Tensile behavior of nanocrystalline Ni–Fe alloy

Tensile deformation and fracture behavior of bulk nanocrystalline Ni–Fe alloy, synthesized by a mechano-chemical method plus hot-isostatic pressing, were investigated by using mechanical tests and fractography. The results indicate that the as-prepared specimen fails prematurely with fracture strength (σf∼1.15 GPa) lower than yield stress evaluated from hardness measurements. However, σf increases monotonously with time in isothermal (850 °C) annealing, in contrast to which microhardness (yield strength) decreases with the annealing time. Fractography indicates that for the as-prepared specimen inter-particle brittle cracking is the main fracture mode. Nevertheless, microscopically local ductile fracture morphology was revealed to exist as manifested by the appearance of nano-dimples. When annealing time ta>∼1.2 h σf surpasses yield strength; correspondingly fractures exhibit more ductile mode, with a large number of both (sub-) micrometer-scale and mesoscale dimples prevailing as ta reaches 24 h. The improvement of mechanical properties and the change of fracture morphology with annealing are discussed.