Void Growth and Coalescence Studied by X-Ray Computed Tomography

Abstract

<p>Void growth and coalescence were investigated by means of in situ X-ray computed tomography coupled with tensile deformation. A variety of model materials were fabricated whereby artificial three dimensional void arrays were embedded in metal matrices. An ultra short-pulsed laser machining system and diffusion bonding technique were used to produce those model materials. Model materials containing a pair of round notches were also produced to investigate the effect of stress triaxiality. In situ X-ray tomography experiments successfully visualized the void growth and coalescence events in the model materials produced. The void growth behavior was quantitatively analyzed by measuring the principal diameters of the internal voids.</p> <p>For several model materials, a new technique called continuous tomography was applied. The experiments were performed at the beamline ID15 of the European Synchrotron Radiation Facility (ESRF), France. This technique allows us to keep taking a series of tomographic scans during non-stop tensile deformation. This new technique enabled us to experimentally capture the plastic strains at the onset of void coalescence for the model materials. The onset of void coalescence here was defined as the point at which the lateral shrinkage of voids terminates. The captured plastic strains at the onset of void coalescence were compared for the first time ever with the existing void coalescence models which are essentially designed to predict the onset of coalescence instead of the simple linkage of voids.</p> <p>Similar experiments were also conducted using a set of notched model materials at the same beamline ID15. A significant influence by the higher stress triaxiality and the strain concentration induced by the notches on the void growth in these model materials was observed. This aspect was confirmed by the FE simulation performed. The captured linkage strains were used to assess the validity of the void coalescence models at a higher stress triviality.</p> <p>Model materials made of Glidcop and brass were also tested using the same methodology. This set of experiments were performed at the beamline BL20XU of SPring-8, Japan. The results obtained from these samples suggested that the higher work hardening exponent in brass stabilized the plastic flow of the ligament while the lower work hardening exponent in the Glidcop materials significantly accelerated the linkage events. Nevertheless, the critical values of work hardening rate normalized by the current flow stresses were quite similar (lower than the critical value of localized necking) in individual materials (copper, Glidcop, brass) even if the plastic strains at linkage and coalescence events of the materials are different.</p>