超细晶纯钛疲劳裂纹的扩展行为

对纯钛进行2道次室温等径弯曲通道变形(ECAP)、等径弯曲通道变形加旋锻复合变形(ECAP+RS)并在旋锻后在300℃和400℃退火1 h，制备出4种具有不同组织的超细晶纯钛。对这4种超细晶纯钛进行疲劳裂纹扩展实验并观察分析超细晶纯钛的显微组织和疲劳断口的形貌，研究了裂纹的扩展行为。结果表明：显微组织对超细晶纯钛的疲劳裂纹扩展门槛值和近门槛区有显著的影响；超细晶纯钛的疲劳裂纹扩展门槛值随着塑性变形量的增大而增大，随着旋锻后退火温度的提高而降低；疲劳裂纹扩展速率曲线因超细晶纯钛晶粒尺寸和强度的影响出现转折，转折前ECAP+RS复合变形纯钛的抗疲劳裂纹扩展能力比ECAP变形强，且随着退火温度的提高而降低；转折后4种超细晶纯钛的疲劳裂纹扩展速率相差较小，呈现出相反的结果。疲劳裂纹扩展寿命中转折前近门槛区裂纹扩展寿命占绝大部分，因而转折前的门槛值与近门槛区的扩展速率对抗裂纹扩展能力更为重要。

Fatigue Crack Propagation Behavior of Ultrafine Grained Pure Titanium

Four kinds of ultrafine grained (UFG) pure titanium were obtained by two-pass equal channel angular pressing (ECAP) at room temperature, ECAP+rotary swaging (RS) and ECAP+RS followed by annealing at 300℃ and 400℃ for 1 h, respectively. The fatigue crack growth tests of different UFG pure titanium were carried out, while their microstructure, the fatigue fracture morphology and the crack growth behavior were investigated by TEM and SEM. Results show that the microstructure has a significant effect on the threshold of fatigue crack growth rate and the near threshold zone of UFG pure titanium. The threshold values of fatigue crack growth rate for UFG pure titanium increase with the increase of strain, and decrease with the increase of annealing temperature after RS. The turning point occurs in the fatigue crack growth rate curve, which is affected by grain size and strength of UFG pure titanium. Before the turning point, UFG pure titanium produced by ECAP+RS has stronger resistance to fatigue crack growth than that produced only by ECAP, and the resistance to fatigue crack growth of UFG pure titanium after ECAP+RS decreases with the increase of annealing temperature. After the turning point, the fatigue crack growth rates of four kinds of UFG pure titanium are slightly different and the opposite result is presented. The threshold value before the turning point and the growth rate of the near threshold zone may play much important role in enhancing the resistance to the crack growth because the growth life of the crack of the near threshold zone before the turning point accounts for a very large part of the fatigue crack growth life.