Bulk metallic glass composites fabricated via high pressure torsion

Lisa Krämer

Research output: ThesisDoctoral Thesis

116 Downloads (Pure)

Abstract

For this thesis high pressure torsion (HPT) was used to fabricate bulk metallic glasses (BMGs) and bulk metallic glass composites (BMGCs) starting from powders. The powders (amorphous and crystalline) were mixed by hand and the subsequent HPT-process leads to consolidation of the particles and refinement of the microstructure. Using this technique gives a high flexibility regarding combination of different materials and their ratios. The evolution of the microstructures was investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), hardness measurements and transmission electron microscopy (TEM). BMGs generated from powders via HPT require a certain amount of applied shear strain to enforce welding of the particles, and a dense and homogenous microstructure can be obtained. This required strain depends on the used amorphous powder. The obtained microstructure of the BMGCs is lamellar and the phase spacing can be varied from micrometers to nanometers by increasing the applied shear strain. Very high deformation led to the formation of a saturation microstructure, which contained for most compositions only one amorphous phase generated by mixing of the initial materials. For one composite containing 80 wt% of Cu, a nanocrystalline supersaturated solid solution of Cu with elongated MG-nanolamellae was formed instead. Mechanical properties were investigated by means of nanoindentation, micropillar compression tests and tensile tests. To determine the onset of plastic deformation, a new method of analysis for strain rate jump tests was developed. Thereby, activation energy and volume can be calculated. The main parameter controlling the onset of shear bands is the testing temperature, while composition, energy state, fabrication method and even the second phase in composites have a minor effect. However, the second phase has a great influence on the propagation of the shear bands, which is shown by micropillar compression tests. The shear bands are affected by the material of the second phase (crystalline or amorphous), by the structural size of the phases and also by the orientation of the lamellar structure regarding the loading direction. In conclusion, BMGs and BMGCs were fabricated via HPT and the microstructural evolution was investigated. Changes in deformation behavior were studied and it was shown that the nucleation of shear bands depends mainly on the homologous temperature of the MG, while their propagation can be influenced by adding a second phase.
Translated title of the contributionSynthese metallischer Massivgläser mittels Hochdrucktorsion
Original languageEnglish
QualificationDr.mont.
Awarding Institution
  • Montanuniversität
Supervisors/Advisors
  • Pippan, Reinhard, Assessor A (internal)
  • Clemens, Helmut, Assessor B (internal)
Publication statusPublished - 2018

Bibliographical note

embargoed until null

Keywords

  • bulk metallic glass
  • bulk metallic glass composite
  • high pressure torsion
  • nanoindentation
  • micropillar compression test
  • microstructural evolution

Cite this