Nanoscale planar faulting in nanocrystalline Ni–W thin films: Grain growth, segregation, and residual stress

Pure Ni and Ni–W thin films with different W contents (<22 at.%) and a thickness of 500 nm have been produced by (co)sputtering. The phase composition, changes in residual stress, crystallite size, microstrain, and texture have been investigated employing in-situ x-ray diffraction measurements (25–550 °C) and ex-situ transmission electron microscopy analyses. For all compositions investigated, W dissolves substitutionally in Ni. The dissolution of W results in a highly columnar nanocrystalline microstructure with grain aspect ratios (height to width) exceeding 10. The Ni(W) solid solution exhibits a very high density of planar (twin and intrinsic stacking) faults oriented perpendicular to the growth direction. Whereas grain coarsening occurs for the nanocrystalline pure Ni thin films already upon heating to temperatures as low as about 125 °C, the microstructure of the nanocrystalline Ni–W thin films remains stable up to much higher temperatures, that is, even exceeding 350 °C. Above 350 °C, a W depletion of the Ni–W layer as a result of W segregation at planar faults occurs, which is accompanied by a change in lattice constant and in-plane stress.