超细晶纯钛的低周疲劳行为

Low-Cycle Fatigue Behavior of Ultrafine-Grained Pure Titanium

摘要:

采用室温4道次等径弯曲通道变形+旋锻制备超细晶纯钛，利用Instron型电液伺服疲劳试验机在室温下分别对粗晶和超细晶纯钛进行应变控制的低周疲劳试验，应变幅范围为0.5%~1.1%。通过透射电子显微镜和扫描电子显微镜研究超细晶纯钛疲劳变形后的组织和断口形貌。结果表明：与粗晶纯钛相比较，超细晶纯钛具有更高的低周疲劳寿命，例如在总应变幅为0.5%时，超细晶和粗晶纯钛的疲劳寿命分别为10 850和4820周次。除总应变幅为0.5%外，超细晶纯钛在疲劳变形过程中表现出显著的循环软化现象。随着总应变幅的增大，疲劳滞后回线所包含的面积增大。断口形貌显示疲劳裂纹萌生于试样表面，疲劳扩展区存在大量疲劳辉纹和微裂纹，随着应变幅的增加，疲劳断裂模式由韧性断裂转变为准解理断裂。超细晶纯钛低周疲劳变形机理为位错滑移。

Abstract:

Ultrafine-grained (UFG) pure titanium was produced by equal channel angular pressing for 4 passes, followed by rotatory swaging at room temperature. The strain-controlled low-cycle fatigue tests of UFG and coarse-grained (CG) pure titanium were conducted by Instron electro-hydraulic servo fatigue testing machine in the strain amplitude range of 0.5%–1.1% at room temperature. Transmission electron microscope (TEM) and scanning electron microscope were used to investigate the microstructure and fracture surface of UFG pure titanium after fatigue tests. Results show that UFG pure titanium exhibits a longer low-cycle fatigue life, compared with the CG pure titanium. For example, at a total strain amplitude of 0.5%, UFG and CG pure titanium has fatigue life of 10 850 and 4820 cycles, respectively. Significant cyclic softening occurs in UFG pure titanium, except in the case of a total strain amplitude of 0.5%. Hysteresis loop area is increased rapidly with the increase in strain amplitude. The fracture surface shows that the fatigue crack is initiated from the specimen surface. A series of fatigue striations and many microcracks exist in the propagation region. With the increase in strain amplitude, the predominant failure mode is transformed from ductile failure into quasi-cleavage failure. Dislocation slip is the main plastic deformation mechanism of UFG pure titanium during low-cycle fatigue deformation.