Structural Stability And Mechanical Behavior Of Ni-Mo Alloys With Nanotwinned And Nanograin/amorphous Structures

Grain boundary(GB)structure and chemistry is crucial in determining deformation mechanism,structure stability and mechanical properties of nanograined(ng)materials,especially when grains are refined to below 10 nm in size.Plastic deformation of extremely fine ng metals will be accommodated by the competition of full dislocation,partial dislocation,GB-mediated activities or even shear banding depending upon grain size,microstructure,chemistry,loading conditions.Moreover,it remains unclear what microstructure would be introduced when the grains of extremely fine ng metals are refined further.In the present work,plastic deformation in an electrodeposited nanotwinned columnar nanograined(nt-ng)Ni-12.5 at.%Mo film with an extremely fine columnar grain size of 811.7 nm and a twin thickness of 1.3±0.5 nm was investigated via uniaxial micro-tensile tests at room temperature.The tensile properties and microstructural evolution were evaluated and characterized.Plastic deformation mechanism was analysed by correlating the texture evolution and yield strength.Besides,pulse electrodeposition was employed to synthesize Ni-Mo alloys with a high Mo content(Ni-26.0at.%Mo).Microstructure characterization and chemistry analysis were performed.Mechanical property was assessed by using microhardness tests,deformation mechanism was revealed by the cross-section microstructural evolution and chemistry redistribution.Additionally,the maximum hardness of the Ni-26.0 at.%Mo alloy was explored by annealing with analysis of the corresponding hardening mechanisms.Key findings include:1.Columnar nt-ng Ni-12.5 at.%Mo film with an extremely fine grain size of 8±1.7 nm and a twin thickness of 1.3±0.5 nm was synthesized by using direct current electrodeposition.The nt-ng Ni-Mo samples exhibited an ultrahigh yield strength of 2.11±0.17 GPa and a noticeable elongation to failure of 13.8%±1.9%when the loading direction is parallel to the twinning planes.Tensile plastic deformation of this extremely fine columnar nt-ng Ni-Mo alloy could be ascribed to correlated threading dislocations due to the extremely small twin thickness.Activities of threading dislocations not only account for the high yield stress but also well explains a concurrent texture evolution from a<111>in-plane fiber texture to an out-of-plane texture,with the latter demonstrating the grain size effect in plastic deformation of nanotwinned columnar nanograins.2.Additives of saccharin and 2-butyne-1,4-diol are negative to the promotion of Mo content in Ni-Mo deposits due to enhanced adsorption competition between additives molecules with ternary intermediate ions on the surface of cathode during off time in pulse current electrodeposition.What’s more,lowering the pH value is beneficial to promoting the Mo content in the deposit.Microstructure characterization revealed that the optimized Ni-26.0 at.%Mo is composed of mixed regions of amorphous and nanograins surrounded by nanocrystalline interface.Chemical analysis uncovered that the composite structure correlated to a chemical fluctuation between mixed regions corresponding to a higher Mo content in a range of 24-28 at.%and the interfaces to a lower Mo atomic percentage with a value of 12.3.Microhardness of the composite structured Ni-26.0 at.%Mo is 7.04±0.11 GPa,which is up to 40%higher than its extremely fine ng counterparts.Microstructure characterization and chemical analysis on the indented sample indicated that significant amorphous crystallization occurs during plastic deformation.This crystallization process was accompanied by atomic diffusion,which not only suppressed GB-mediated activities but also reduced the tendency of strain localization by shear banding.Moreover,significant annealing-hardening was observed in Ni-26.0 at.%Mo alloy with the maximum hardness appeared at an annealing temperature of 575℃,which is 50℃higher than that of Ni-21.5 at.%Mo.The microhardness reached as high as 12.87±0.21 GPa,being 1.5 GPa higher than that of Ni-21.5 at.%Mo.Multiple twins,stacking faults and HCP structures observed in the as-indented Ni-26 at.%Mo alloy annealed at 575℃indicate that partial dislocation emission from GBs might contribute to the ultrahigh hardness.The appearance of the second phase Ni3Mo might also promote the GB stability.In-depth analysis is still needed to understand the phenomenon.