High temperature fatigue and creep deformation behavior under static and cyclic loading in ceramics containing a viscous grain boundary phase
Dey, Nishit
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Permalink
https://hdl.handle.net/2142/19112
Description
Title
High temperature fatigue and creep deformation behavior under static and cyclic loading in ceramics containing a viscous grain boundary phase
Author(s)
Dey, Nishit
Issue Date
1995
Doctoral Committee Chair(s)
Socie, Darrell F.
Department of Study
Materials Science and Engineering
Discipline
Materials Science and Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Engineering, Materials Science
Language
eng
Abstract
Fatigue and creep deformation behavior of a commercial grade of alumina with 6 wt % of grain boundary viscous phase was investigated. Two failure mechanisms were observed at 1000 C under static loading. At high stresses, fracture was dictated by slow growth of a single dominant flaw along the grain boundaries; at low stresses fracture occurred due to the nucleation, growth and linkage of multiple macrocracks. In the stress range used in the fatigue experiments, fracture under cyclic loading was dominated by the slow crack growth of a single dominant crack. In the slow crack growth regime, the lifetime under cyclic loading was greater than that under static loading (for the same maximum stress) by approximately a factor of 30. Extensive crack surface bridging by viscous grain boundary phase behind the crack tip was observed and is believed to be the most important factor for retarding crack growth during cyclic loading.
A mechanistic model has been developed that takes into account the effects of grain size, width and viscosity of the viscous grain boundary layer and strain rate to predict the ratio of static to cyclic loading lifetime, for the stress temperature regime where the material fails due to the growth of a single dominant crack. A single dimensionless scalar parameter has been identified as a critical parameter to determine whether cyclic loading will be equally damaging or less damaging than the corresponding static loading at high homologous temperatures. Model predictions agree with experimental results of various authors.
In the temperature range 1000 to 1175 C, both under static and cyclic loading, creep was due to time-dependent changes in elastic properties of the material by nucleation and growth of flaws along grain boundaries. Under cyclic loading, the material had the capacity to accumulate larger amount of creep deformation without failure as compared to that under static loading. Microstructural observation provided evidence that under static loading accumulation of creep damages are more heterogeneous and localized, as compared to that under cyclic loading, leading to easy development of macrocracks and early failure. The ratio of steady state creep rate under static loading to that under cyclic loading was approximately equal to 72 at 1100 C but decreased to 7 at 1175 C. Viscous bridging by the grain boundary second phase was responsible for improved lifetime and lower creep deformation rate under cyclic loading.
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