Due to its specific strength and  good biocompatibility, Ti-6Al-4V alloy find many applications in  medical and aeronautic industries.

At room temperature, the studied TA6V alloy presents a bi-modal microstructure consisting of hcp alpha phase together with bcc beta phase. When subjected to high cooling rates such as in welding processes, a lamellar hcp alpha’ phase appears, which modifies the plastic and damage behavior of the alloy.  In order to better understand the role of microstructure on failure of the material, a clear connection must be made between microstructural parameters such as interfaces, grain or lath orientation, grain boundaries, morphologies and failure initiation.  

The deformation behavior of both welds and base metals were characterized by in situ EBSD tensile tests.  Slip and twin systems were identified for bimodal and fully martensitic microstructure. Deformation paths were followed upon deformation, and related to original laths or grain orientation, see Figure 1. At higher deformation levels, damage starts and cavity appear equally at grain boundaries, phase boundaries, or inside grains. However, if a large number of information is available from these 2D observations, 3D characterization is required for a quantitative damage study as most of the cavities initiate in the volume and not at the surface. The same samples were then characterized by in situ tensile X Ray Tomography tests conducted at ESRF Grenoble.

For several deformation levels, a quantification of the cavity number and size was performed  and a clear description of the cavity initation, growth, and coalescence was obtained in 3D, as reported on Figure 2.

Thanks are due to Aymeric Migliarini, Lea Noirot Coisson and Cyril Lavogiez for their active implication during the project.

Figures:

Figure 1: In situ EBSD maps of the same area of the weld ( IPF Tensile Axis) ( a) before and (b) after 15% deformation. A clear rotation of the central lath is observed, while laths with different initial orientation were sheared, or twinned. The local deformation was measured from image correlation and related to microstructural features and deformation mechanisms.

Figure 2 : In situ X Ray Tomography volumes prensenting the cavity initiation, growth and coarsening in the base metal.

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EMC Abstracts - https://emc-proceedings.com/abstract/an-in-situ-multiscale-approach-for-the-characterization-of-plasticity-and-damage-in-a-ta6v-alloy/