A Strain-controlled Experimental Study Of Cyclic Deformation Behavior Of Extruded Magnesium Alloy

Magnesium(Mg)alloys is widely applied in traffic industry,3C products,automobile and aerospace industry,due to the potential advantages of low density,high specific strength,high stiffness and excellent recyclability.As a structural material,magnesium alloys are mostly work under cyclic loading and fatigue is the main failure forms.In addition,magnesium alloy products are mainly for casting magnesium alloys,thus the research for extruded magnesium alloy and especially for the new type of high-strength magnesium alloy(such as ZK60 magnesium alloy)is still in the foundation stage.Magnesium alloy and its alloys have a hexagonal close packed(hcp)crystal structure.At room temperature,activation of independent slip systems are few leading to low plasticity for magnesium alloy.Furthermore,magnesium alloy showed the anisotropic of mechanical properties obviously due to the polarity of twinning.Therefore,it is great important to understand the fatigue behavior and damage mechanism of magnesium alloys for the reliability and durability of the application by magnesium alloys.In this paper,the monotonic compression and tension evolution experiments and cyclic loading fatigue experiment for both extruded ZK60 ED and TD the specimens were conducted at room temperature.The mechanical properties were obtained from these experiments.In addition,the microstructure evolution of magnesium alloy was measured by EBSD,XRD and SEM which can be analyzed by combining with the mechanical properties.Detailed studies and achievements are as follows:(1)The effect of initial orientation on microstructural evolution for extruded ZK60 magnesium alloy were studied at room temperature.ED specimens were compressed to 3%,9% strain until fracture and tensioned to 5% strain until fracture.The microcosmic testing techniques such as XRD,EBSD and SEM were used to analysis the change of microstructure and texture,such as grain orientations and twin types.As the result,the stress-strain curve of ED specimens exhibited a pronounced tension-compression asymmetry which is relevant to the activation of twinning.Under compression,the stress-strain curve exhibited the cave-up shape.Under tension,the stress-strain curve exhibited the cave-down shape.(2)TD specimens were compressed to 1%,4%,7%,10% strain until fracture and tensioned to 1%,7% strain until fracture.In addition,grain orientation and fracture morphology were observed XRD and SEM for qualitative and quantitative analysis.As the result,the stress-strain curve of TD specimens exhibited almost symmetrical.Twinning is hard to be activated for TD specimens under both compression and tension,and dislocation slips is the dominant deformation mechanism.(3)The stress-strain curve and the strain-life curve of ED specimens were obtained from strain-controlled(the strain ratio is R1?(28)-)cyclic loading experiments.As the result,the stress-strain curve was approximately symmetrical when strain amplitude is greater than 2% due to complete twinning-detwinning.When strain amplitude is between 0.35% and 2%,the stress-strain curve became asymmetric due to the partial twinning-complete detwinning.When strain amplitude is lower than 0.35%,the dominant deformation mechanism is dislocation slips.The strain-life curve of ED specimens existed obvious kink points at the strain amplitude of 3% and 0.35%,revealing different deformation mechanism in different segments.(4)The stress-strain curve and the strain-life curve of TD specimens were obtained from strain-controlled(the strain ratio is R1?(28)-)cyclic loading experiments.Macroscopic mechanical properties and fatigue characteristics of TD specimens shown significant differences as well as the microstructure and fracture morphology.For TD specimens,the main deformation mechanism were slip at all strain amplitude.For TD specimens,the strain-life curve shown no kink point.When strain amplitude is within the range of 0.45% and 1%,the strain-life curve of TD specimens is consistent with the strain-life curve of ED specimens.When strain amplitude is lower than 0.45%,the fatigue lifes of TD specimens are shorter than ED specimens.(5)The traditional SWT model of fatigue life prediction was used to fit of three parametric equation for both ED and TD orientations.All PFSWT parameters obtained from loading under the condition of extruded ZK60 magnesium alloy into three parameter equation to calculate the fatigue life prediction.Comparing the fatigue life prediction results with the experiment measured the fatigue life,the results show that the traditional SWT model can better predict the fatigue life loading along different orientations.The original SWT fatigue model prediction results: 48% of the experimental datas(20 datas)are in the scope of the three times the error factor,all of the experimental datas(42 datas)are in the scope of the five times the error factor.