Grain refinement of NiTi shape memory alloy thin films by W addition

The present research explores the novel approach to achieve grain refinement in NiTi shape memory alloy thin films by adding W in the matrix of NiTi. It involves production of NiTiW shape memory alloy thin films by Co-sputtering of NiTi and W targets. The grain size of B2-NiTi decreases with increasing W content, due to the immiscible W layer obstructing its grain growth. Moreover addition of W induces the B2–R single step transformation by suppressing thermally induced martensite phase due to grain size refinement below 40 nm. With W content ranging from 2.6 at% to 4.5 at%, the films are strengthened and can reach highest hardness and elastic modulus of 32.8±2.7 GPa and 167.83±8.64 GPa, respectively. With further increase in W content, the mechanical properties of films decrease gradually. This behavior can be explained in terms Hall–Petch theory and lattice distortion of NiTi crystals with increasing the W content.

► Grain refinement of NiTi thin films by adding W in its matrix.
► Phase transformation from B2–R–B19′ to single step B2–R with grain refinement.
► Highest hardness (32.8±2.7 GPa) and elastic modulus (167.83±8.64 GPa) due to grain refinement.
► Gradual decrease in mechanical properties due to grain size reduction (below ∼15 nm).