Cyclic deformation and fatigue of extruded AZ31B magnesium alloy under different strain ratios

Abstract

Cyclic deformation and fatigue behavior of extruded AZ31B magnesium (Mg) alloy were investigated under strain-controlled loading along the extrusion direction at different strain amplitudes with three strain ratios (ε = 1,). With a strain ratio of ε = , partial twinning-partial detwinning occurs at all strain amplitudes, leading to accumulation of deformation twins as the loading cycle is increased. With strain ratios being ε = and 1, partial twinning-complete detwinning is the cyclic deformation mechanism when the strain amplitude is higher than 0.35%. At a strain amplitude lower than 0.35%, dominant cyclic deformation mechanism is dislocation slips irrespective of the strain ratio. Compared with the cases of ε = and ε = , stronger cyclic hardening is exhibited at all the investigated strain amplitudes for ε =. Fatigue fracture surfaces show regions with lamellar-like and dimplelike features. Lamellar-like feature existing in the crack initiation and stable propagation region are mainly due to twinning-detwinning during cyclic deformation. The dimple-like feature formed in the unstable crack propagation and final rupture region mainly arises from severe intragranular plastic deformation by dislocation slips. At the same strain amplitude, the lamellar-like traces become more aggravated with a lower stain ratio. The strain-fatigue life curves exhibit a distinguishable kink at strain ratios ε = 1 and. The fatigue life with ε = shows the highest and that with is the lowest at the same strain amplitude. The Smith-Watson-Topper (SWT) fatigue criteria can correlate well the fatigue experiments of AZ31B Mg alloy under different strain ratios.

Effect of initial texture on fatigue properties of extruded ZK60 magnesium alloy

“…However, detwinning is easy to be complete during the tensile reversal because of the lower twin capacity. This type of twinning‐detwinning behavior is referred to as “partial twining‐complete detwinning” . From Figure D, both the tensile peak and compressive peak stress slowly increase with increasing loading cycles, indicating a moderate cyclic hardening.…”

“…To confirm this speculation, Figure shows the results measured by XRD on the RD‐TD plane in the TD specimens which were obtained from 3/4 loading cycle with compression first followed by tension (0 → −1% → +1% → 0, as indicated by strain process I) and tension first followed by compression (0 → +1% → −1% → 0, as indicated by strain process II), respectively, and 1/4 fatigue life. The degree of residual twins can be represented by the inverse value of λ = using the integrated intensity of and (0002) peaks, where the parameter λ of the cyclically deformed specimens was further normalized by the λ value of the initial material and the normalized λ value is denoted by λ Norm . A higher value λ Norm (>1) indicates that a higher degree of tension twins remained in the sample. It can be seen that the specimen tested in strain process I (0 → −1% → +1% → 0) displays a λ Norm value less than 1, indicating minimal twins left in the tension phase.…”

“…This type of twinning-detwinning behavior is referred to as "partial twining-complete detwinning". 22,25,31,32 From Figure 3D, both the tensile peak and compressive peak stress slowly increase with increasing loading cycles, indicating a moderate cyclic hardening. A previous study shows that the deformation at 1% compressive strain corresponds to a nucleation period of 1012 È É tension twinning.…”

“…[17][18][19][20] Loading magnitude plays an important role in the deformation mechanisms of Mg alloys under cyclic loading. Earlier studies [21][22][23][24][25][26][27][28][29][30][31][32] reveal that in the strain life fatigue Nomenclature: ED, extrusion direction; TD, transverse direction; ED −9.4% , precompressed to 9.4% along the ED; XRD, X ray diffraction; SEM, scanning electron microscope; R ε , strain ratio; λ, twin fraction parameter; λ Norm , normalized λ value curves of pure Mg and Mg alloys (AZ31B, AZ61, and ZK60), there exist a kink point that demarcates the dominant deformation mechanisms. Twinning contributes significantly to material deformation when the strain amplitude is above the kink point.…”

“…Loading magnitude plays an important role in the deformation mechanisms of Mg alloys under cyclic loading. Earlier studies reveal that in the strain life fatigue curves of pure Mg and Mg alloys (AZ31B, AZ61, and ZK60), there exist a kink point that demarcates the dominant deformation mechanisms. Twinning contributes significantly to material deformation when the strain amplitude is above the kink point.…”