Evolution of high-cycle fatigue behavior of extruded AZ91 alloy by artificial cooling during extrusion

The influence of artificial cooling on the high-cycle fatigue behavior of extruded AZ91 alloy is investigated via fully reversed stress-controlled fatigue tests of alloy samples indirectly extruded with and without artificial cooling using water. Application of artificial cooling leads to a decrease in the size of recrystallized grains and an increase in the amount of dynamically formed Mg17Al12 precipitates of the extruded alloy; this consequently results in an improvement in its tensile and compressive strengths. The artificially cooled sample has superior fatigue resistance to the non-cooled sample because the fatigue damage per cycle in the former is smaller than that in the latter. The dominant cyclic deformation mechanism of the non-cooled sample varies from twinning-detwinning to dislocation slip with cycling owing to the occurrence of cyclic hardening; this variation, in turn, causes the initial asymmetric hysteresis loop to become gradually symmetric with an increase in the number of cycles. On the other hand, the artificially cooled sample has a completely symmetric hysteresis loop from the first cycle owing to the suppression of twinning in compression by the high compressive strength. In addition, the shape of the hysteresis loop, the tensile and compressive peak strains, and the total strain amplitude of the artificially cooled sample remain almost unchanged throughout the entire lifetime because the sample is cyclically stable. These different cyclic deformation behaviors of the two samples are strongly related to their different dislocation densities in the as-extruded state.