Thermal stability of nanocrystalline nickel–18 at.% tungsten alloy investigated with the tomographic atom probe

The microstructure of commercially available electrodeposited and thermally aged Ni–W layers with a composition of 18 at.% W was studied by means of field ion microscopy and the tomographic atom probe. In comparison with standard Ni–P or hard chrome coatings, Ni–W layers have a promising application field owing to their specific tribological and electro-erosion properties and in particular because they are manufactured at low cost without harm to the environment. The as-plated state is characterized by the presence of nanocrystalline grains of the Ni-rich fcc phase, with the nanocrystalline structure being preserved up to 700 °C. At this temperature the formation of the ordered Ni4W-phase (D1a structure) is observed and finally, after aging at 800 °C, the specimens are completely ordered. Whereas in Ni–P the continuous segregation of P and the grain boundaries is responsible for the thermal stability, in Ni–W grain growth is inhibited by the low mobility of the W atoms.