Evaluation and mathematical modeling of asymmetric tensile and compressive creep in aluminum alloy ZL109

This study investigates the effects of tension/compression asymmetry during creep deformation under different conditions. The asymmetry is found to be dependent on stress and temperature. At high temperatures (350 degrees C, 70 MPa) or high levels of stress (250 degrees C, 130 MPa), the ratio between the tensile and compressive creep rates can be as large as 10. This ratio is smaller at lower temperatures (200 degrees C, 90 MPa) and lower levels of stress (300 degrees C, 30 MPa). Scanning electron microscopy (SEM) visualization of different microdefects indicates that the size and volume of microcavities are dependent on the level of stress applied. Similarly, transmission electron microscopy (TEM) is used to visualize dislocations and twinning. The differences in microcavity size and volume in tensile and compressive creep appear to be larger under higher temperature and stress, but no difference in dislocation is observed and no twinning crystals are found. Cavity nucleation appears to be the cause of the asymmetry in creep behavior, which is determined by temperature and stress. A new mathematical model for creep is constructed and validated considering the different asymmetric mechanisms of tensile and compressive creep. (C) 2015 Elsevier B.V. All rights reserved.