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|Title:||Investigation of the Lanthanide Sesquioxides as High Temperature Transformation Toughening Agents|
|Author(s):||Jero, Paul Daniel|
|Doctoral Committee Chair(s):||Kriven, W.M.,|
|Department / Program:||Ceramics Engineering|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(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.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.
|Date Available in IDEALS:||2014-12-16|