Influence of ECAP Processing on Fatigue Crack Nucleation on ZK60A Magnesium Alloy

Abstract

Low-cycle fatigue tests were conducted on ZK60A magnesium alloy that were processed by equal-channel angular pressing (ECAP) method. The ECAP processing was carried out on a hot-extruded ZK60A alloy at a die temperature of 473K. Total pass number was four and the processing route was Route Bc. One of the ECAPed sample was annealed at 573K to induce grain coarsening. The low-cycle fatigue tests were conducted under a constant plastic strain amplitude of εpl=2×10-3, in air at room temperature. A hysteresis loop shape during the low-cycle fatigue tests depended on the sample: a tension-compression asymmetric shape was recorded in the as-drawn specimen having the tensile axis parallel to the extruction direction, while a symetric shape was seen in case of the tensile axis perpendicular to the extrusion direction. Deformation twinning during fatigue was most probably attribute to the asymmetric hysteresis loop shape. Because the asymmetric hysteresis loop was absent in the ECAPed specimen, it is possible that grain refinement suppressed the deformation twinning in the fatigue test. In the as-drawin specimen having the tensile axis parallel to the extrusion direction, a small crack was observed near a deformation twin. By contrast, a fatigue crack in the as-ECAPed specimen was generated along a shear band that was inclined to the tensile axis at 45°. After the ECAPed specimen was annealed at 573K, an intense shear banding and a subsequent crack nucleation were moderated. Accordingly, it can be said that the low-temperature annealing on the ECAPed ZK60A magnesium alloy reduced the fatigue crack nucleation sites.