Date of Award

12-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Rack, Henry J.

Committee Member

Hudson , JoAn

Committee Member

Luo , Jian

Committee Member

Pennington , William T.

Abstract

The present dissertation has investigated the effect of oxygen on the ù and á phase stability in metastable Ti-Mo â titanium alloys using thermal analysis, hardness measurements, electron microscopy, and x-ray diffraction.
Single crystal x-ray diffraction has shown that oxygen atoms are located in the tetrahedral interstitial lattice sites in the rapidly cooled bcc Ti crystal structure, interfering directly with the reversible displacive formation of ù, with this transformation involving collapse of the bcc lattice along <111>â. Subsequent thermal exposure of reversible ù, as occurring during slower cooling, heating, and aging, prompts short range diffusion and the formation of chemical altered irreversible ù.
X-ray diffraction particle size analysis based on the Warren-Averbach approach has shown that the continued irreversible ù phase evolves in four stages during isothermal aging, initial growth followed by size stabilization, coarsening, and dissolution. The latter stages of ù evolution are controlled by elastic residual stresses surrounding these particles. Ultimate stress relaxation is based on secondary formation and growth, promoting coarsening and dissolution of ù. All of the aforementioned stages can be accelerated by increasing both the oxygen content and isothermal aging temperature.
The hardness response parallels this evolution and is dependent upon the ù and á phase evolution. The initial hardness increase is due to the growth of ù. The hardness plateau is based on stabilized ù size and fine á precipitation. The overaging hardness response is due to continuous secondary á formation and growth combined with ù coarsening and dissolution. Hardness increases with increasing interstitial content as a result of solid solution strengthening and á particle refinement.
Isochronal and isothermal thermal analysis has shown that increasing oxygen content promotes the á phase formation thereby increasing the ù instability. Grain boundary and primary á morphology growth are associated with the growth and thickening of sideway á platelets at low aging temperatures and longer aging times. The highest oxygen levels investigated, 2 weight percent (wt%), promotes earlier intragranular á nucleation.

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