In recent years, near-nano (submicron) and nanostructured materials have attracted increasingly more attention from the materials community. Nanocrystalline materials are characterized by a microstructural length or grain size of up to about 100 nm. Materials having grain size of ∼0.1 to 0.3 μm are classified as submicron materials. Nanocrystalline materials exhibit various shapes or forms, and possess unique chemical, physical or mechanical properties. When the grain size is below a critical value (∼10–20 nm), more than 50 vol.% of atoms is associated with grain boundaries or interfacial boundaries. In this respect, dislocation pile-ups cannot form, and the Hall–Petch relationship for conventional coarse-grained materials is no longer valid. Therefore, grain boundaries play a major role in the deformation of nanocrystalline materials. Nanocrystalline materials exhibit creep and super plasticity at lower temperatures than conventional micro-grained counterparts. Similarly, plastic deformation of nanocrystalline coatings is considered to be associated with grain boundary sliding assisted by grain boundary diffusion or rotation. In this review paper, current developments in fabrication, microstructure, physical and mechanical properties of nanocrystalline materials and coatings will be addressed. Particular attention is paid to the properties of transition metal nitride nanocrystalline films formed by ion beam assisted deposition process.

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Nanocrystalline materials and coatings

This study reviews densification behaviour, mechanical properties, thermal, and electrical conductivities of the ZrB2 ceramics and ZrB2-based composites. Hot-pressing is the most commonly used densification method for the ZrB2-based ceramics in historic studies. Recently, pressureless sintering, reactive hot pressing, and spark plasma sintering are being developed. Compositions with added carbides and disilicides displayed significant improvement of densification and made pressureless sintering possible at ≤2000 °C. Reactive hot-pressing allows in situ synthesizing and densifying of ZrB2-based composites. Spark plasma sintering displays a potential and attractive way to densify the ZrB2 ceramics and ZrB2-based composites without any additive. Young's modulus can be described by a mixture rule and it decreased with porosity. Fracture toughness displayed in the ZrB2-based composites is in the range of 2–6 MPa m1/2. Fine-grained ZrB2 ceramics had strengths of a few hundred MPa, which increased with the additions of SiC and MoSi2. The small second phase size and uniform distribution led to higher strengths. The addition of nano-sized SiC particles imparts a better oxidation resistance and improves the strength of post-oxidized ZrB2-based ceramics. In addition, the ZrB2-based composites showed high thermal and electrical conductivities, which decreased with temperature. These conductivities are sensitive to composition, microstructure and intergranular phase. The unique combinations of mechanical and physical properties make the ZrB2-based composites attractive candidates for high-temperature thermomechanical structural applications.

Densification of ZrB2-based composites and their mechanical and physical properties: A review

The Role of Al and Li Concentration on the Formation of Cubic Garnet Solid Electrolyte of Nominal Composition Li7La3Zr2O12

On this seventy-fifth anniversary of the first scientific report of true superplastic flow, it is appropriate both to look back and examine the major developments that established the present understanding of superplasticity and to look to the future to the new opportunities that are made possible by new processing techniques, based on the application of severe plastic deformation, that permit the production of fully dense bulk materials with submicrometer or nanometer grain sizes. This review proposes a minor modification to the present definition of superplasticity, it provides an overview of the current understanding of the flow of superplastic metals and ceramics and then it examines, and gives examples of, the new possibilities that are now available for achieving exceptional superplastic behavior.

Seventy-five years of superplasticity: historic developments and new opportunities

High-strain-rate superplasticity describes the ability of a material to sustain large plastic deformation in tension at high strain rates of the order of 10-2 to 10-1 s-1 and is of great technological interest for the shape-forming of engineering materials. High-strain-rate superplasticity has been observed in aluminium-based and magnesium-based alloys. But for ceramic materials, superplastic deformation has been restricted to low strain rates of the order of 10-5 to 10-4 s-1 for most oxides and nitrides with the presence of intergranular cavities leading to premature failure. Here we show that a composite ceramic material consisting of tetragonal zirconium oxide, magnesium aluminate spinel and alpha-alumina phases exhibits superplasticity at strain rates up to 1 s-1. The composite also exhibits a large tensile elongation, exceeding 1,050 per cent for a strain rate of 0.4 s-1. The tensile flow behaviour and deformed microstructure of the material indicate that superplasticity is due to a combination of limited grain growth in the constitutive phases and the intervention of dislocation-induced plasticity in the zirconium oxide phase. We suggest that the present results hold promise for the application of shape-forming technologies to ceramic materials.

A high-strain-rate superplastic ceramic

Superplasticity in SiO{sub 2}-doped tetragonal zirconia polycrystal (2.5Y-TZP) is investigated by means of tensile testing in a temperature range 1,200--1,500 C. The grain boundary SiO{sub 2} phase reduces the flow stress and greatly enhances the tensile ductility in TZP. The stress and grain size exponents take a value close to 2 and 3, respectively, in this temperature range, but there is an abrupt change in activation energy at 1,380 C in SiO{sub 2}-doped TZP. The activation energy for superplastic flow above and below this temperature is estimated to be 182 kJ/mol and 635 kJ/mol. The enhancement of superplasticity due to SiO{sub 2}-doping is explained in terms of the accelerated plastic flow in the grain boundary SiO{sub 2} phase to accommodate the stress concentration generated by grain boundary sliding. The elongation to failure in the SiO{sub 2}-doped TZP is phenomenologically described as a function of the Zener-Hollomon parameter. At high temperatures and low strain rates, the cavities tend to align and interlink parallel to the tensile axis, and the huge elongation in excess of several hundred percent is obtained.

The enhancement of superplastic flow in tetragonal zirconia polycrystals with SiO2-doping

High-temperature plastic flow of high-purity A1 2 O 3 and MgO-doped A1 2 O 3 has been examined in a temperature range 1300 ∼ 1450°C. High-purity A1 2 O 3 with an initial grain size of about 0.9 μm is characterized by large strain-hardening rate and small tensile ductility up to 20%. The addition of 0.1 wt% MgO into A1 2 O 3 results in a decrease of strain-hardening rate and improves tensile ductility. The tensile elongation up to 70% is obtained in MgO-doped A1 2 O 3 . The difference in plastic flow behavior between pure and MgO-doped A1 2 O 3 is caused by the difference in grain growth during high-temperature deformation between the two materials. The grain growth inhibition caused by the MgO addition is very important to improve the tensile ductility. The tensile ductility of MgO-doped A1 2 O 3 is slightly but further improved in a temperature-increasing test. The role of grain size on tensile ductility is discussed in this paper.

Improvement of tensile ductility in high-purity alumina due to magnesia addition

Superplasticity in SiO2-containing tetragonal zirconia polycrystal

The Gibbs energy difference between cubic and tetragonal ZrO2 is assessed in terms of a mathematical expression which is then used to calculate the t + c two-phase field in ZrO2Y2O3 system. In order to obtain agreement with experimental data it was necessary to modify the first assessment. The calculation of the phase diagram illustrates that the t + c two-phase field should be regarded as a miscibility gap with a sharp maximum and with a spinodal. Neglecting the effect of coherency stresses one would expect two spontaneous reactions in a cubic material cooled below the spinodal, the appearance of a tetragonality and phase separation by spinodal decomposition.

Thermodynamic modeling of the c → t transformation in ZrO2 alloys

High-temperature plastic flow in WC-13wt%Co with a carbide grain size of 13 μm was examined by compression testing at temperatures of 1100–1250 °C The materials is highly deformable in this temperature range, and plastic strains of more than 100% can easily be obtained The log-log plot of stress vs  strain rate is represented by a sigmoidal relationship and is divided into three regions at each temperature The low and high strain rate regions are characterized by an activation energy close to that of tungsten diffusion in WC, and by a strain rate sensitivity of about 03 In the intermediate region, the activation energy is about 3 4 of that in the other two regions, and the strain rate sensitivity takes the highest value of around 06 The deformation mechaniss in each is discussed in this paper

T. Sakuma

Papers

The enhancement of superplastic flow in tetragonal zirconia polycrystals with SiO2-doping

Improvement of tensile ductility in high-purity alumina due to magnesia addition

Superplasticity in SiO2-containing tetragonal zirconia polycrystal

Thermodynamic modeling of the c → t transformation in ZrO2 alloys

Plastic flow in WC-13wt.%Co at high temperatures