Ultrasonic fully reversed tension fatigue tests have been performed in the Very High Cycle Fatigue (VHCF) regime (NR>107−108cycles) on Ti-6Al4V specimens containing a controlled internal notch. Two sets of samples have been used. The first one contains a central chimney along the specimen longitudinal axis which brings air to the internal notch; in the second series the notches are not connected to the surface. The microstructure present below the fracture surface of the broken specimens has been studied by electron microscopy (EBSD, TKD and TEM). The formation of nanograins and nanovoids was observed below the surface of the cracks growing in a vacuum environment but not below the surface of cracks connected with ambient air. In the latter case extensive striations were observed. Below each striation the formation of tensile {101̄2} twins was observed.

中文翻译：

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Ti 合金在真空和空气环境中内部疲劳裂纹尖端的变形机制


在极高周疲劳 （VHCF） 状态 （NR>107-108cycles） 下，对包含受控内部缺口的 Ti-6Al4V 试样进行了超声全反向拉伸疲劳试验。已使用两组样本。第一个包含沿试样纵轴的中央烟囱，将空气引入内部缺口;在第二个系列中，槽口不连接到表面。通过电子显微镜（EBSD、TKD 和 TEM）研究了破损试样断裂表面下方的微观结构。在真空环境中生长的裂纹表面下方观察到纳米颗粒和纳米空隙的形成，但在与环境空气相连的裂纹表面下方未观察到。在后一种情况下观察到广泛的条纹。在每个条纹下方观察到拉伸 {101̄2} 双胞胎的形成。