MICRO-SCRATCH CHARACTERIZATION OF STRENGTH IN NANOCRYSTALLINE METALS
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abstract
Tensile testing can provide the detailed plastic behavior from deformation at yielding to the ultimate strength. The power-law dependence of strength on strain rate provides a measure of the rate sensitivity. In general, tensile strength increases for many cubic metals as grain size decreases from the micro-scale into the nano-scale regime. However, many nano-crystalline metals are prone to localized plastic deformation or even brittle failure. As such, tensile strengths may appear well below the true upper-bound values. This drawback can make a quantitative interpretation of the strain-rate sensitivity quite difficult. As an alternative, the method of microscratch testing is used to evaluate the upper bound strength through micro-hardness measurements. By varying the velocity of the indent test under constant load, the width of the scratch reveals the rate dependence of hardness. New test results are presented for nanocrystalline cubic metals as gold alloys with grain sizes less than 10 nm.
