Microsample tensile testing of nanocrystalline metals

A novel non-contact strain measurement technique has been employed to measu re the tensile properties of extremely small 'microsamples' of pure high-de nsity ultrafine-grained Al (ufg-Al) nanocrystalline Cu (n-Cu) and nanocryst alline Ni (n-Ni). These microsample tests confirmed the absence of Young's modulus variations for metals with grain sizes approaching 25 nm. Significa nt strength enhancements were associated with the nanocrystalline specimens ; the tensile stresses achieved in these microsample tests were measured to be an appreciable fraction of the theoretical shear strength for these met als. The ufg-Al samples (diameter, 250 nm) exhibited extensive plasticity w hile deformation in the n-Ni (diameter, 28 nm) remained almost entirely ela stic up to failure at 1500 MPa. The n-Cu samples were found to have a multi scale grain structure that produced an attractive balance of strength and d uctility. Transmission electron microscopy investigations of deformed n-Ni failed to produce any evidence of dislocation activity. In the absence of d islocation motion, the tensile strength of truly nanocrystalline metals is remarkably high but currently dominated by intrinsic porosity and mesoscale microcrack coalescence.