Inverse Hall-Petch and nanocrystal-amorphous transition of broad-spectrum W content NiW alloys

Fabricating metallic materials with grain sizes below 10 nm, particularly in the range between 2.4 and 7.2 nm, poses a challenge. However, it is crucial to accurately identify Hall-Petch breakdown within this range. Moreover, there are differing opinions regarding the change in strength that occurs when nanocrystals transform into amorphous materials. This study involved the electroplating preparation of amorphous or nanometer NiW coatings spanning 1.9–15.8 nm by adjusting current density and temperature. The microstructure under tungsten atoms solid solution was characterized and measured surface hardness. The results revealed that the tungsten solid solution in FCC Ni alloy refines grain size and the strength follows the Hall-Patch relationship while the hardness of nanocrystal NiW alloy with 12.3 at% W regional maximum at 7.79 GPa at the critical point of 5.9 nm, significantly surpassing the 5.29 GPa hardness of pure nickel at 22.0 nm. After that, reverse Hall-Petch abnormal phenomenon begin. The annealed alloy of Ni-15.0 at% W had the similar trend and critical point at 5.0 nm. Following the transformation into an amorphous state, hardness further rises, reaching a pinnacle of 8.95 GPa of Ni-20.9 at%W. The values of solid solution hardening under various content of tungsten atoms were calculated. It suggests that the solid solution limitation of nanocrystal NiW which grain size bellowed 10.0 nm expanded. After removing the hardening by atoms solid solution, the similar Hall-Petch breakdown phenomenon was observed while the critical point was expansion offset to 8.6 nm which was approaching the critical point of nickel electrodeposited coating.