Structural Reorganization During Cyclic Deformation

Annika Martina Diederichs

    Research output: Book/ReportPh.D. thesis

    142 Downloads (Pure)


    A major failure reason for structural materials is fatigue‐related damage due to repeatedly changing mechanical loads. During cyclic loading, dislocations self‐organize into characteristic ordered structures, which play a decisive role for the materials lifetime. The synchrotron technique High Resolution Reciprocal Space Mapping (HRRSM) using high energy X‐rays was successfully applied to characterize these heterogeneous deformation structures evolving during cyclic deformation of commercially pure, polycrystalline aluminium AA1050. Insight into the structural reorganization within single grains embedded in the bulk material is gained by in‐situ monitoring of the microstructural evolution during individual tension‐compression load cycles and after selected numbers of cycles along tension‐tension or tension‐compression cycling sequences. By High Resolution Reciprocal Space Mapping individual subgrains can be resolved in the bulk of polycrystalline specimens and their fate, their individual orientation and elastic strains, tracked during different loading regimes. With this approach, the evolution of the intragranular structure in selected grains was followed.
    Four or five grains were monitored during each of in total four weeks of beam time at Argonne Photon Source and PETRA and a detailed analysis of their subgrain structure is presented for selected grains. Initially, the microstructural changes during tension‐tension cycling were investigated, where the azimuthal maps and radial profiles of in total four grains were analyzed during a cycling sequence of 7350 cycles. It was possible to follow the same subgrain over the entire cycling sequence. It is concluded that the microstructure is stable during the saturation stage of cyclic deformation, since only minor microstructural changes where observed in azimuthal maps and radial profiles during cycling sequences. It was however shown that major changes are occurring during the first cycles after tensile loading possibly due to structural reorganization for adaptation of the cyclic deformation condition. During in total three weeks of beam time tension‐compression cycling was investigated, where finally up to 60 acquisitions were done for individual grains monitoring the microstructural changes during tension‐compression cycling and along five subsequent tension‐compression load cycles. A characteristic behavior of the peak profile width and peak profile asymmetry was revealed. It was observed for the first time that the maximum asymmetry does not occur at the maximum tension and compression, but around the yield points. This is attributed to a size effect of the subgrains. The elastic back strains of subgrains are Gaussian distributed with larger subgrains showing larger back strains implying a size effect. Four subgrains were followed during three subsequent load cycles with fourteen HRRSM acquisitions along each hysteresis. The subgrains showed different behaviors of the local elastic strains. All investigated grains showed different values for the profile position, width and asymmetry as well. It is therefore concluded that the local environment is of high importance for the behavior of grains and subgrains under applied stress.
    The detailed characterization of the microstructure during cyclic loading by in‐situ monitoring of the internal structure within individual grains facilitates the understanding of the material behaviour during cyclic deformation by providing experimental findings for the development of models predicting the materials performance.
    Original languageEnglish
    PublisherDanmarks Tekniske Universitet (DTU)
    Number of pages256
    ISBN (Electronic)978-87-7475-566-1
    Publication statusPublished - 2019


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