M. Jiménez-Melendo

The role of a threshold stress in the superplastic deformation of fine-grained yttria-stabilized zirconia polycrystals

Zirconia-based ceramics have been studied extensively over the past two decades because of their excellent properties of strength and toughness, which derive from the stress-induced martensitic transformation of the metastable tetragonal phase to a phase with monoclinic symmetry. In this paper the authors present a detailed study of the superplastic behavior of 4 mol% yttria-stabilized zirconia polycrystals as a function of the stress. The observed variation in the apparent creep parameters n and p have been rationalized in terms of the presence of a threshold stress below which grain boundary sliding cannot operate.

Grain size and temperature dependence of the threshold stress for superplastic deformation in yttria-stabilized zirconia polycrystals

Abstract The influence of the threshold stress on the value of the activation energy ( Q ) and stress exponent ( n ) measured during creep tests in fine-grained Y 2 O 3 -doped ZrO 2 polycrystals has been investigated in the low-stress region at intermediate temperatures. A significant increase of the measured value of Q and n with decreasing stress has been observed, consistent with the existence of a threshold stress for superplastic deformation.

Mechanical and microstructural aspects of the high temperature plastic deformation of yttria-stabilized zirconia polycrystals

Abstract The high-temperature plastic deformation of 6 mol% Y2O3-stabilized ZrO2 polycrystals with grain sizes of 1.8, 3.4 and 6.3 μm has been studied in compression between 1350 and 1450 °C in air at constant strain rate (between 1 × 10−5 and 2 × 10−4s−1) and under constant load (between 5 and 90 MPa). Two mechanical behaviours were observed depending on strain rate or stress levels: grain boundary sliding controlled by cation bulk diffusion, with an activation energy of 560 kJ/mol, and intergranular cavitation without plastic deformation of the grains.

Creep of nanocrystalline Y-SZP ceramics

Abstract Fully dense nanocrystalline Yttria-Partially Stabilized Zirconia with a grain size of 40 nm has been crept in compression at temperatures ranged between 1100°C and 1200°C and strain rates between 5 × 10 −7 s −1 and 10 −4 s −1 . The creep parameters n = 1.4 and Q = 660 kJ/mol were obtained and the microstructure was observed before and after deformation. The microscopic features and macroscopic parameters showed that these nanocrystalline ceramics probably cannot be deformed superplastically.

Like-metal superplasticity of fine-grained Y2O3-stabilized zirconia ceramics

Abstract The superplastic deformation of fine-grained yttria-stabilized zirconia polycrystals (YSZPs) has been revised through a detailed analysis. The results indicate that high purity YSZPs (residual impurity content below 0.1 wt%) display a transition in the apparent stress exponent n ap from 2 (region II) to higher values (region I) with decreasing stress; a further change towards 1 (region 0) is found at very low stresses. A continuous variation in the apparent activation energy and grain size exponent with stress, grain size and temperature is found in region I. By contrast, low-purity YSZPs display only region II over the entire stress range. The threshold stress approach used to explain the conventional and high-strain-rate superplasticity of metallic alloys accounts for the mechanical characteristics of high-purity YSZP ceramics. The constitutive equation for superplastic flow is indeed identical with that found for metals when lattice diffusion is rate controlling. Yttrium segregation at grain b...

HETEROGENEOUS JUNCTION OF YTTRIA PARTIALLY STABILIZED ZIRCONIA BY SUPERPLASTIC FLOW

Layers of different composition and/or grain size of yttria partially stabilized zirconia (YPSZ) have been compressed, with the stress perpendicular to the interface of the layers and temperature of 1400 °C, in order to produce a joint using the microstructural feature of superplasticity found in fine-grained ceramics. The pieces joined have been characterized by scanning electron microscopy (SEM), showing a clean interface with no cavitation. The stiffness of the junction was checked using Vickers indentation at room temperature at the interface and compression tests at the same conditions ( T = 1400 °C in air) used for the joining and the stress parallel to the interface. The observation and comparison between the cracks developed around the indents at the interface and in the bulk of the pieces joined as well as the absence of cavities along the interface in the samples compressed parallel to the interface shows that this technique is useful to produce a joint with a clean and strong interface.

Diffusional and dislocation creep of NiO polycrystals

Abstract Polycrystals of NiO have been deformed in compression under nominal stresses between 5 and 90 MPa, at temperatures between 1100 and 1450°C. The oxygen activity was changed by the flowing air and an oxygen-argon mixture. At low stresses, the deformation occurs by grain switching, the creep rate being controlled by oxygen self-diffusion along grain boundaries. At high stresses, dislocation recovery creep is the dominant mechanism of deformation; the creep rate is related to oxygen bulk diffusion. The same point defect is responsible for diffusion in the bulk and the grain boundaries.

Current understanding of superplastic deformation of Y-TZP and its application to joining

A review of the high-temperature, steady-state creep of yttria tetragonal zirconia polycrystals (Y-TZP) as a function of grain size, strain rate, stress, and temperature will be presented in this paper. Data have been analyzed using the standard creep equation incorporating a threshold stress. Microstructural observations of deformed samples indicated that deformation was achieved primarily by grain boundary sliding (GBS). This microscopic characteristic was used to join pieces of these materials. When two pieces of ceramics are compressed in the superplastic regime, GBS will produce a junction between the pieces. Several types of strong junctions have been produced, i.e. between pieces of the same material having different grain sizes, and between pieces of different materials. Scanning electron microscopy of the interfaces showed that each interface was cavity- and crack-free. Vickers indentations and high-temperature mechanical tests indicated that the interfaces were as strong as the matrix. This joining technique allows the construction of a complex shape or a functional gradient material.

High temperature mechanical behavior of aluminium titanate–mullite composites

Abstract Compressive deformation tests at high temperatures (1300–1450°C) have been performed on aluminium titanate (undoped and modified with MgO and Fe 2 O 3 )–10 wt.% mullite composites obtained by reaction sintering. Two deformation regimes have been observed, depending on the experimental conditions. At high stresses/strain rates, the samples failed intergranularly at very small failure strains. At low stresses/strain rates, large macroscopic strains were reached without significant microstructural changes. MgO and Fe 2 O 3 additions have a pronounced effect on the brittle-ductile transition. In contrast, they have no effect on the steady-state flow, which is characterized by a stress exponent close to unity and an activation energy of 650 kJ/mol. Experimental results are compatible with a mechanism of grain boundary sliding controlled by cation bulk diffusion.

Effect of layer interfaces on the high-temperature mechanical properties of alumina/zirconia laminate composites

Abstract High-temperature plastic deformation of laminar composites containing layers of Al 2 O 3 and a mixture of 85 vol.% Al 2 O 3 +15 vol.% 3 mol% Y 2 O 3 -stabilized tetragonal ZrO 2 (ZTA) produced by sequential slip casting is examined in uniaxial compression testing. The mechanical behavior is compared with that of monolithic Al 2 O 3 and ZTA synthesized by the same technique. The layered composites exhibit better creep properties at low strain rate than either of the two constituent materials. The good interfacial adhesion of the layers imparts creep resistance and ductility simultaneously to the laminates.