Microstructure and fracture toughness of electrodeposited Ni-21 at.% W alloy thick films

Nanocrystalline Ni-W has been shown to exhibit properties superior to nanocrystalline Ni and coarse-grained materials, but studies regarding its fracture behavior are limited. This study investigates the fracture behavior of Ni-W, establishes structure-property relationships via correlation to the microstructure, and assesses the suitability of the micro-mechanical testing approach. As-deposited and various heat-treated Ni-21 at.% W films, processed by electrodeposition in a sulfate-citrate bath, were evaluated by in-situ microcantilever bend testing. Due to non-negligible plastic yielding, conventional linear elastic fracture mechanics was insufficient, and novel elastic-plastic fracture mechanics was necessary. Periodic partial unloading was implemented along with J-integral interpretation to monitor crack growth and quantify the fracture toughness. Each alloy was also examined with aberration-corrected electron microscopy to develop a complete understanding of the various microstructures and facilitate their correlation to the fracture behavior. Overall, fracture toughness measurements varied between 5.1 and 8.9 MPa√m depending on the heat treatment. Annealing significantly increased the microhardness but decreased the fracture toughness, thus implying a trade-off between the two.