Doctoral Dissertations

Date of Award

8-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

George M. Pharr

Committee Members

Easo P. George, John D. Landes, Carl J. McHargue

Abstract

Oxide dispersion strengthened (ODS) ferritic steels are currently being investigated as candidate materials for nuclear applications due to their increased high temperature strength and low activation characteristics. Recent studies have shown that ODS ferritic steels containing Ti exhibit enhanced high temperature properties due to the formation of a very fine dispersion of nanometer-sized oxide clusters based on Ti, Y, and O. Studies are currently underway to examine so called 14YWT alloys with nominal compositions of Fe-14Cr-3W-0.4Ti (wt. %) mechanically alloyed with 0.25 (wt.%) Y2O3. The focus of this study was to investigate how the early stages of processing of 14YWT alloys during mechanical milling, heat treatment, and consolidation affect the structure and properties of the alloys.

The 14YWT alloys were milled at different times up to 80 hours, along with alloy powder compositions of Fe-14Cr + 0.25 wt.% Y2O3 (14Y) and Fe-14Cr without Y2O3 (Fe-14Cr). The evolution of the microstructure and mechanical properties during milling was examined with a combination of optical metallography, x-ray diffraction, electron microscopy, atom probe tomography, and nanoindentation. Alloy powders were also heat treated and studied using high temperature x-ray diffraction and differential scanning calorimetry methods. Special attention was paid to milling parameters and temperature ranges which lead to the formation of nano-sized oxide clusters in the alloys. Finally, the microstructure of consolidated alloys was examined and related to milling and heat treatment methods.

Mechanical properties and microstructure during milling were similar in the three alloy powders examined regardless of dispersoid or alloy addition. Mechanical mixing of the alloy powders was inefficient after 40 hours of milling. Milling did not produce bulk amorphous phases but quickly reduced the crystallite size to ~10-20 nm. Milling also resulted in relatively uniform dissolution of Y2O3. Thermal studies in the range of 600-850 °C showed precipitation of oxide clusters occurred before consolidation which blocked recrystallization and grain growth. Results also showed retention of sub-micron sized grains at high temperature in both the annealed powders and extruded material was seen to be direct evidence of nano-cluster formation. Alloy extrusions conducted at 1175 °C showed coarse oxide precipitation while extrusions at 850 °C showed nanometer-sized clusters. The results indicated that the heat treatment temperature before consolidation was very important for producing the most favorable microstructure.

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