Microstructure and mechanical behavior of nanocrystalline metals

The intrinsic mechanical properties of nanocrystalline metals are masked by imperfections introduced during synthesis or later heat treatment. These imperfections include gas-filled pores, flaws, spatially abrupt variations in grain size and recrystallized regions. Desorption studies show the presence of gases trapped in the pores at sufficient pressure to cause blistering during annealing. Larger flaws are associated with regions of imperfect bonding of the nanocrystalline clusters. TEM examination shows the presence of recrystallized grains, their volume fraction varying with sample exposure to elevated temperature. Tensile tests on miniature samples of nanocrystalline Cu showed yield stresses of 450–600 MPa and strains to failure of 2–3% in nanocrystalline Cu, but only elastic deformation in nanocrystalline Ni with fractures stresses of 1200 and 1500 MPa. This difference in plasticity may be attributed to the smaller volume fraction of recrystallized grains in the Ni samples.