Investigation into the mechanical character of grain boundaries in technically pure and micro-doped molybdenum

Severin Jakob

Research output: ThesisDoctoral Thesis

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Abstract

Technically pure Mo has a broad field of applications. It has a high melting point of 2620°C with no phase transformation. Therefore, its physical and mechanical/technological properties experience no abrupt changes up to elevated temperatures. Furthermore, the high electrical and thermal conductivity in combination with a low thermal expansion coefficient offers unique functional applications. The limiting factor as a structural material is, beside a brittle-to-ductile transition somewhere around room temperature, the tendency for brittle intercrystalline failure at low temperatures. Modern grain boundary engineering and in particular segregation engineering offers the possibility to mitigate this Achilles’ heel. The addition of B and/or C in small amounts to technically pure Mo is known to enhance the cohesion of grain boundaries and changes the fracture mode from intercrystalline to transcrystalline failure. So far, only this qualitative comparison has been described.
In this thesis, the chemical composition of the recrystallization front on semi-recrystallized samples is explored by atom probe tomography and diffusion considerations. The main part of this work deals with the mechanical response of grain boundaries. Investigations were carried out with micromechanical testing methods, such as nanoindentation, pillar compression and micro-sized bending beams on technically pure Mo and B micro-doped Mo. The possible influences during instrumented hardness testing at or near grain boundaries are elucidated and the crucial impact of indenter shape and rotation angle in combination with grain orientation is discussed. Pillar compression is performed to analyze slip transfer at the grain boundary and criteria for a comparison between material variants are presented. Experimentation on micro-sized bending beams is carried out on cantilevers with and without the introduction of a sharp notch. The amount of plastic deformation before fracture at a grain boundary might give insight into the resistance to tensile loading of the interface.
Moreover, a new meso-scale approach to examine crack initiation on the tensile-loaded extreme fiber of mm-sized bending samples is presented. In this way, a large number of grain boundaries are loaded and the influence of individual grain orientation pairings is negligible. The length of separated grain boundaries is measured on scanning electron micrographs and put into context to the total boundary length per sample area. The addition of B reduces the relative length of separated boundaries to one third, when comparing samples with similar microstructures. Furthermore, the grain boundaries' propensity to fracture is analyzed concerning possible slip transmission at the interface and crosschecked with the appearance of cracks. A distinct differentiation of regimes is apparent at 30° misorientation of the grain boundaries. 90% of cracks are at higher misorientation, which also correlates with a reduced potential for slip transmission.
The meso-scale testing approach was applied to industrial-scale quality control. Samples from semi-finished mill products were tested at the equipment of the industrial partner and appearing cracks were investigated. They show the same tendency towards lesser relative crack length in samples with B additions.
Translated title of the contributionUntersuchung der mechanischen Eigenschaften von Korngrenzen in technisch reinem und mikrodotiertem Molybdän
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Motz, Christian, Assessor B (external)
  • Maier-Kiener, Verena, Supervisor (internal)
  • Clemens, Helmut, Assessor A (internal)
  • Pippan, Reinhard, Co-Supervisor (external)
Publication statusPublished - 2021

Bibliographical note

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Keywords

  • molybdenum
  • grain boundaries
  • segregation engineering
  • atom probe tomography
  • interface cohesion
  • slip transfer
  • mechanical properties
  • micromechanical testing
  • meso-scale bending

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