On the Mechanical Behavior of Nanocrystalline Ni-Based Alloys

The fabrication and characterization of a ternary nanocrystalline (NC) stabilized Ni-Cu-P alloy was reported. The complexities of fabrication a low contamination, equiaxed NC material was discussed. For comparison, elemental Ni was prepared via sputter deposition, highlighting the difficulty in fabricating an idealized structure within the confines of the parametric space. An exchange between the ideal grain size distribution and film coalescence was discussed with the ramifications of substrate heat, deposition pressure, and deposition rate considered. These results were then compared to a kinetic model to assess the validity of the model and its ability to capture the complex growth behavior noted between the Ni films.A series of ternary NC Ni-Cu-P thin films were fabricated, characterized, and loaded via in situ annealing and nanoindentation. Examination of the thermal stability behavior revealed differences in precipitation behavior as a function of Cu and P solute content. Furthermore the Ni-40Cu-0.6P (at.%) alloy was noted as the only NC stabilized composition at 550 °C. The Ni-40Cu-0.3P deposit developed nanoscale precipitates upon annealing to temperatures of 550 °C which ultimately resided in the matrix. It was found that the ternary Ni-Cu-P films are harder than previously studied binary Ni-1P and Ni-4P (at.%) systems. Adding Cu to the Ni-P system promoted solid solution strengthening which mitigated softening in those binary alloys previously reported.A MEMS device was then employed for in situ thermomechanical testing of the stabilized Ni-40Cu-0.6P film for comparison with a binary Ni-40Cu counterpart. Digital image correlation (DIC) was employed to data mine microstructural evolutions that occurred while the films were loaded. Increased fracture strength was reported with the P solute addition, irrespective of the loading temperature. The ramifications of adding this stabilizing solute were discussed with respect to the fracture profile and microstructural stability.