预扭转对挤压AZ31镁合金超高周疲劳性能的影响

Effect of Pre-torsion on very High Cycle Fatigue Properties of Extruded AZ31 Alloy

目的研究各向异性和预扭转处理对挤压AZ31镁合金组织演变和超高周疲劳性能的影响规律,探索不同角度预扭转处理下AZ31镁合金的超高周疲劳断裂机制。方法设计狗骨形和三喉部试件,沿挤压方向(ED)和径向方向(RD)对狗骨形试件进行超声疲劳实验,对两方向三喉部试样的主喉部进行预扭转,并对预扭转后的三喉部试件进行超声疲劳实验,采用金相显微镜和扫描电镜(SEM)对疲劳实验后的试件进行显微组织观察,通过分析不同状态下的疲劳断口形貌,探索合金的超高周疲劳机制。结果狗骨形试件沿ED和RD方向单轴超声疲劳的S-N曲线为连续下降的直线,两直线接近平行,其疲劳寿命减小的趋势一致。当试件加载相同应力幅值时,ED-AF试件疲劳寿命高于RD-AF试件。与初始样相比,预扭转后沿ED方向试样的疲劳寿命大于3.6256×10^(5)周次,疲劳寿命提高2倍以上;沿RD方向试样疲劳寿命大于5.1285×10^(5)周次,疲劳寿命提高10倍以上。结论ED-AF试件初始疲劳变形机制为位错滑移-孪生,随着循环周次累积后转为孪生-退孪生循环;RD-AF试件疲劳变形机制为孪生-退孪生循环。挤压形成的流线型微观结构降低了疲劳裂纹扩展的速率,增加了裂纹的阈值水平,造成两方向的S-N曲线不同。预扭转变形引入的{1012}拉伸孪晶改变了超高周疲劳加载过程中的疲劳机制,延长了退孪生过程,抑制了试样沿c轴方向拉伸时的位错滑移;使ED和RD方向上的疲劳性能均得到改善。

The work aims to investigate the effects of anisotropy and pre-torsion treatment on microstructure evolution and super high cycle fatigue properties of extruded AZ31 magnesium alloy and explore the very high cycle fatigue fracture mecha-nism of the alloy after pre-torsion treatment at different angles.Dog bone and three-throat specimens were designed,and very high cycle fatigue tests were conducted on dog bone specimens along the extrusion direction(ED)and the radial direction(RD).Pre-torsion treatment was performed on the main throat of the three-throat specimens in both directions,and very high cycle fa-tigue test was conducted on the three-throat specimens after pre-torsion.The microstructure of the fatigue specimens was ob-served by metallographic microscope and scanning electron microscope(SEM),and the very high cycle fatigue mechanism of the alloys was explored by analyzing the fatigue fracture morphology under different states.The S-Ns of uniaxial ultrasonic fa-tigue of dog bone specimen along ED and RD directions were straight lines that continuously decreased,and the two straight lines were nearly parallel,and the fatigue life decreased in the same trend.The fatigue life of ED-AF specimen was higher than that of RD-AF specimen under the same stress amplitude.Compared with the as-received specimen,the fatigue life of the specimen along the ED and RD direction after pre-torsion are greater than 3.6256×10^(5)cycles and 5.1285×10^(5)cycles respec-tively,and the fatigue life are increased by more than 2 times and 10 times.The initial fatigue deformation mechanism of ED-AF specimens is dislocation slip-twinning,which changes to twin-detwinning cycle with the accumulation of cycles.The fa-tigue deformation mechanism of RD-AF specimens is twin-detwinning cycle.The streamlined microstructure formed by extru-sion reduces the crack propagation rate,increases the crack threshold level,and results in different S-N curves in both directions.The tensile twins{1012}introduced by the pre-torsion deformation change the fatigue mechanism during very high cycle fa-tigue loading,prolong the detwinning process,and inhibit the dislocation slip of the specimen along the c-axis.The fatigue properties in both ED and RD directions are improved.