FRACTURE-BEHAVIOR OF DIFFUSION-BONDED BIMATERIAL TI-AL JOINTS

Failure modes of constrained metal foils between two elastic solids ar e rathel different from those in the unconstrained condition. If the i nterface adhesion is strong between materials, a lower strength thin m etal (plastic) foil between two much higher strength metals (elastic) can undergo substantial plastic deformation and fail with high triaxia lity induced ductile fracture. Experiments have been conducted to expl ore the modes of failure and the factors governing fracture in such a constrained metal interlayer. In the present work, the effects of soft interlayer thickness and brittle reaction layer on the fracture behav iour of four point bend specimens have been investigated. A series of solid state diffusion bonds were produced between 25 x 25 mm section t itanium bars using pure aluminium foils of different thickness (50, 45 7, 914, and 2000 mu m) as the soft constrained interlayer. All four po int bend specimens containing an similar to 2 mu m thick intermetallic reaction layer TiAl3 between the titanium and aluminium failed in duc tile fracture made within the soft aluminium interlayer next to the in terface. A number of void formations were observed ahead of the crack tip next to the interface. No evidence of interface debonding was obse rved. However, the specimens containing an 8 mu m thick TiAl3 layer fa iled by brittle fracture along the interface between the titanium subs trate and the TiAl3 layer. It was found that decreasing the soft inter layer thickness from 2000 to 457 mu m increased the load carrying capa city and decreased the fracture toughness caused by constrained plasti c deformation (high a triaxiality) of the interlayer.