Ronald S. Gordon

Creep of polycrystalline alumina, pure and doped with transition metal impurities

Grain size effects were used to evaluate the relative contributions of aluminium lattice and oxygen grain boundary diffusion to the high temperature (1350 to 1550° C) steady state creep of polycrystalline alumina, pure and doped with transition metal impurities (Cr, Fe). Divalent iron in solid solution was found to enhance both aluminium lattice and oxygen grain-boundary diffusion. Large concentrations of divalent iron led to viscous Coble creep which was rate-limited entirely by oxygen grain-boundary diffusion. Nabarro-Herring creep which was rate-limited by aluminium lattice diffusion was observed in pure and chromium-doped material. Chromium additions had no effect on diffusional creep rates but significantly depressed non-viscous creep modes of deformation. Creep deformation maps were constructed at various iron dopant concentrations to illustrate the relative contributions of aluminium grain boundary, aluminium lattice, and oxygen grain-boundary diffusion to the diffusional creep of polycrystalline alumina.

Stress-relaxation technique for deformation studies in four-point bend tests: application to polycrystalline ceramics at elevated temperatures

The advantages of the stress-relaxation technique can be effectively realized by applying the procedure to conventional four-point bend tests usually employed in the deformation studies of ceramic materials. A test system and procedure for determining plastic strain rate-stress relationships at elevated temperatures (up to 1600° C) by the stress-relaxation method is described. An analysis to calculate true plastic strains and stresses from measured deflections and loads is presented and it is shown that such an analysis requires minimum assumptions regarding the materials behaviour. Preliminary results obtained on an iron-doped MgO specimen are discussed and compared with the constant load test results obtained on identical specimens. Sources of error in the four-point bend stressrelaxation tests and the methods to minimize them are also discussed.

Creep mapping in a polycrystalline ceramic: application to magnesium oxide and magnesiowustite

The construction of deformation mechanism maps for a polycrystalline ionic solid in which anion and cation transport are coupled has been demonstrated. Because of anioncation ambipolar coupling, two regimes of Coble creep are possible in systems where anion grain boundary transport is rapid: (1) rate-controlled at low temperatures and small grain sizes by cation grain-boundary diffusion, and (2) rate-limited at high temperatures and large grain sizes by anion grain-boundary diffusion. A new type of deformation mechanism map was introduced in which the temperature and grain size were primary variables. This map was shown to be particularly useful for materials which deform primarily by diffusional creep mechanisms. Ambipolar diffusional creep theory was used to construct several deformation mechanism maps for polycrystalline MgO and magnesiowustite over wide ranges of stress, grain size, temperature and composition.

Impurity and Grain Size Effects on the Creep of Polycrystalline Magnesia and Alumina

Steady state, constant load, creep experiments have been conducted on pure and doped, polycrystalline magnesium and aluminum oxides to determine: 1) the effect of solid solution impurities (e.g. Fe, Cr, Ti) on deformation mechanisms, particularly as they relate to the concentration of lattice defects 2) the effects of impurities, atmosphere, and grain size on the relative contribution to viscous deformation of lattice and grain boundary diffusion of both cationic and anionic species 3) the relative roles of viscous and non-viscous deformation mechanisms.

Analysis of mass transport in the diffusional creep of polycrystalline MgO-FeO-Fe2O3 solid solutions

An analysis of mass transport in the diffusional creep of iron-doped, polycrystalline MgO was conducted. Creep regimes in which magnesium grain boundary, oxygen grain boundary, and magnesium-lattice diffusion were rate-controlling were identified. An analytical procedure was developed for the estimation of the diffusion constants for these three processes.

Enhancement of the diffusional creep of polycrystalline Al2O3 by simultaneous doping with manganese and titanium

The effect of simultaneous doping with manganese and titanium on diffusional creep was studied in dense, polycrystalline alumina over a range of grain sizes (4–80μm) and temperatures (1175–1250° C). At a total dopant concentration of 0.32–0.37 cation %, diffusional creep rates were enhanced considerably such that the temperature at which cation mass transport was significant was suppressed by at least 200° C compared to that observed in undoped material. The Mn-Ti (and Cu-Ti) dopant couple was far more effective in enhancing creep rates and suppressing sintering temperatures than the Fe-Ti couple. The enhanced mass transport kinetics are believed to be caused by significant increases in both aluminium lattice and grain-boundary diffusion. When aluminium grain-boundary diffusion is enhanced by increasing the concentration of divalent impurity (Mn2+, Fe2+) or by creep testing at low temperatures, creep deformation is Newtonian viscous.

High temperature steady-state creep of polycrystalline rutile, pure and doped with tantalum

The steady state creep of polycrystalline (5–60 μm) rutile, which is doped with 1 cation % tantalum, is controlled by a Nabarro-Herring lattice diffusion process at 1100 to 1200° C. Doping with tantalum significantly depresses the steady state creep rate by lowering the concentrations of titanium interstitials and oxygen vacancies. The concentrations of these defects, and hence the steady state creep rate of doped rutile, can be increased by decreasing the oxygen partial pressure below 10−7 to 10−8 atm at 1200° C. Tentative evidence is presented in support of the hypothesis that the steady state creep of polycrystalline, undoped rutile at 950 to 1100° C is controlled by interfacial defect creation and/or annihilation at grain boundaries. Interfacial controlled deformation rates are probably due to the large concentrations (and perhaps high mobilities) of cation and anion lattice defects which are present in pure rutile equilibrated in both oxidizing and reducing atmospheres. The steady state creep rate was a very weak inverse function of the grain size and essentially independent of the oxygen partial pressure.

Processing and Characterization of Polycrystalline β″-Alumina Ceramic Electrolytes

In this paper recent developments in the processing and characterization of polycrystalline β″-alumina ceramic electrolytes will be discussed, β″-alumina ceramics have potential uses in diverse devices such as the electrolyte or sodium-ion separator in (1) the sodium-sulfur energy storage battery 1 designed for either electric utility load leveling or automotive propulsion, (2) a high temperature sodium heat engine or thermoelectric device.2 (3) a high electrowinning of sodium metal from molten sodium salts, (4) the purification of sodium metal and (5) various electrochemical devices for measuring sodium and perhaps even oxygen activities.

Effects of anelastic deformation on high-temperature stress relaxation of polycrystalline MgO and Al2O3

High-temperature (1160 to 1450‡ C) deformation of dense polycrystalline (10 to 90 Μm) Al2O3 and MgO doped with Fe (up to 2.65 cation %) was studied by stress relaxation, dead-load creep and creep recovery. In some cases, all three deformation tests were conducted on a single specimen. A comparison of strain rate-stress data calculated from both stress relaxation and dead-load creep experiments revealed discrepancies in both the magnitude of the strain rates and the dependence between the strain rate and stress. These differences were attributed to the existence of anelastic deformation effects. The correction of stress relaxation data in the low stress regime for linear anelasticity led to strain rate-stress data in reasonably close agreement with results obtained from dead-load creep tests conducted in the viscous creep regime. Creep recovery experiments indicated that anelastic deformation in these ceramic materials was relatively insensitive to changes in temperature and grain size over the range of variables studied.

Creep of Doped Polycrystalline Magnesium and Aluminum Oxides

It is well known that under conditions of small stress, fine grain size and enhanced lattice and/or grain boundary diffusion polycrystalline ceramics will deform at elevated temperatures by one or more diffusional creep mechanisms.[1–9] In this paper creep deformation maps will be utilized to illustrate the role of soluble (aliovalent) dopants, grain size, and oxygen fugacity on the relative contributions of (1) lattice diffusion, (2) grain boundary diffusion, (3) interfacial defect reactions at grain boundaries, and (4) power law or dislocation deformation mechanisms on the creep deformation of polycrystalline magnesium and aluminum oxides.