Mechanical properties of nanocrystalline metals and alloys studied via multi-step nanoindentation and finite element calculations

The mechanical behavior of metals is strongly governed by the grain-size variation. Like the tensile strength, the fatigue limit increases with decreasing grain size in the microcrystalline regime. In this paper the properties of plastically graded nanocrystalline Ni–W alloy produced via electrodeposition have been analyzed by multi-step instrumented nanoindentation. The variation in the measured hardness of the different materials have been measured in the range 50–400 mN indentation load and the results compared with those of pure nickel and cobalt nanocrystalline metals produced by electrodeposition. Finally, the problem of a crack approaching a plastically graded microstructure along its path was analyzed. alysis has been conducted by employing ABAQUS simulations on nanocrystalline Ni–W alloys with 20 and 100 nm constant grain size; the results were compared with those of the plastically graded sheets with negative graded configuration in which the grain size varies between 20 and 100 nm from the surface to the bottom in a linear way and with positive graded configuration in which the grain size varies between 100 and 20 nm from the surface to the bottom in a linear way. nite element study has conducted to the conclusion that the grade in the microstructure and consequent plastic mechanical properties of nanocrystalline Ni–W alloys leads to several changes in the driving force for fracture demonstrated by the different behavior of the effective J-integral at the crack tip in the different studied configurations.