Preparation and sintering of fine composite precursors of mullite-zirconia by chemical copolymerization of metal alkoxides

Composite precursors of mullite-zirconia were prepared by mixing a mullite precursor prepared by the partial hydrolysis method and zirconium alkoxide. The mullite precursor was reacted with zirconium alkoxide in order to achieve a uniform dispersion of fine zirconia particles in the mullite matrix. The composite precursors were hydrolysed and heat-treated to examine the properties of the precursors. X-ray diffraction data showed that a large amount of tetragonal zirconia existed in the composite powders in spite of the high temperature calcination below 1600° C. These results suggested that transformation toughening of mullite-based ceramics prepared by firing the composite precursor compacts could be attained if the total densification occurred at low temperature.     

Transferred arc plasma processing of mullite-zirconia composite from natural bauxite and zircon sand

Low cost mullite-zirconia composites were prepared from the mixtures of natural bauxite and zircon sand by using transferred arc plasma processing. In this paper, a mixture of natural bauxite and zircon in the ratio of 7:3 by weight (based on composition of 3:2 mullite) was ball milled for 4 h and melted in the transferred arc plasma for 2 and 4 min. Argon was used as plasma forming gas. The torch was operated at 5 kW input power. The phase and microstructure formation of melted samples were investigated by XRD and SEM images. The results show that the processing time is a key factor to get a single phase mullite-zirconia composites with required microstructure.     

Superplasticity in an alulllinium alloy 2124/SiCp composite

Abstract

The superplastic potential of an aluminium alloy 2124/SiC_p composite, fabricated by powder metallurgy techniques, has been investigated. Instead of any special thermomechanical processing or hot extrusion, simple warm rolling has been employed to obtain a fine grained structure before superplastic testing. Constant strain rate tests were performed to characterise the superplastic behaviour of the composite. All tests were performed in air at temperatures of 743–783 K and in the strain rate range 10^-3-10^-1 S^-l. A maximum elongation of 425% was achieved at a temperature of 763 K and a strain rate of 8.3 × 10^-2 S^-1. The highest value obtained for the strain rate sensitivity index (m) was 0.41. Differential scanning calorimetry was used to ascertain the possibility of any partial melting in the vicinity of optimum superplastic temperature. These results suggested that no liquid phase existed where maximum elongation was achieved and deformation took place entirely in the solid state. Optical and electron microscopy were used to examine the materials microstructure before and after superplastic testing.     

Crystallization of gel-derived mullite-zirconia composites

The crystallization of gel-derived mullites containing ZrO₂ content up to 20wt% had been studied. The formation of a metastable solid solution between ZrO₂ and mullite was established. A model was proposed to account for the formation of this solid solution. Grain refinement of mullite grains was observed in the composites containing at least 7wt% ZrO₂.     

Coprecipitation-derived mullite and mullite-zirconia composites

Precursor powders of mullite-zirconia (0–40 wt% ZrO₂) were prepared by a hydroxide coprecipitation method and their behaviour during calcination between room temperature and 1500 °C was studied using thermal analysis, X-ray diffraction and electron microscopy. The only crystalline phases present in the precalcined powders were bayerite and gibbsite, and these were stable up to 250 °C. Powders containing ZrO₂ were initially amorphous, but on calcination between 250 and 850 °C produced different crystalline phases at temperatures which depended on the amount of zirconia present. Thus in the case of mullite-40 wt% ZrO₂, zirconia crystallized at about 850 °C and was stable up to 1200 °C, when it reacted with free silica to form zircon (ZrSiO₄). Mullite formed above 1250 °C at the expense of zircon and remained stable at higher temperatures. The oxide powders were very homogeneous, and on sintering produced ceramics with a fine-grained uniform microstructure. The powders were very reactive and could be sintered conventionally to near-theoretical density at 1600–1700 °C without sintering aids. The fracture strength of mullite was about 275 MPa, and this could be improved to 350 MPa by hot isostatic pressing the presintered bodies. Addition of zirconia enhanced the sintering kinetics as well as the fracture strength of mullite.     

Superplasticity

Superplasticity is the phenomenon of extraordinary ductility exhibited by some alloys with extremely fine grain size, when deformed at elevated temperatures and in certain ranges of strain rate. To put the phenomenology on a proper basis, careful mechanical tests are necessary. These are divided into (i) primary creep tests, (ii) steady state deformation tests, and (iii) instability and fracture tests, all of which lead to identification of macroscopic parameters. At the same time, microstructural observations establish those characteristics that are pre-requisites for superplastic behaviour. Among the macroscopic characteristics to be explained by any theory is a proper form of the equation for the strain rate as a function of stress, grain size and temperature. It is commonly observed that the relationship between stress and strain rate at any temperature is a continuous one that has three distinct regions. The second region covers superplastic behaviour, and therefore receives maximum attention. Any satisfactory theory must also arrive at the dependence of the superplastic behaviour on the various microstructural characteristics. Theories presented so far for microstructural characteristics may be divided into two classes: (i) those that attempt to describe the macroscopic behaviour, and (ii) those that give atomic mechanisms for the processes leading to observable parameters. The former sometimes incorporate micromechanisms. The latter are broadly divided into those making use of dislocation creep, diffusional flow, grain boundary deformation and multimechanisms. The theories agree on the correct values of several parameters, but in matters that are of vital importance such as interphase grain boundary sliding or dislocation activity, there is violent disagreement. The various models are outlined bringing out their merits and faults. Work that must be done in the future is indicated.     

Combustion synthesis and properties of mullite-zirconia composites

Mullite-zirconia composite powders were prepared by the combustion of an aqueous heterogeneous redox mixture consisting of Al(NO₃)₃, Zr(NO₃)₄/ZrO(NO₃)₂, silica fume and urea/diformyl hydrazine at 500 °C. X-ray diffraction data showed that a large amount of tetragonal zirconia existed in the composite powders in spite of high temperature calcination. Milled composite powders showed enhanced densification compared to the unmilled powders and the microstructure of the sintered (1600 °C) compacts showed the presence of spherical zirconia grains in intergranular positions along with elongated mullite grains.     

超塑变形机制

本文介绍了(细晶)超塑性变形机制研究的历史和现状,重点讨论了晶界滑移模型。指出:单一机制不能描述整个超塑变形的持征,多重机制将成为今后研究的方向。     

Superplasticity in ceramics

It is now recognized that superplasticity is a potential deformation process in ceramics. This review summarizes the major characteristics of superplasticity and examines the reports of both transformation and structural superplasticity in ceramic and other non-metallic materials. It is shown that there are both similarities to and differences from metals. Similarities include the variation of strain rate with stress and grain size, but an important difference is the necessity to consider the role of intergranular glassy phases in ceramics. Superplasticity is also important in intermetallic compounds, and in geological materials where there is evidence for superplastic deformation both in laboratory experiments and in natural deformation.     

陶瓷材料的超塑性

虽然陶瓷材料在本质上是一种脆性材料；然而研究已表明细晶陶瓷材料具有超塑性，在高温下能产生很大的拉伸形变．本文综述了超塑性的特征和Y2O3稳定四方ZrO2多晶体这种典型的超塑性陶瓷材料的形变机理，形变特征以及动态晶粒生长、玻璃相和产生孔穴对其超塑性形变的影响，此外，还总结了其他陶瓷材料，包括Al2O3、Al2O3－Y2O3稳定四方ZrO2、纳米陶瓷和玻璃陶瓷的超塑性行为和特征．     

陶瓷材料超塑性研究进展

超塑性是细晶陶瓷在高温下的固有属性。本文综述了陶瓷材料超塑性的一般特征和氧化钇稳定四方相氧化锆多晶陶瓷(Y-TZP)的形变机理及最新研究进展。解释了不同纯度Y-TZP陶瓷在Ⅰ区存在巨大差异的原因以及杂质特征对应力指数的影响。从能量的观点进一步分析了陶瓷材料超塑变形过程中的控速机制。对共价键陶瓷Si₃N₄、SiC的超塑性特征以及晶间玻璃相在超塑变形中的作用进行了概括。此外,还总结了其它陶瓷材料,包括Al₂O₃及其复合陶瓷、纳米陶瓷的研究进展及发展方向。     

Superplasticity of AlMgSi alloys

Commercial AlMgSi alloy sheets produced by thermomechanical treatment are found to be superplastic between 500 and 570°C at strain rates of 10^–5–10^{–3 –1} The strain rate sensitivity,m, is about 0.4. It was found that the highly alloyed sample contains pre-existing cavities in higher volume fraction than the alloy of lower concentration. An exponential growth of cavity volume fraction was found during superplastic deformation which is characteristic of plasticity controlled cavitation. The growth rate of the cavity volume fraction can be decreased by applying back pressure.     

Superplasticity of ZrO2 and ZrO2/Al2O3 composite consisting of nano-sized grains

 Y₂O₃-stabilized ZrO₂ and Y₂O₃-stabilized ZrO₂/10 mol%Al₂O₃ composite both consisting of homogeneous nano-sized grains were successfully fabricated by a pulse electric current sintering through solution chemistry technique. The relative density was above 97% for both the obtained materials, and the grain size was less than 90 nm and 40 nm for the ZrO₂ monolith and the ZrO₂/Al₂O₃ composite, respectively. The materials showed superplastic behavior in compression at temperature of 1200°C. Received: 2 March 1998 / Accepted: 10 March 1998     

Mg-Li系合金超塑性研究进展

超塑性是材料在一定温度和应变速率下表现出异常高塑性的能力。Mg-Li合金具有超轻的密度、高比刚度和良好的电磁屏蔽能力,可望在航天、军事、汽车、3C电子等领域获得应用。综述了国内外Mg-Li合金超塑性研究现状,介绍了轧制、挤压、等通道转角挤压、搅拌摩擦加工、差速轧制、高压扭转和多向锻造方法获得的超塑性。指出了Mg-Li合金超塑性存在的问题和今后进一步研究的方向。