Characterization of Strain-Rate Sensitivity and Grain Boundary Structure in Nanocrystalline Gold-Copper Alloys

Abstract

The power-law dependence of strength on strain rate provides a measure of the strain-rate sensitivity. In general, strength increases as grain size decreases from the microscale into the nanoscale regime for many cubic metals. The method of microscratch testing is used to measure microhardness in order to evaluate material strength. The strain-rate dependence of hardness is measured by varying the microscratch velocity. New results for nanocrystalline gold alloys show that the exponent (m) of the power-law dependence of stress on strain rate increases to 0.20 as grain size decreases to values less than 10 nm. A high-resolution electron microscopy examination of grain boundary structure reveals that an increase in the strain-rate sensitivity exponent (m) is found with an increase in the grain boundary misorientation.

Anomalies in Hall-Petch strengthening for nanocrystalline Au–Cu alloys below 10nm grain size

“…a yielding of the surface, representative of strength that can be conducted over a wide range of equivalent strain rates where tensile tests are not compatible with a mode of ductile failure. Micro-scratch test results are reported recently [7] that produce strain-rate sensitive measurements comparable with tensile test results.…”

“…In the micro-scratch hardness method, a diamond stylus travels at a velocity (υ) across the specimen surface under a normal incident load (P) producing a scratch with width (ω). It is derived empirically [7] that the strain rate (έ) equals the following:…”

“…grain size (h g ), is determined [15][16][17] from the (111) reflections using the standard Debye-Scherrer formulation [19], where the measured peak broadening is corrected for instrument broadening (in addition to the broadening contributions that are attributable to crystallite size). The instrument broadening is determined [7,15] by calibration of the X-ray peak results with measurements from bright-field transmission electron micrographs of specimens with 30-100 nm grain size. The X-ray spectra for nanocrystalline Au-Cu specimens [16] contain all of the fundamental reflections for a face-centered cubic crystal with relative peak intensities that correlate with the standard powder-diffraction file (PDF no.…”

“…The scratch-hardness test method is shown as useful to assess [7][8][9] the mechanical behavior of electrodeposited foils with micro-and nanocrystalline grain structures. Micro-scratch testing gives a measure of hardness, i.e.…”

On the Tapping Mode Measurement for Young’s Modulus of Nanocrystalline Metal Coatings

“…Nanoindentation normal to the surface is routinely used to measure the hardness and Young's modulus. Triboindentation tests are used [5][6][7] to measure both hardness and shear strength as well as quantify strain-rate sensitivity [8,9] effects in the evaluation of deformation mechanisms in nanocrystalline alloys. The standard approach [10][11][12] to determine the elastic modulus during nanoindentation evaluates the load ( ) versus displacement (ℎ) curve during unloading after plastic deformation.…”