Alexandria Digital Research Library

Study of the Structure, Composition, and Stability of Yttrium-Ti-Oxygen nm-Scale Features in Nano-Structured Ferritic Alloys

Cunningham, Nicholas John
Degree Grantor:
University of California, Santa Barbara. Mechanical Engineering
Degree Supervisor:
G. Robert Odette
Place of Publication:
[Santa Barbara, Calif.]
University of California, Santa Barbara
Creation Date:
Issued Date:
Engineering, Mechanical and Engineering, Materials Science
Nanostructured Ferritic Alloy
Atom Probe Tomography
Small Angle Neutron Scattering
ODS steels
Thermal Stability
Alternative Processing
Dissertations, Academic and Online resources
Ph.D.--University of California, Santa Barbara, 2012

This work advances the understanding of the Y-Ti-O nanofeatures (NFs) in nanostructured ferritic alloys (NFAs); a class of high temperature, oxide dispersion strengthened iron alloys with applications in both advanced fission and fusion reactors. NFAs exhibit high creep strength up to 800ºC and a remarkable radiation damage tolerance and He management. However, the NFs, which are responsible for these properties, are not fully understood. This work addresses key questions including: a) what is the NF structure and composition and how are they affected by alloy composition and processing; b) what is the NFA long-term thermal stability; c) and what alternative processing paths are available to reduce costs and produce more uniform NF distributions?

A detailed study using small angle neutron scattering (SANS), transmission electron microscopy (TEM-group member Y. Wu), and atom probe tomography (APT) evaluated the NF average size (), number density (N), volume fraction (f), composition, and structure in two heats of the commercial NFA MA957. The and N were &ap;2.6 nm and &ap;5x1023 m-3 , respectively, for both heats, with TEM indicating the NF are Y 2Ti2O7. However, SANS indicates a mixture of NF compositions or atomic densities with a difference between the heats, while APT shows compositions with &ap; 10% Cr and a Y/Ti ratio < 1. However, microscope artifacts such as preferential undercounting of Y and O or trajectory aberrations that prevent resolving Ti segregation to the NF-matrix interface could account for the discrepancy.

The microstructure and NFs in MA957 were stable for long times at temperatures up to 900&ordm;C. Notably, Ti in the matrix and some from the NFs migrates to large, Ti-rich phases. Aging at higher temperatures up to 1000&ordm;C for 19.5 kh produced modest coarsening for &ap; 3.8 nm and &ap;30% increase in grain size for a corresponding 13% reduction in microhardness. A coarsening model shows no significant NF coarsening will occur at temperatures less than 900&ordm;C.

A number of 14YWT NFAs with different Y, Ti, and O content showed that increases in Y and Ti produce higher N and f, but the and the compositions are fairly insensitive. The O content has a more dramatic affect with low O (0.065 wt.%) producing low microhardness due to large grains (>10 mum) and large NFs with higher Y/Ti ratios. High O (0.249 wt.%) produced the smallest grain size, small NF, and highest microhardness.

An alternative alloying technique adds Y during the gas atomization process, but the Y was phase separated, requiring 20 to 40 h attritor milling for adequate mixing. He atmosphere atomization produced more uniformly distributed Y compared to Ar and Ar/O atmospheres, and the Ar/O powder required longer milling. After milling, the Y-Ti-O solutes were concentrated on dislocation cell or grain boundaries, but were not highly associated with each other. The grain size distribution was bimodal for all annealing or consolidating at 1150&ordm;C and unimodal when consolidating at 850&ordm;C. The NF systematically increased and the N and f decreased with annealing temperature, and iso-thermal annealing tended to produce slightly smaller NFs. The final alloy produced (PM2) had a very high tensile strength with a low DBTT compared to other NFAs, but creep performance was scattered and requires additional testing.

Physical Description:
1 online resource (351 pages)
UCSB electronic theses and dissertations
Catalog System Number:
Inc.icon only.dark In Copyright
Copyright Holder:
Nicholas Cunningham
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