Fatigue crack growth of nanocrystalline and ultrafine-grained metals processed by severe plastic deformation

Thomas Leitner

    Research output: ThesisDoctoral Thesis

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    Ultrafine-grained (UFG) and nanocrystalline (NC) metals exhibit extraordinary mechanical properties, such as very high yield strengths and increased fatigue limits. However, in order to safely use these materials for engineering applications, the investigation of their fracture behavior under quasi-static and cyclic loading is essential. The present thesis focuses on the fatigue crack growth behavior of UFG and NC metals generated by severe plastic deformation (SPD) techniques. The aim of this work is to study the influence of SPD processing and the resulting grain refinement on crack propagation processes. For the present thesis a broad variety of materials, ranging from pure metals (iron and nickel) to complex industrially used alloys (austenitic steel, pearlitic steel and a shape memory alloy), were deformed by high pressure torsion and equal channel angular pressing to obtain UFG and NC microstructures. Fatigue crack growth experiments were performed on samples of these metals in their un-deformed and SPD processed state. Different specimen orientations in respect to the deformation process were tested for the investigation of the fatigue crack growth anisotropy. Additionally, extensive fracture surface analyses were carried out to get further information about the mechanisms of fatigue crack propagation. It will be shown, that the deteriorated fatigue crack growth behavior of UFG and NC materials, which is often reported in literature, can be attributed to a reduction of crack closure contributions. Intergranular fracture, which is typical for the failure of SPD metals under cyclic loads, is found to have a lower crack growth resistance, compared to a transgranular crack growth. However, at high mean stresses, where crack closure contributions play only a minor role, grain refinement can increase the intrinsic fatigue crack growth resistance. This improvement is a result of the increased strength of the material, but is only observed when the fracture mode is not changed. Dislocation patterns on the fracture surfaces, as well as estimations of the fatigue crack growth rate from deformation based models indicate that fatigue crack propagation is governed by a blunting and re-sharpening process along the grain boundaries in the investigated UFG and NC materials. Furthermore, it will be proven that the anisotropic fatigue crack growth behavior of the SPD processed metals originates from the elongated grains of SPD processed metals. This effect can be positively exploited by using the material in an orientation, with the longer axis of the grains perpendicular to expected crack growth directions, as this results in an increased crack growth resistance.
    Translated title of the contributionErmüdungsrisswachstum in hochverformten nanokristallinen und ultrafeinkörnigen Metallen
    Original languageEnglish
    • Clemens, Helmut, Assessor B (internal)
    • Pippan, Reinhard, Assessor A (internal)
    Publication statusPublished - 2017

    Bibliographical note

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    • Fatigue crack growth
    • severe plastic deformation
    • SPD
    • high pressure torsion
    • HPT
    • ECAP
    • ultrafine-grained
    • nanocrystalline
    • fracture mechanics

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