Investigation of the Lanthanide Sesquioxides as High Temperature Transformation Toughening Agents
Jero, Paul Daniel
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https://hdl.handle.net/2142/71711
Description
Title
Investigation of the Lanthanide Sesquioxides as High Temperature Transformation Toughening Agents
Author(s)
Jero, Paul Daniel
Issue Date
1988
Doctoral Committee Chair(s)
Kriven, W.M.,
Department of Study
Ceramics Engineering
Discipline
Ceramics Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Abstract
The lanthanide sesquioxides were investigated as possible high temperature transformation toughening agents. Specifically, the monoclinic to cubic (B $\Rightarrow$ C) transformation in Gd$\sb2$O$\sb3$ and Tb$\sb2$O$\sb3$ was examined. This transformation involves an 8-10% volume expansion and a shape change ($\Delta\beta$) of $\sim$10$\sp\circ$. Techniques used in the investigation of the transformation behavior included: grinding, cooling in liquid nitrogen, DTA, dilatometry, ambient and high temperature Vickers indentation, and notched-beam three point flexure.
A distinct change in B $\Leftrightarrow$ C transformation kinetics occurred on moving from Gd$\sb2$O$\sb3$ to Tb$\sb2$O$\sb3$. Tb$\sb2$O$\sb3$ exhibited a rapid B $\Rightarrow$ C transformation, whereas Gd$\sb2$O$\sb3$ remained in the B phase after high temperature processing. This difference is believed to result from the operation of different transformation mechanisms in these materials. Specifically, the occurrence of a rapid B $\Rightarrow$ C transformation in Tb$\sb2$O$\sb3$ at temperatures as low as 200$\sp\circ$C indicates that it can transform by a diffusionless mechanism.
The B $\Rightarrow$ C transformation in Gd$\sb2$O$\sb3$ was not observed, except in a thin surface layer on Gd$\sb2$O$\sb3$ specimens which were ground and subsequently annealed at temperatures $\geq$700$\sp\circ$C for extended periods of time. Specimens of pure Tb$\sb2$O$\sb3$ fired above 1700$\sp\circ$C shattered on cooling through the B $\Rightarrow$ C transformation. Laser melting and roller quenching, however, resulted in thin flakes which were primarily B phase. Incorporation of the Tb$\sb2$O$\sb3$ into a matrix of MgO or 2Tb$\sb2$O$\sb2\cdot$Al$\sb2$O$\sb3$ allowed preparation of bulk specimens. Extremely rapid cooling was required in order to retain the Tb$\sb2$O$\sb3$ in the high temperature B form. Cooling rate was observed to be the dominant factor in retention of the high temperature form.
The B $\Rightarrow$ C transformation could not be induced in Tb$\sb2$O$\sb3$-MgO specimens at room temperature. Elevated temperatures were required in order to overcome the nucleation barrier to transformation. The B $\Rightarrow$ C transformation was observed on the fracture surfaces of specimens broken at $\geq$1075$\sp\circ$C. Spontaneous transformation was observed on roughly ground surfaces at 1300$\sp\circ$C, and in the bulk at 1400$\sp\circ$C. In Tb$\sb2$O$\sb3$-Al$\sb2$O$\sb3$ specimens, transformation on fracture surfaces was observed at 1150$\sp\circ$C and spontaneous surface and bulk transformation at 1300$\sp\circ$C. High temperature mechanical testing showed an increase in toughness with temperature in these specimens.
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