In situ nanomechanical testing of magnetron sputtered RCCA films: The effect of RCCA composition on room temperature ductility

Refractory metal complex concentrated alloys (RCCAs) have excellent high temperature strength but limited ductility at room temperature. Literature has shown that composition plays an important role in determining RCCA strength and ductility. More specifically, the inclusion of Group IV elements (Ti, Zr, Hf) tends to lower strength and increased ductility while the inclusion of Group VI elements (Cr, Mo, W) has the opposite effect.

Our experiments systematically study how the balance of Group IV, V, and VI elements in magnetron sputtered RCCA films affect ductility and fracture toughness. In order to minimize the grain boundary effects on the mechanical performance, the RCCA films were grown epitaxially on MgO (100) substrates and the magnetron sputtering processing conditions were chosen to promote large grains.

The performance of the RCCA films is evaluated using nanoindentation hardness, microcantilever beam bending, and in situ observations of crack tip opening. The small scale of the RCCA films made tensile tests impractical, so the microcantilever beam geometry was chosen so that elongation-to-fracture could be evaluated. Microcantilever beams with were prepared using a focused ion beam and notches were milled to initiate cracks. A Hysitron PI 89 SEM PicoIndenter was used to deform the beams and in situ SEM imaging enabled the observation bending and crack opening.