Failure modes of constrained metal foils between two elastic solids are rather different from those in the unconstrained condition. If the interface adhesion is strong between materials, a lower strength thin metal (plastic) foil between two much higher strength metals (elastic) can undergo substantial plastic deformation and fail with high triaxiality induced ductile fracture. Experiments have been conducted to explore the modes of failure and the factors governing fracture in such a constrained metal interlayer. In the present work, the effects of soft inter layer thickness and brittle reaction layer on the fracture behaviour of four point bend specimens have been investigated. A series of solid state diffusion bonds were produced between 25 × 25 mm section titanium bars using pure aluminium foils of different thickness (50, 457, 914, and 2000 μm) as the soft constrained inter layer. All four point bend specimens containing an ∼ 2 μm thick intermetallic reaction layer TiAl3 between the titanium and aluminium failed in ductile fracture mode within the soft aluminium interlayer next to the interface. A number of void formations were observed ahead of the crack tip next to the interface. No evidence of interface debonding was observed. However, the specimens containing an 8 μm thick TiAl3 layer failed by brittle fracture along the interface between the titanium substrate and the TiAl3 layer. It was found that decreasing the soft interlayer thickness from 2000 to 457 μm increased the load carrying capacity and decreased the fracture toughness caused by constrained plastic deformation (high triaxiality) of the interlayer.