High Strength Al-zn-mg-cu Alloy Fatigue Fracture Properties And Microstructure Of The Study

The fatigue and fracture properties and microstructure of the fatigue specimen have been investigated by using SEM (including EBSD (Electron Back-scattering Patterns) analysis techniques) and TEM techniques, which reveals the micromechanisms of fatigue crack initiation and growth. After further analysis the conclusions come as follow:1. The fracture toughness of overaged Al-Zn-Mg-Cu alloy is mainly affected by the second phase particles and the fracture mode of this kind of aluminum alloy is dimple fracture caused by coarsed second phase particles. The variation of strength of the alloy matrix and precipitation at grain boundaries give rise to a discrepancy between matrix and grain boundaries, which has an extreme impact on fracture mode and fracture toughness of alloys.2. The fatigue damage of Al-Zn-Mg-Cu alloys are usually caused by Fe-rich intermetallic particels at or near the surface of the specimen, which are approximately 10×12～17μm2 in size. Micro cracks usually initiate in defects such as fracture particles, interface between partiles and matrix, grain boundaries or surface pits. After initiation fatigue cracks propagate further towards the center of the specimen, and finally fatigue failure occurs.3. The specimen was subjected to a tensile test with stress amplitude of 330Mpa. In a region of approximately 170×190μm2 no striations were observed. As the fatigue cracks propagate secondary some typical fracture morphology like cracks and striations could be observed. In regions where striations are visible a uniform increase in striation distance as the crack length increased.4. There are 3 stages of the crack propagation, when△K<16MPa×m1/2, it is the first stage, during which a great number of micro cliffs on the fracture surface can be observed. Afterwards when 16MPa×m1/2<△K<33MPa×m1/2, striation is the characteristic morphology. Thirdly, under the condition of△K> 33 MPa×m1/2, fracture failure occurs, lots of dimples and fracture particles are visible, the fracture surface is familiar with that of static tention.5. The propagation mode of Al-Zn-Mg-Cu alloys is a mixed pattern of intergranular and transgranular ones. The relationship between misorientation of adjacent grains and propagation of fatigue crack obeys the crystal deformation mechanism of crack tips. When the interfacial angle of adjacent grains is relatively wide, fatigue cracks can't propagate through the grain boundaries and is apt to be intergranular.6. When fatigue cracks cross the grain boundaries and enter an adjacent grain, compared with twist, deflection of the crack tip is more likely to happen.7. For experimental alloys in tesile tests, with the increase of stress amplitude, the distance between the crack source and the free surface of the specimen decreases, and the size of the region without striations is smaller. At the same time the striation distance, the growth rate of cracks and the size of the fatigue rupture region increase.