Fractographic Features of the Fatigue Fracture of Nitinol Alloy
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FRACTOGRAPHIC FEATURES OF THE FATIGUE FRACTURE OF NITINOL ALLOY V. P. Iasnii,1, 2 H. M. Nykyforchyn,3 O. Z. Student,3 and L. M. Svirska3
UDC 620.193.81: 620.197.6
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. Keywords: shape-memory alloy, martensitic transformation, fractography, hydrogen effect, plastic deformation.
Alloys with the properties of pseudoelasticity [1] and shape memory [2] are now used in various branches of engineering more and more extensively [3–6]. Among these alloys, we can especially mention nitinol (Ni–Ti alloy). Its ability to resist to cyclic loads is regarded as one of its most important mechanical characteristics [7] specifying its serviceability as a functional material with hyperelasticity effect. Numerous specific features of pseudoelastic nitinol under the conditions of low-cycle loading were determined in [8]. In particular, these features include a rapid decrease in the amplitude of deformation and an increase in the level of residual strains observed for about ten loading cycles and is terminated by the region of stabilization of these characteristics. Note that the pseudoelastic behavior of these alloys is explained by the direct and inverse austenite-martensite transformations. Under the influence of deformation caused by loading, we observe the direct phase transformation. In the course of unloading, we observe a subsequent recovery of deformation of the metal even at temperatures higher than the temperature of termination of the austenitic transformation. As a result of these structural and phase transformations, the Ni–Ti alloy may recover up to 6-8% of the level of strains. The signs of embrittlement of nitinol are connected, first of all, with a decrease in the energy consumption of the fracture processes. The fractographic analysis of the fracture surfaces reveals these specific features. In particular, this is well visible in th
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