Al Maharbi, Majid H. (2009-12). Mechanical Flow Response and Anisotropy of Ultra-Fine Grained Magnesium and Zinc Alloys. Doctoral Dissertation. Thesis uri icon


  • Hexagonal closed packed (hcp) materials, in contrast to cubic materials, possess
    several processing challenges due to their anisotropic structural response, the wide
    variety of deformation textures they exhibit, and limited ductility at room temperature.
    The aim of this work is to investigate, both experimentally and theoretically, the effect
    os severe plastic deformation, ultrafine grain sizes, crystallographic textures and number
    of phases on the flow stress anisotropy and tension compression asymmetry, and the
    mechanisms responsible for these phenomena in two hcp materials: AZ31B Mg alloy
    consisting of one phase and Zn-8wt.% Al that has an hcp matrix with a secondary facecentered
    cubic (fcc) phase. Mg and its alloys have high specific strength that can
    potentially meet the high demand for light weight structural materials and low fuelconsumption
    in transportation. Zn-Al alloys, on the other hand, can be potential
    substitutes for several ferrous and non-ferrous materials because of their good
    mechanical and tribological properties. Both alloys have been successfully processed
    using equal channel angular extrusion (ECAE) following different processing routes in order to produce samples with a wide variety of microstructures and crystallographic
    textures for revealing the relationship between microstructural parameters,
    crystallographic texture and resulting flow stress anisotropy at room temperature. For
    AZ31B Mg alloy, the texture evolution during ECAE following conventional and hybrid
    ECAE routes was successfully predicted using visco-plastic self-consistent (VPSC)
    crystal plasticity model. The flow stress anisotropy and tension-compression (T/C)
    asymmetry of the as received and processed samples at room temperature were
    measured and predicted using the same VPSC model coupled with a dislocation-based
    hardening scheme. The governing mechanisms behind these phenomena are revealed as
    functions of grains size and crystallographic texture. It was found that the variation in
    flow stress anisotropy and T/C asymmetry among samples can be explained based on the
    texture that is generated after each processing path. Therefore, it is possible to control
    the flow anisotropy and T/C asymmetry in this alloy and similar Mg alloys by
    controlling the processing route and number of passes, and the selection of processing
    conditions can be optimized using VPSC simulations. In Zn-8wt.% Al alloy, the hard
    phase size, morphology, and distribution were found to control the anisotropy in the flow
    strength and elongation to failure of the ECAE processed samples.

publication date

  • December 2009