Influence of precipitates and dislocations on the acoustic nonlinearity in metallic materials

The influence of precipitates and dislocations on the acoustic nonlinearity was experimentally investigated in thermally-aged ferritic CrMo steel and low-cycle-fatigued Cu. The variation in the relative acoustic nonlinearity (A₂/A₁²) was interpreted as resulting from microstructural changes, where was supported by the electron microscopy data. The acoustic nonlinearity of CrMo steel increased abruptly in the initial stage of degradation and then changed little due to the coarsening of carbide and to the precipitation of stable M₆C carbide during thermal aging. Due to the evolution of the dislocation cell substructure, the acoustic nonlinearity of Cu increased with the number of fatigue cycles. These experimental results show that the acoustic nonlinearity is severely affected by the lattice strain caused by precipitates and dislocations.