Observation of two stage dislocation dynamics from nonlinear ultrasonic response during the plastic deformation of AA7175-T7351 aluminum alloy

Systematic nonlinear ultrasonic (NLU) studies using longitudinal and guided surface acoustic waves (SAW) were conducted on AA7175-T735 aluminum alloy specimens that were plastically deformed to varying levels of residual strain. The variation of nonlinear ultrasonic (NLU) parameter (A2/A12) as a function of residual plastic deformation, suggests an increase in NLU parameter at a slow rate up to a certain level of deformation and thereafter there is a significant increase in NLU parameter with further increase in the strain level. X-ray diffraction methods to characterize line broadening and etch pit dislocation distribution studies suggest that the smaller rate of increase in NLU parameter could be due to the increase in the dislocation density and higher rate of increase could be caused by formation of cellular dislocation pattern. Thus, our study shows that it is possible to study the two stage dislocation dynamics observed during residual plastic deformation of Al alloy 7175-T7351 using nonlinear ultrasonic technique.