Fatigue crack growth through partially stabilized zirconia at ambient and elevated temperatures

Fatigue crack growth through magnesia stabilized zirconia at 20, 450 and 650°C has been observed dynamically in a high temperature loading stage for the scanning electron microscope. Crack tip micromechanics parameters were measured using the stereoimaging technique. Fatigue crack growth at ambient temperature was found to be very similar to crack growth through metallic alloys. With increasing temperature, the stress intensity levels in which stable fatigue crack growth could be sustained were found to narrow significantly, until fatigue is expected to not be a valid mechanism of crack growth above about 750°C. Measured crack tip parameters were used to derive the low-cycle fatigue and the stress-cycles to failure characteristics. The latter agreed with measured SN curves. Deformation within the plastic zone was shown to account for the measured value of fracture toughness. The mechanisms of crack growth are discussed.     

The resistivity of refractory compounds at high temperatures

Summary The authors measured the temperature dependence of the electrical resistance of carbides of titanium, zirconium, hafnium, niobium, molybdenum and tungsten and borides of titanium and zirconium. The thermal coefficients of the electrical resistance were calculated for the measured carbides and borides. Attempts were made to correlate the values of the electrical resistances with the electron structure of the compounds.     

Fast separations at elevated temperatures on polybutadiene-coated zirconia reversed-phase material

This paper describes the results of thermal stability studies of polybutadiene-coated zirconia reversed phases and application to fast HPLC separations at elevated temperatures and high flow rates. The thermal stability of the material was evaluated at temperatures up to 200 degrees C, and the rapid analyses of polyaromatic hydrocarbons and typical reversed-phase test mixtures were carried out at 100 degrees C and a flow rate of 5 mL/min. We found that the material is completely stable at 200 degrees C for at least 1300 column volumes. Analysis time can be decreased about 18-fold at high temperatures and flow rates without any significant loss in resolution relative to that at conventional temperatures and normal flow rates. For the separation of a five-component reversed-phase test mixture, the analysis time was only 50 s.     

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O₂ ZrO₂(CaO) N₂ - O₂ (P’tO₂ = 1 atm) solid electrolyte (P″O₂ = 0.39 - 10^10-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O₂ ^1/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-_? ^b+ = 0.28- 5100/T and logP_b+ =-0.87 + 15,400/T where σ-_? ^b+ is the þ-@#@ type electronic conductivity at P_{O 2} = 1 atm, and P_b+ is the oxygen activity at which the þ-@#@ type electronic conductivity and the ionic conductivity are equal.     

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O₂ ZrO₂(CaO) N₂ - O₂ (P’tO₂ = 1 atm) solid electrolyte (P″O₂ = 0.39 - 10^10-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O₂ ^1/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-_‡ ^b+ = 0.28- 5100/T and logP_b+ =-0.87 + 15,400/T where σ-_‡ ^b+ is the t-@#@ type electronic conductivity at P_{O 2} = 1 atm, and P_b+ is the oxygen activity at which the t-@#@ type electronic conductivity and the ionic conductivity are equal.     

The oxidation behavior of some Ni-Cr-Al alloys at high temperatures

Oxidation of ternary Ni-Cr-Al alloys containing different Cr/Al ratios has been studied in the temperature range 800° to 1300°C. Most of the studies were performed in 1 atm oxygen or air, but the oxygen pressure dependence for one of the alloys was also investigated. The experimental methods included thermogravimetric measurements of oxidation rates and studies on reacted specimens by means of X-ray diffraction, metallographic techniques, electron microprobe analysis, and electron microscopy. In general, the oxidation rates decrease faster with time than that for an ideal parabolic behavior. The major reaction products were NiO, Cr₂O₃,α-Al₂O₃, and Ni(Cr,Al)₂O₄. The relative amounts of these were a function of composition, temperature, oxygen pressure, and reaction time. The Ni-9Cr-6Al alloy has the best oxidation resistance due to the formation ofα-Al₂O₃ at all temperatures investigated. The oxidation mechanism of the alloy is discussed.     

The deformation of silicon at high temperatures and strain rates

Polycrystalline and 〈100〉 single crystalline semiconductor grade silicon samples have been subjected to uniaxial compression at strain rates from 10⁻⁵ to 12 s⁻¹ at temperatures ranging from 1100 to 1380 °C. Both intrinsic and p-type polycrystalline material and p-type single crystalline material were tested. Except at the highest temperature and lowest strain rate, no steady state deformation was observed for the polycrystalline material. In all other cases strain hardening was observed which increased with increasing strain rates. The polycrystalline material could be compressed by as much as 50 pct at 1380 °C and a strain rate of 7 s⁻¹ without cracking. An axial stress of approximately 170 MPa produces a strain rate of 5 s⁻¹ at 1380 °C. The stress necessary to produce a given strain rate increases rapidly with decreasing temperature while the ductility rapidly decreases. A preliminary forming limit diagram has also been determined for the polycrystalline material at 1380 °C. The deformation rate-controlling process in the polycrystalline material at high stresses could be the production of vacancies on jogged dislocations. Formerly with the Department of Materials Science and Engineering, University of Pennsylvania     

Electronically driven transport of oxygen from liquid iron to CO + CO2 gas mixtures through stabilized zirconia

Transport of oxygen in the following electrochemical system was investigated;O (liquid iron) Oⁱⁿ²⁻ (in ZrO₂2−CaO) O₂ (CO + CO₂) An alumina crucible was charged with liquid iron containing 580 ± 10 ppm oxygen. A calcia-stabilized zirconia tube (closed at one end) was immersed in the liquid iron. The inside of the zirconia tube was flushed with a stream of CO + CO2 gas mixture. Oxygen was removed from liquid iron to the CO + COO₂ gas mixture without application of an external current. Kinetics of oxygen transport in this system are discussed in terms of mixed ionic and electronic conduction of the zirconia, and also diffusion of oxygen in liquid iron. The rate controlling step for this oxygen removal process was found to be transport of oxygen across a boundary layer in the melt at the melt/electrolyte interface. M. IWASE, on leave from the Department of Metallurgy, Kyoto University, Kyoto, Japan M. TANIDA, Formerly Graduate Student at Kyoto University     

Steady-state deformation at intermediate and high temperatures

Experimental data of steady-state deformation obtained either in creep or in constant strain-rate experiments, which extend from high to intermediate temperatures, become increasingly available in the literature. While in many cases the high-temperature regime can be explained by the well-known power-law models, the incorporation of the results at intermediate temperatures is still under discussion. It is the aim of the present work to show by a careful analysis of the above mentioned data, that the jog-dragging model put forward by Barrett and Nix can account for the whole temperature range considered here and that the activation enthalpy of steady-state deformation is that of lattice self-diffusion.     

氧化镁稳定氧化锆纳米粉末的制备与物相分析

以草酸为络合剂,采用溶胶–凝胶法制备一系列氧化镁稳定氧化锆粉末Zr1 xMgxO2 x(0.04≤x≤0.10),利用X射线衍射(XRD)、场发射扫描电镜(FESEM)等分析技术对粉末进行表征。结果表明,掺杂氧化镁后,低温350~450℃煅烧产物晶型为四方相(t-ZrO2),随煅烧温度升高,t-ZrO2逐渐向m-ZrO2转变。在550℃下煅烧时,少部分四方相转变为单斜相(m-ZrO2),转变比例随掺杂量增加而降低。Mg2+取代Zr4+产生氧缺陷是ZrO2晶体结构稳定的主要因素。随煅烧温度从350℃升高到650℃,Zr0.92Mg0.08O1.92粉末中t-ZrO2晶粒尺寸从42 nm长大到100nm;随Mg掺杂量从0.04增加到0.10,t-ZrO2晶粒尺寸从110 nm减小到97.8 nm,而纳米尺寸晶粒有利于t-ZrO2稳定。     

High-temperature thermal properties of yttria fully stabilized zirconia ceramics

In this study,the dependences of yttria content,porosity and grain size on the thermal properties of Y2O3 stabilized ZrO2 (YSZ) ceramics were investigated.YSZ ceramics were synthesized by the solid state reaction method.The phase,microstructure and thermal properties of YSZ ceramics were characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM),differential scanning calorimetry (DSC) and laser-flash apparatus (LFA),respectively.The results indicated that the specific heat capacity of YSZ increased with the increase of temperature and decreased with the increase of yttria content.As the temperature increased,the thermal diffusivity and conductivity of YSZ ceramics were decreased,whereas their variations for 16YSZ,18YSZ and 20YSZ were much less pronounced than those for 12YSZ and 14YSZ.At a given temperature,the thermal conductivity of YSZ was opposite to yttria content.The thermal conductivity of YSZ ceramics almost linearly decreased with the increase of porosity.In addition,the grain size also had a great influence on the thermal conductivity.