DIFFUSION BONDING AND FRACTURE IN Ti6Al4V SUPERALLOY

Diffusion bonded specimens of Ti6Al4V were prepared as tensile test coupons by wire-cutting. Fracture tests were performed at room temperature and temperatures up to 800ºC. The results of both the ultimate tensile stress limits and visual features by Scanning Electron Microscopy on the fracture surfaces were studied. It was found that the stress limit decreases almost linearly with temperature. The SEM surface photos showed families of intersecting lines resembling slip lines on as- fractured surfaces. An Instron machine with a heating chamber and a LEO 440i SEM were used. Preliminary analyses indicated that the fracture occurs by multiple slip of certain planes intersecting the bond surface, rather than by generation of voids and vacancies. These defects were not apparent under the magnification used, for these particular specimens. By contrast, in ductile fracture, voids are clearly seen at high magnification. Normally, these combine to create sites along which fracture initiates. In this particular experiment, ledges produced by planar slip probably initiated misfit along the bond planes. The spacing between slip lines varied with the temperature at which fracture occurred. Cracks were not visible, indicating that fracture occurred by some precursor mechanism.