In situ TEM investigation on void coalescence in metallic materials

More precise modeling on ductile fracture of metals at the scale from micron to meter is still urgently needed in many engineering applications. Due to the variety of metal and its alloys, a lack of understanding on the mechanisms and quantitative experimental data impede building and assessing mechanics models for ductile fracture at different length scales. In this paper, in situ tensile tests are carried out in Transmission Electronic Microscope (TEM) on copper, high entropy alloy and aluminium alloy. We examine the full process of void growth and coalescence of neighboring voids under the view of the TEM. Nanotwins on the ligament between neighboring voids lead to a new coalescence mechanism for copper and high entropy alloy. Necking and shearing coalescence of aluminium alloys are clearly illustrated, which are demonstrated by the samples with manually drilled hole before by SEM (Scanning Electronic Microscope). The quantitative data on the evolution of void geometry are recorded, and are then used to verify the existing coalescence models in the literature. It is found that the McClintock model for void coalescence provides a better prediction than the Brown-Embury model (including the modified Brown-Embury model). A multiscale homogenization framework for in situ experiments can be further used to extract the stress state around the voids, and explaining the importance of stress-state on cavitation/nucleation, growth and coalescence of voids.