Observations of Tension/Compression Asymmetry in the Cyclic Deformation of Aluminum Alloy 7075

The cyclic deformation of AA7075 in the T6 and T651 tempers has been investigated at room temperature under plastic strain control at amplitudes from 0.1% to 1.0%. Although both tempers represented peak aged conditions, their cyclic deformation behavior differed. The T6 material exhibited a slight amount of cyclic hardening to saturation, with softening evident only at the highest amplitudes. At low amplitudes, the peak compression stresses were higher than the tension peaks throughout the test. The extent of this asymmetry in peak stresses diminished at higher amplitudes. In contrast, the peak compression stresses for the T651 temper were initially lower than the tension peaks. The tension peaks remained relatively stable throughout each test, but the compression peaks exhibited hardening such that they became greater in magnitude than the tension peaks. A nonlinear elastic strain contribution is shown to account for asymmetries in hysteresis loop shape, but not the asymmetric peak stresses. We hypothesize that the tension/compression asymmetry results from strain localization caused by the inhomogeneous microstructure created during quenching and aging. In addition, stretching prior to aging gives rise to the unusual behavior of the T651 material.