ELASTIC PROPERTY DEPENDENCE ON LAYER PERIODICITY IN CU/NI SUPERLATTICES

Cu/Ni superlattices are prepared by magnetron sputter deposition and structurally characterized with X-ray diffraction measurement. A 1.2-4.5 nm range of layer pair spacings is produced in a series of 1-2-mu m-thick films which have a [111] textured growth. Uniaxial tensile testing is used to produce load-displacement curves from free-standing Cu/Ni films and calibration standards of Cu and Ni films. Direct measurement of the yield and ultimate stresses as well as Young's modulus is performed for the Cu/Ni films. The measured Young's modulus shows a bimodal variation with layer pair spacing. A maximum increase of 50% in Young's modulus, above the rule-of-mixtures value, is measured for a 2-nm-thick layer pair sample. The yield stress behavior follows the modulus trend with layer pair spacing, whereas the ultimate stress inversely follows the trend, that is, the stiff est samples are the most brittle. Isothermal annealing of the 2-nm-thick Cu/Ni sample progressively homogenizes the layered structure and diminishes the modulus enhancement to the rule-of-mixtures value.