Molecular dynamics analysis of oxidation, segregation and stress corrosion failures of refractory alloys

Open Access
Author:
Verners, Osvalds
Graduate Program:
Mechanical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 10, 2014
Committee Members:
  • Adrianus C Van Duin, Dissertation Advisor
  • Adrianus C Van Duin, Committee Chair
  • Md Amanul Haque, Committee Member
  • Zoubeida Ounaies, Committee Member
  • Sulin Zhang, Committee Member
Keywords:
  • oxidation
  • segregation
  • stress corrosion cracking
  • hydrogen embrittlement
  • alloys
Abstract:
The focuses of the thesis are heating induced segregation/mixing of refractory alloys, along with oxidation and stress corrosion properties of selected fcc metals and thin oxide layers formed on the surfaces thereof. The particular studies include segregation and oxidation simulation of Mo3Ni alloy clusters. These reveal favorable stabilizing oxidation resistance properties due to the Ni component, which diffuses during annealing to the surface of the clusters. A comparative study has been done for different sized Al grains in Fe or Ni bulk matrices. Its results indicate that Ni matrix is favorable due to the grain dissolution and energetic stability properties upon heating and cooling of the structures. Oxidation simulation of the same structures in slab structures indicate that unmixed metals oxidize first and the alloy layer, which forms only for the Ni matrix, eventually segregates to single-metal layers, which oxidize subsequently. The stress corrosion properties of Al oxide slab/thin film structures in water, noble gas and vacuum environments have been studied with the aim of subsequent stress corrosion simulation of alloys or metals with protective surface oxide layers. The obtained results indicate brittle type failures, which involve shear deformation and localized amorphization. The plasticity enhancing fluid environment effects are found to be similar for both reactive and nonreactive species, which indicates significant pressure effects and passivated reactivity of surfaces. Parallel to the corrosion study, strain rate effects and cyclic loading behavior for slab structures in vacuum have been characterized and compared at different temperatures. These indicate time dependent deformation mechanisms including temperature enhanced local amorphization prior to crack formation. Complementary analyses include extended timescale crack behavior of a slab structure in vacuum using parallel replica dynamics and steady state analysis of a slab structure in water using a Grand Canonical Monte Carlo method. The latter allow for validation of the high strain rate and limited timescale results obtained with classical molecular dynamics. Hydrogen embrittlement studies have been conducted for Al metal slabs with oxidized surfaces. These have provided evidence for possible grain boundary decohesion and void formation related embrittlement mechanisms at different loading conditions with different initial defects. Studies of stress corrosion properties of Ni metal slab structures in water at different temperatures have been performed. The results indicate reduction of dislocation nucleation barriers due to the water reactions on material surfaces and increased temperature, resulting in reduction of material ductility. Significant effects of stress triaxiality are also observed.