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 (sigma(f) similar to 1.15 GPa) lower than yield stress evaluated from hardness measurements. However, sigma(f) increases monotonously with time in isothermal (850 degreesC) 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 t(a) > similar to1.2 h sigma(f) 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. (C) 2003 Elsevier B.V. All rights reserved.