Critical Voltage Effect in Cu-based Alloys Containing Al or Au

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Abstract
The critical voltage Vc of Cu-based alloys with 15 at%Al and 5 or 15 at%Au were measured by JEM-1000 HVEM to study the effect of composition, temperature and short range order on the critical voltage effect. The addition of 15 at%Al to Cu causes an increase in Vc for the 222 and 400 reflections by 15 and 32 kV, respectively, whereas 5 at%Au decreases Vc for those reflections by 44 and 77 kV. A decrease as much as 176 kV occurs for the 400 reflection by addition of 15 at%Au. These results agree with the expectation that the addition of solute element with a smaller (or larger) scattering factor than that of the solvent will produce increase (or decrease) of Vc. The mean square displacement (msd) of atoms can be determined from the measured values of Vc by using the corrected atomic scattering factors of alloys. The effects of thermal motion and static displacement of atoms to the msd can be separated from each other by measuring Vc at various temperatures. Reasonable values for the static displacement of atoms and the Debye temperatures were obtained, and the latter was found to depend on the measuring temperature. These results are also in good agreement with the theoretical prediction by Shirley and Fisher’s formula if the parameter τ is assumed to be 1.1 and 1.8 for Cu–Al and Cu–Au, respectively. In order to investigate further the effect of short range order on Vc, the difference in Vc for Cu-15 at%Au between the specimens furnace cooled and quenched from 673 or 1073 K was measured and the change of 5 or 12 kV was obtained for the 400 reflection.