We study the macro- and microfractographic features of the mechanism of initiation and propagation of fatigue cracks in the nitinol alloy after its testing for low-cycle fatigue and analyze possible influence of the structural and phase transformations caused by the cyclic deformation of nitinol on the fractographic features of its fatigue fracture. Thus, almost parallel transcrystalline facets of brittle cleavage located in almost mutually perpendicular planes along the entire length of martensite crystals are observed within the boundaries of separate grains (first of all, in the early stages of fracture). The signs of shallow fatigue striations are detected (but rarely) in the zone of the stable growth of the fatigue crack. The spacing of these striations approximately corresponds to a crack-growth rate of 8 ∙ 10 7 m/cycle. It is suggested that the deformation transformation of austenite into martensite can also distort the classical deformation mechanism of formation of the fatigue striations. In the zones of fractures with uncontrolled crack growth, the elements of ductile pit topography are predominant, which is typical of the fracture surfaces of specimens destroyed under active loading.

中文翻译：

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镍钛合金疲劳断裂的断口特征

我们研究了镍钛合金低周疲劳试验后疲劳裂纹萌生和扩展机制的宏观和微观断口特征，并分析了镍钛合金循环变形引起的结构和相变对断口形貌的可能影响。其疲劳断裂特征。因此，在分离晶粒的边界内（首先，在断裂的早期阶段）观察到沿马氏体晶体的整个长度位于几乎相互垂直的平面中的脆性解理的几乎平行的跨晶面。在疲劳裂纹稳定增长的区域检测到浅疲劳条纹的迹象（但很少）。这些条纹的间距大约对应于 8 ∙ 10 7 m/周期的裂纹增长速率。这表明奥氏体向马氏体的变形转变也可以扭曲疲劳条纹形成的经典变形机制。在裂纹扩展不受控制的断裂带中，韧性坑形貌元素占主导地位，这是主动载荷下试样破坏的典型断裂面。