Plastic deformation in impure nanocrystalline ceramics
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Plastic deformation in impure nanocrystalline ceramics Rachman Chaim Department of Materials Engineering, Technion – Israel Institute of Technology, Haifa 32000 Israel (Received 5 August 1998; accepted 26 January 1999)
Plastic deformation behavior of impure nanocrystalline ceramics (NCC) was modeled using the percolative composite model in conjunction with models for plastic deformation by grain boundary sliding. The “glass transition temperature” concept was used to determine the threshold strain rate criterion below which the impure nanocrystalline ceramic would deform plastically. Threshold strain rate is stress independent. It increases with the temperature increase and with the grain size decrease. Using the dissolution-precipitation model, dependence of the strain rate on temperature, stress, and grain size in the nanometer regime for impure NCCs was calculated. As an example, the critical conditions for plasticity in impure yttria-tetragonal zirconia polycrystals (Y-TZP) were evaluated. At 600 ±C, strain rates as high as 1024 s21 were expected in 10 nm impure Y-TZP. Comparison of the published data extrapolated into the nanometer range to the calculated threshold level showed that increase in the applied stress is associated with increase in the grain size and strain rate onsets for plastic deformation.
I. INTRODUCTION
In the last decade, a vast amount of effort was devoted to development of nanocrystalline metallic and ceramic materials due to their special properties relative to their conventional counterparts.1–4 Several interesting observations were made with respect to the mechanical properties of these materials. Generally, regardless of the bonding type, the elastic modulus was found to decrease with decrease in the grain size.5–8 However, part of these results were interpreted as due to the residual porosity in the compacted and sintered powders.9–11 With respect to the plastic deformation behavior, inverse Hall–Petch relations in yield stress and hardness were reported for some nanocrystalline materials.12–15 Numerous models were developed for description of this abnormal behavior in the nanocrystalline regime.16–22 However, these trends show that the Hall–Petch relation is not universal for the description of the plastic deformation, especially in nanocrystalline materials. This aspect was theoretically discussed by Christman23 who showed the exponents of the equations to be crystal structure dependent, and different from 0.5. Experimental work of Savader et al.24 has shown an inverse grain size dependence of the hardness in the nanocrystalline iron films. Consequently, the Hall–Petch behavior is not necessarily the only way for the description and understanding of plastic deformation in nanocrystalline materials. On the other hand, some nanocrystalline ceramics exhibited plasticity or superplasticity at temperatures far below the onset plasticity/superplasticity temperatures in their conventional counterparts.25–27 Recently, a model based on grain bo
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