Elastic and tensile behavior of nanocrystalline copper and palladium

The elastic and tensile behavior of high-density, high-purity nanocrystalline Cu and Pd was determined. Samples with grain sizes of 10–110 nm and densities of greater than 98% of theoretical were produced by inert-gas condensation and warm compaction. Small decrements from coarse-grained values observed in the Young's modulus are caused primarily by the slight amount of porosity in the samples. The yield strength of nanocrystalline Cu and Pd was 10–15 times that of the annealed, coarse-grained metal. Total elongations of 1–4% were observed in samples with grain sizes less than 50 nm, while a sample with a grain size of 110 nm exhibited > 8% elongation, perhaps signifying a change in deformation mechanism with grain size. Hardness measurements followed the predictions of the Hall-Petch relationship for the coarse-grained copper down to ≈ 15 nm, and then plateaued. Hardness values (divided by 3) were 2–3 times greater than the tensile yield strengths. Processing flaws may cause premature tensile failure and lower yield strengths. The size and distribution of processing flaws was determined by small-angle neutron scattering. Tensile strength increased with decreasing porosity, and may be significantly affected by a few large processing flaws.