Bin Gan

Ultrahigh Strength of Dislocation-Free Ni3Al Nanocubes

Individual Ni(3) Al nanocubes under pressure are investigated by comparing the compressive strength of both dislocation-free and irradiated Ni(3) Al nanocubes. The results are dicussed in light of the size-dependent and size-independent strength of face-centered cubic (fcc) nanocrystals in the framework of dislocation nucleation at free surfaces. This study sheds more light on the understanding of fundamental deformation mechanisms and size-affected strength in dislocation-free metallic nanocrystals.

In situ characterization of the martensitic transformation temperature of NiTi shape memory alloys via instrumented microindentation

A novel, instrumented microindentation technique was successfully used to measure the temperature associated with the martensitic transformation leading to the recovery of plastic strain in a Nickel–Titanium (NiTi) shape memory alloy. Following a standard indentation cycle, the indenter was partially unloaded such that a good contact was maintained between indenter and specimen surface. The onset and finish temperature of the martensitic transformation, the associated volume contraction, and the amount of the recovered plastic deformation were determined by quantifying the indenter displacement as a function of temperature. These experiments were compared to conventional measurements of the transformation temperature by differential scanning calorimetry and compression testing.

Rate dependence of the serrated flow in Ni-10Pd during high temperature instrumented microindentation

Instrumented indentation tests were conducted on a pure nickel and a Ni-10Pd solid solution at 450 °C with loading rates varying from 62.5 to 1000 mN/s. The load–depth curves from the pure nickel exhibited a smooth and continuous transition; while the load–depth curves from the Ni-10Pd were initially smooth and then became serrated after reaching a critical load. Increases in loading rates resulted in an earlier occurrence of the serrated flow with a higher load threshold. The mechanism responsible for the serration was delineated by accounting for the reconfiguration of dislocation substructures and the interactions between solutes and forest dislocations.

Influence of pre-deformation on jerky flow in Ni–10Pd during high-temperature instrumented indentation

During instrumented microindentation at 450 °C, prominent and limited serrations were exhibited in the load–depth curves corresponding to an annealed Ni–10Pd and a deformed Ni–10Pd specimen, respectively. After fully recrystallizing the deformed Ni–10Pd sample and minimizing the stored strain energy, the characteristic jerky flow behaviour was restored in the Ni–10Pd sample during instrumented nanoindentation at 450 °C. The effect of pre-deformation on the serrated flow behaviour is rationalized using a modified cavity expansion model that accounts for the reconfiguration of dislocation substructures and solute–dislocation interactions.

Compositional dependence of serrated flow in nickel binary solid solutions during high-temperature microindentation

At 450 °C, pure nickel and nickel binary solid solutions, containing Ru, Rh, W, Re, Ir and Pt, have a smooth load–depth curves. In contrast, the alloying addition of Nb, Mo, Pd and Ta results in the occurrence of serrations in their load–depth profiles. Analyses show the load increment of the quasi-elastic segments scales linearly with the load at their starting point. The slope of such linear relationships was normalized by the solute concentration and then plotted against the diffusion rate of transition metal solutes within the nickel lattice and along dislocation cores. Linear relationships were observed in both cases. Results suggest that the solute diffusion along dislocation cores coupled with the atomic size misfit are mainly responsible for such serrations.