Superplastic flow of mullite/2Y-TZP composites.

To explore the possibility of fine-grained superplasticity in mullite/zirconia composites, a systematic study of microstructural development and constitutive flow relations has been conducted. Powder processing and firing procedures have been developed to vary the volume fractions of mullite and 2Y-TZP (2 m/o yttria-stabilized tetragonal zirconia polycrystal), the sizes of matrix grains and inclusions, and the grain shape of mullite. Deformation of composites of a variety of microstructures allowed access to various flow regimes in two-phase superplastic flow. Microstructural evolutions (grain growth, cavitation and texture) during the superplastic deformation are examined. Results from such a study indicate that the mullite/(2Y-TZP) composites are indeed superplastic with a strain rate sensitivity of up to 0.8 at a moderate temperature and flow stress. Mullite, at all compositions, has a much higher flow stress, by two orders of magnitude, than 2Y-TZP. The flow softening and flow hardening behaviors of these two-phase composites are well described by an analytical continuum model due to Chen. In particular, in zirconia-rich composites, superplastic flow may be regarded as controlled by a soft (zirconia) matrix hardened by rigid (mullite) inclusions. Likewise, in mullite-rich composites, superplastic flow may be regarded as controlled by a hard (mullite) matrix softened by fluid-like (zirconia) inclusions. Grain growth and cavitation are not observed for most deformation of these composites. Texture is found in the tetragonal zirconia when the zirconia-rich composites are deformed at $\dot\varepsilon$ = 5 $\times$ 10$\sp{-3}$/sec, but not at a slower strain rate. Mullite is more texture prone even in the superplastic regime. Possible causes of texture formation in mullite and tetragonal zirconia phases are identified to complement the deformation analysis.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/102962/1/9023677.pdfDescription of 9023677.pdf : Restricted to UM users only