in the cross-section of the hyper-deformed surface (figure 2) in order to quantify this mechanical property gradient in-depth. However, the yield strength increment observed with this technique does not reveal the different micro-structural contributions (grain size effect, dislocation hardening, etc…) on the increase of mechanical properties. Indeed, the main purpose of this work is to correlate the mechanical properties gradient with the local microstructural evolution produced by the impact-based severe plastic deformation. For these purpose, EBSD mapping (figure 1) is used to determine the grain size distribution and the local “Kernel Average Misorientation” (KAM) in the cross section. A qualitative estimation of the geometry necessary dislocation density could be done from this latter estimation. With this analysis, the Hall-Petch and dislocation strengthening contributions could be correlated and compared with the experimental results from micro-pillar compressions (figure 2).

Figures:

Figure 1: EBSD map (indexation step size 300 nm) in the cross-section of a micro-percussion test imprint: 10000 impacts, 15° et 150 mm/s.

Figure 2: Micro-pillars for in-situ compression tests in different regions of the imprint cross-section: TTS zone and single crystal region.

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EMC Abstracts - https://emc-proceedings.com/abstract/microstructural-and-mechanical-properties-of-hyper-deformed-surfaces-in-situ-micro-pillar-compression-and-ebsd-investigations-in-%ce%b1-iron/