Structural instabilities in nanostructured metals

Marlene Kapp

Research output: ThesisDoctoral Thesis

Abstract

Nanocrystalline (NC) and ultrafine-grained (UFG) materials have attracted extensive attention due to their superior strength under monotonic loading but also during high cycle fatigue. Such high strength materials allow for constructions requiring less material volume and can therefore promote light-weight efforts for the automotive or aviation industry. Still some difficulties have to be overcome to make this material class reliable aspirants for technical applications. Firstly, their propensity to deform via shear bands during monotonic or cyclic loading causes the initial structure to locally collapse what deteriorates the material’s ductility or lifetime. Secondly, during cyclic loading the initial NC or UFG structure tends to coarsen what degrades the strength. The knowledge about the underlying driving forces and mechanisms for such instabilities is still at its infancy, but its understanding the key to overcome them. By using different experimental methodologies, involving static and cyclic loading conditions, intriguing insights into both phenomena have been gained. Cyclic micro bending experiments have been performed on UFG copper revealing structural modifications by a growth of larger grains at the expense of adjacent smaller ones. Thereby the continuous migration of high angle grain boundaries was identified as the basic mechanisms of cyclic grain growth. This migration procedure is not just thermally assisted, proven by cyclic high pressure torsion experiments under cryogenic conditions on UFG nickel where a distinct coarsening of the structure took place. Irrespective of the deformation temperature grain coarsening was amplified within shear bands, thus, regions of strain concentration. This emphasizes the crucial role of the cyclic strain in triggering the migration of a grain boundary. Because the hydrostatic pressure prevents failure of the sample the cyclic softening portion stemming exclusively from grain growth could be quantified for the first time. Additionally, grain coarsening as well as the propagation of shear bands could be studied up to large cyclic strains, which allow the conclusion that cyclically induced growing of grains is not an everlasting process as it levels off at a certain grain size. This final, coarsened grain size is determined by the nominal strain amplitude. Strain localizations affecting the mechanical behaviour occur also under static loading conditions, which was investigated on a pearlitic nanocomposite exhibiting a lamellar arrangement of ferrite and carbon rich cementite. Micro compression experiments revealed different types of strain localization, depending on the loading direction with respect to the lamellar orientation of the harder cementite. Perpendicular loading leads to shearing of the cementite into the shear band direction, parallel loading to buckling within a kink band and inclined loading localizes the strain by confined layer slip parallel to the cementite lamellae, without deforming it. Not only the lamellar orientation but also the strain path has an influence on the deformation behavior, as for instance during bending the strain gradient prohibits the formation of catastrophic shear bands. Also during cyclic bending homogeneous deformation occurs and in addition pronounced grain growth is prevented by the layered architecture. Thus, for cyclically loaded nanolamellar architectures structural instabilities can be avoided, which enhances the reliability for their in-service use.
Translated title of the contributionStrukturelle Instabilitäten in nanostrukturierten Metallen
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Pippan, Reinhard, Assessor A (internal)
  • Clemens, Helmut, Assessor B (internal)
Publication statusPublished - 2017

Bibliographical note

embargoed until null

Keywords

  • ultrafine-grained
  • nanolamellar
  • instability
  • grain growth
  • shear band
  • strength

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