Cyclic Plastic Deformation Behavior Of Extruded ZK60 Magnesium Alloy

For Mg alloys employed as load-bearing structural components, it is of great importance to understand the cyclic deformation mechanisms and fatigue properties of Mg alloys so that the reliability and durability of the products can be ensured and fatigue failure can be prevented by applying appropriate fatigue design principles. Wrought Mg-6Zn-0.5Zr(ZK60) alloy is a high strength commercial Mg alloy with a strong basal texture which has potential applications in the load-bearing components. First, the current work investigated the cyclic deformation and low cycle fatigue properties of extruded ZK60 alloy, and studied the influence of precipitates and initial twins on the cyclic deformation and low cycle fatigue properties of extruded ZK60 alloy. Second, by Electron Backscatter Diffraction(EBSD) analysis, the current work studied the twin evolution behavior during monotonic tension, monotonic compression and cyclic deformation with a 4% strain amplitude. Third, a detailed analysis was conducted on the stress-plastic strain curve in the uniaxial cyclic deformation, and the evolution of cyclic plastic characteristics was investigated. Finally the plastic deformation mechanism of the strong textured magnesium alloy was clarified.The results of monotonic tension and compression experiments show that both the extruded and aged ZK60 display pronounced tension-compression asymmetry. The aging treatment has a significant influence on static mechanical properties under both monotonic tension and monotonic compression. 56% tension twins can be found in the initial microstructure of the 5% pre-compressed alloy. The tension-compression asymmetry of the pre-compressed alloy is reduced.The results of cyclic deformation of extruded ZK60 show that the alloy displays different cyclic plastic deformation mechanisms under different strain amplitudes. When the strain amplitude is less than or equal to 0.32%, dislocation slips dominates the plastic deformation. A typical feature is the symmetrical stress–strain hysteresis loops with zero mean stresses and insignificant cyclic hardening. When the strain amplitude ranges from 0.32% to 1.5%, twins develop under compression and detwinning occurs in the tensile reversal in a loading cycle. The stress–strain hysteresis loops display a concave-down shape under compression and a sigmoidal shape under tension that is typically reported for Mg alloys with a basal texture. A distinct characteristic of cyclic deformation is the sustentation of the asymmetric stress–strain hysteresis loops and the positive mean stress with increasing loading cycles. The cyclic plastic deformation mechanism is referred to as ‘‘partial twining-detwinning completed”. When the strain amplitude is larger than 1.5%, twinning is exhausted under the compressive reversal and non-basal slip can occur with a high compressive stress. On the tensile reversal part, most grains twinned under compression are detwinned and non-basal slips can occur at a high tensile stress that results in an increased slope in the stress–strain curve. The mean stress decreases quickly with increasing loading cycles and the stress–strain hysteresis loops tend to become more and more symmetric. The cyclic plastic deformation mechanism is referred to as ‘‘twining exhausted-detwinning completed”.The results of cyclic deformation of aged ZK60 show that at strain amplitudes up to 0.35%, cyclic deformation aged ZK60 Mg alloy are identical to that of as-extruded. This indicates that the aging process which introduces Mg-Zn precipitates has no effect on basal slips of the Mg alloy. For strain amplitudes larger than 0.35%, the aging treatment has an influence on cyclic deformation only in the early period of loading cycles. With increasing number of loading cycles, the stress amplitudes and hysteresis stress-strain loops of as-extruded ZK60 tend to become identical to those of the aged ZK60.The results of cyclic deformation of 5% pre-compressed ZK60 show that when the strain amplitudes ranges from 0.35% to 1.5%, detwinning is not completed in the tensile reversal. The stress-strain hysteresis loops display symmetric shapes. The cyclic plastic deformation mechanism is twinning-detwinning, and both the twinning process and detwinning process are not exhausted.The results of low-cycle fatigue experiments show that there exists two kink points in the strain-life curves for extruded, aged and pre-compressed ZK60 alloys. The lower kink points correspond to the strain amplitudes of 0.32%(extruded) and 0.35%(aged and pre-compressed). This kink point demarcates the influence of twinning-detwinning deformation on fatigue. The higher kink point corresponds to a strain amplitude of 3% for the three alloys. When the strain amplitudes are larger than 3%, the strain amplitudes and fatigue lives display an approximate linear relationship in log-log scale. The fatigue specimens with strain amplitudes larger than 3% were finally fractured due to compression in the last loading cycle. The aging treatment has marginal influence on fatigue properties of ZK60 alloy. The initial twins can improve the fatigue properties only at the strain amplitudes range from 0.35% to 2%.The results of twin evolution in the monotonic loading experiments show that when the tension strain is 5%, {101 1} compression twins and {101 1}-{101 2} double twins can be found. With increasing strain amplitudes, the size and amount of twins increase. In the monotonic compression, {101 2} tension twins nucleate, propagate and grow. When the compressive strain are 1%, 2%, 4%, 5%, 6.4%, 8%, 10% and 15%, the twin volume fraction are 6.1%, 24%, 29%, 56%, 79%, 90%, 91% and 92%. {101 1}-{101 2} double twins can be found when the compressive strain reaches 10%.The results of twin evolution in the cyclic loading experiments with a strain amplitude of 4% show that twinning and detwinning dominate the cyclic plastic deformation in the compression reversal and tension reversal, respectively. Cyclic hardening caused by twinning/detwinning and dislocation slips increases twin nucleation sites but inhibits twin growth and twin shrinkage. The enhanced twin nucleation sites result in an increase in the number of twin lamellae and an increase in the twin volume fraction with increasing number of loading cycles. The inhibition of twin growth and shrinkage leads to serrated boundaries of fragmented twins. Double twins and compression twins can be introduced by cyclic loading and the formation of these twins does not lead to immediate fracture of the material. With increasing number of loading cycles, more and larger sized residual tension twin lamellae can be detected by EBSD, but the total volume fraction of the residual twins is trivial.The results of the analysis of on the stress-plastic strain curve in the uniaxial cyclic deformation show that the elastic limit range depends on the microstructure at the peak stress. If the microstructure at the peak stress displays a strong basal a-texture, yielding during unloading is dominantly associated with the activation of basal slips, which has an elastic limit range of 100 MPa. If the microstructure at the peak stress contains tension twins, the elastic limit range during unloading reflects the activation stress of detwinning or retwinning process. The activation of detwinning or retwinning involves the gliding of twin boundaries of the twin bands. Therefore, the elastic limit range in this case reflects the stress needed to activate the gliding of twin dislocations. The stability of twin dislocations is influenced by twin volume fraction, twin morphology, and cyclic hardening.