Stabilized nanograin boundaries in nickel-molybdenum alloys result in increased hardness with decreasing grain size.

Annealing hardening has recently been found in nanograined (ng) metals and alloys, which is ascribed to the promotion of grain boundary (GB) stability through GB relaxation and solute atom GB segregation. Annealing hardening is of great significance in extremely fine ng metals since it allows the hardness to keep increasing with a decreasing grain size which would otherwise be softened. Consequently, to synthesize extremely fine ng metals with a stable structure is crucial in achieving an ultrahigh hardness in ng metals. In the present work, direct current electrodeposition was employed to synthesize extremely fine ng Ni-Mo and Ni-P alloys with a grain size of down to a few nanometers. It is demonstrated that the grain size of the as-synthesized extremely fine ng Ni-Mo and Ni-P alloys can be as small as about 3 nm with a homogeneous structure and chemical composition. Grain size strongly depends upon the content of solute atoms (Mo and P). Most importantly, appropriate annealing induces significant hardening as high as 11 GPa in both ng Ni-Mo and Ni-P alloys, while the peak hardening temperature achieved in ng Ni-Mo is much higher than that in ng Ni-P. Electrodeposition is efficient in the synthesis of ultrahard bulk metals or coatings.

Conventionally, nanograined metals and alloys can be stabilized through segregating foreign elements at grain boundaries (GBs). Yet such an effect may be offset by formation of second phase at elevated temperatures. In this paper, by introducing minor W into a binary Ni-Mo alloy, we found precipitation of intermetallic phases was suppressed, enhancing thermal stability of the nanograined structure. Characterized faceted GBs and a high-fraction of Σ3 coincidence site lattice (CSL) boundaries illustrate that GB structures are relaxed by formation of copious annealing twins. Adding W reduces stacking fault energy of the solid solution and facilitates the thermally-triggered GB relaxation. Suppressed precipitation of the intermetallic phases may be attributed to depletion of solutes at relaxed GBs.

A Ni-26 at. %Mo alloy with a composite structure of nanocrystalline and amorphous was synthesized by pulse electrodeposition. The composite structure was composed of mixed regions of amorphous and nanograins divided by a nanocrystalline interface network, which significantly suppressed grain coarsening and shear banding that would otherwise deteriorate mechanical properties of extremely fine nanograined metal. Plastic strain induced significant crystallization accompanied by Mo diffusion from mixed regions to nanograined interfaces. As a result, the Ni-26 at. %Mo alloy exhibited a superior hardness to its nanograined counterparts. The present work demonstrates an example of enhancing mechanical performance with hybrid structures crossover from nanocrystalline to amorphous.

The performance of 2-Mercapto-5-benzimidazolesulfonic acid (MBIS) in a Watts bath in crack electro-healing was investigated. Potentiostatic and galvanostatic voltammetry, current efficiency measurements were performed for electrochemical investigation. The morphology of the healed crack indicates that the healing crystals grow in a controllable manner with the addition of MBIS under forced convection with the healing crystals fill preferentially in the crack tip to the crack sidewalls. No obvious defects were observed along the interface between the substrate and the healing crystals, which is attributed the much higher current efficiency of MBIS. Annual-ring like defects are observed but with a much longer width compared with PEI. The formation mechanism of annual ring is discussed. Possible approach to get defect-free crack healing is also prospected.