טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentEliash Tal
SubjectCorrelation between Nano and Macro Mechanical Properties of
Poly Crystalline Molybdenum
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Eugen Rabkin
Full Thesis textFull thesis text - English Version


Abstract

Grain Boundaries (GB) have a strong effect on properties of polycrystalline solids. The increase of hardness and of yield stress with decreasing average grain size is known as a Hall-Petch relationship. However, little is known about the microscopic plasticity mechanisms in the vicinity of individual GBs and their triple junctions. In this work we used the depth-sensing indentation technique to study the nanoscale mechanical properties of molybdenum in the vicinity of GBs and their triple junctions. We tested both the tricrystals prepared by electron beam floationg zone melting technique and ultrafine grained samples obtained by severe plastic deformation technique.

Main findings of this research are:

  • A region with increased hardness is revealed in the vicinity of all GBs studied. The width of this region is about 2 mm and the maximal relative increase of hardness at the GB is about 30 %. We propose a phenomenological model that treats the GBs as efficient slip barriers and thus explains their hardening behaviour.
  • The observed increase of nanohardness in the vicinity of triple junctions (as compared with the GBs) can be understood in terms of the proposed model.
  • Incipient plasticity is observed at various levels in both the GB and the grain interior as a displacement bursts (Pop-Ins) in the Load-Displacement curves. The Pop-Ins are discussed in terms of homogeneous dislocation nucleation.
  • The hardening effect of GBs is consistent with the increased nanohardness of ultrafine grain molybdenum.
  • We also observed an indentation size effect and discussed it in terms of the model of Nix and Gao.