Institute for Applied Materials – Materials Science and Engineering

Deformation Mechanisms in FCC and BCC High Entropy Alloys Under Various Conditions

M.Sc. Aditya Srinivasan Tirunilai

Motivation

The aim of the proposed project is to reveal the underlying mechanisms of deformation in ductile high entropy alloys (CoCrFeMnNi and HfNbTaTiZr) at low temperature or at high strain rates. For this purpose, the alloys have to be prepared in according microstructural conditions in a reproducible manner as well as disorder has to be verified bridging several length scales. Subsequently, the contributions of dislocation slip and mechanical twinning will be quantitatively evaluated at low temperatures and high strain rates based on microstructural investigations. This will especially lead to a better understanding of the not intensively investigated deformation mechanisms in HfNbTaTiZr with its rather unexpected high ductility at room temperature. The gained knowledge will be later on used to investigate the interaction of dislocations with lattice distortion and local disorder induced by the mixed elements by suppression of thermal activation at even lower temperatures (temperature of liquid helium, 4.2 K). In order to reveal these interactions, the fundamentally different physical properties - low thermal conductivity and very low specific heat - have to be quantitatively taken into account since these may result in lack of coupling to the cooling medium and in retarded heat dissipation from zones of localized deformation. In the end, a holistic understanding of the interaction of dislocations and lattice distortion is aimed by further analyzing its most important macroscopic manifestation: solid solution hardening. For this purpose, a recently proposed theory for describing solid solution hardening in concentrated alloys will be verified within the CoCrFeMnNi system and tested for possible transfer to HfNbTaTiZr.

Objectives

  • synthesis of face-centered and body-centered cubic solid solutions (CoCrFeMnNi and HfNbTaTiZr)
  • optimization of the microstructure by thermo-mechanical treatment (with our partners at IFW Dresden)
  • characterization of the temperature dependence of mechanical properties (with our partners at CryoMaK)
  • investigation and modeling of the texture und microstructural changes during uniaxial loading under various conditions
  • revealing deformation mechanisms as well as their relations to the mechanical properties of the materials

Methods

  • synthesis of the material by arc melting and subsequent thermo-mechanical treatment
  • macroscopic deformation tests under various conditions
  • characterization of deformation by analytical scanning electron microscopy (orientation imaging by electron backscatter diffraction), transmission electron micrscopy as well as texture determination

Partner

Funding

This project is part of the DFG priority program SPP 2006 "CCA-HEA".