The ductility of bcc-Materials is often insufficient for structural applications at low temperatures. The transition to brittle fracture below a critical temperature is likely caused by two possible mechanisms. Either the dislocation mobility is limited or the nucleation sources cannot be activated. In order to find out, semibrittle materials with dislocation gradients were investigated with microcantilevers.
In order to apply achieve a specific initial dislocation density, indentations were made on a sample surface. The resulting plastic strain field in the material was analyzed by FEM in order to identify areas with homogeneous deformation for fracture tests. The cantilevers for the fracture tests had a diameter of 3000 nm and were milled with a FIB. Testing was achieved by two different approaches. On the one hand an in situ setup with a micromanipulator was used to directly observe crack growth and tip blunting. On the other hand an ex-situ nanoindenter setup was used, which offered better displacement resolution and a larger load range.
The measurements show the dependence of the transition in the fracture behavior on the availability and activity of dislocations sources. Nevertheless the fracture toughness is also dependent on the mobility of dislocations at different temperatures.