NEAR-THRESHOLD FATIGUE CRACK-GROWTH AT ROOM-TEMPERATURE AND AN ELEVATED-TEMPERATURE IN AL-LI ALLOY-8090

The effects of slip distribution and crack tip shielding mechanisms on the near-threshold fatigue crack growth of the Al-Li alloy 8090 have been studied at both room temperature and an elevated temperature. The slip distribution has been varied by changing the distribution of the S phase, through prior stretching or by means of a duplex heat treatment. Fatigue crack growth (FCG) tests were conducted at a high stress ratio to reduce possible effects due to crack closure. At room temperature the changes in FCG rates are interpreted as arising from the changes in the degree of planarity of slip in the materials. At 150-degrees-C, the microstructural changes due to the long exposure to elevated temperature appear to dominate the effects observed. At lower DELTA-K, where the time at temperature is greatest, lower DELTA-K thresholds than those found at room temperature are obtained. These have been attributed to increased slip homogenization due to the increased precipitation and coarsening of the incoherent S phase together with loss of toughness due to the growth of coarse grain boundary phases and the formation of the associated delta' precipitate free zone. At higher DELTA-K, where the time at temperature is low and microstructural changes are minimal, slower FCG rates than those found at room temperature are obtained. These are explained in terms of increased crack tip shielding which arises because of the increase in tortuosity of the crack path, the increased slip homogenization and the climb and cross-slip within the crack tip plastic zone.