Creep of ceramics: Part 1 Mechanical characteristics

This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of ∼ 5 and ∼ 3, respectively. It is concluded that the behaviour with an exponent of ∼ 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of ∼ 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.

Creep of ceramics

Wave profile and equation of state (EOS) data are reported for low-porosity polycrystalline magnesium oxide under shock compression. The Hugoniot equation of state between 14 and 133 GPa is U_S = 6.87(10) + 1.24(4)u_p, where the numbers in parentheses are one standard deviation uncertainties in the last digit(s). Reverse-impact wave profiles constrain the compressional sound velocity, V_p, at 10–27 GPa to ±2%. Measured V_p values are consistent with ultrasonic data extrapolated from 3 GPa. By combining the Hugoniot results with ultrasonic data, the adiabatic bulk modulus and its first and second pressure derivatives at constant entropy are 162.5(2) GPa, 4.09(9), and −0.019(4) GPa^(−1). The shear modulus and its first and second pressure derivatives are 130.8(2) GPa, 2.5(1), −0.026(45) GPa^(−1). Polycrystalline MgO has a compressive yield strength of 1–1.5 GPa at the elastic limit which increases to 2.7(8) GPa along the Hugoniot and is similar at unloading. Wave profiles for MgO at 10–39 GPa are described using a modified elastic-plastic model. There are significant differences in the dynamic response of single-crystal and polycrystalline MgO.

/pdf/compressional-sound-velocity-equation-of-state-and-43l2ytbhg4.pdf

Compressional sound velocity, equation of state, and constitutive response of shock-compressed magnesium oxide

The use of the average or maximum grain size, G, in G-tensile strength, σ, relations and the resultant character of the σ-G−1/2 plots have been considered. Experimental and literature data (mainly at 22°C) show that using the size of exaggerated grains in Al2O3, β-Al2O3, B4C, SiC, Si3N4 and probably TiO2 (i.e. mainly non-cubic materials), is appropriate, provided such grains are the source of failure and are larger than or equal to the flaw size, which is most probable at intermediate G, otherwise an average G, Ga, is appropriate. Larger grains or clusters of larger grains do not always act as fracture origins, because associated flaws, mainly cracks (e.g. from machining) or pores are required. Ga was found to be appropriate for materials (e.g. MgO, Y2O3, ZrO2 and MgAl2O4) not typically having exaggerated grains. The slopes of the larger and finer G branches of the σ-G-1/2 plots are shown to be less than for polycrystalline KIC and more than 0, respectively, contrary to assertions of other workers using maximum G, Gm. The latter slope indicates that the R-curve effects and related mechanisms have much more limited effects on σ than is commonly recognized. Better measurement and characterization needs are identified, i.e. (1) basic consistency between σ and G values (violated in the past use of Gm), and (2) addressing the spatial distribution of larger grains and other defects, e.g. via fractography. Use of an average (possibly weighted) grain diameter for G instead of a linear intercept, is recommended.

Ceramic tensile strength-grain size relations: grain sizes, slopes, and branch intersections

Resume Nous presentons une revue sur la deformation plastique a haute temperature mettant en jeu le deplacement des dislocations dans les oxydes. Les liaisons chimiques et les structures cristallines sont plus complexes que dans les metaux; aussi, les dislocations ne sont pas encore bien decrites ni dans les oxydes binaires (NiO, Cu2O …) ni dans les oxydes ternaires (MgAl2O4…). Ces composes, meˆme purs, contiennent de grandes quantites de defauts ponctuels pouvant provoquer des ecarts a la stoechiometrie; une bonne connaissance des concentrations et de la mobilite de defauts est indispensable pour expliquer les proprietes plastiques. Les essais mecaniques habituels sont la flexion et surtout la compression; ils presentent de nombreux inconvenients, en particulier ils conduisent a une deformation heterogene des eprouvettes. Le but de ces tests est d'etablir la loi qui relie la vitesse de deformation aux autres parametres (contrainte, temperature …). Une grande quantite de resultats existe et est rassemblee...

Mouvement des dislocations et plasticité à haute température des oxydes binaires et ternaires

The creep of pure magnesia (99.9 +% MgO) was tested in transverse bending at temperatures from 1200° to 1500°C, strain rates near 10−2%/hr, and grain sizes of 4 to 50μ. In most cases, grain growth during the test affected the apparent creep behavior more than all the other variables combined. An analytical graphical method was used to separate the grain growth effect from other effects and to obtain more meaningful creep data. Creep occurred primarily by a viscous mechanism (Nabarro-Herring type, cation-lattice-diffusion controlling) with a minor amount of plastic creep (dislocation climb). The agreement with previous creep data was good.

High‐Temperature Creep of Polycrystalline Magnesia: I, Effect of Simultaneous Grain Growth

The creep of magnesia doped with 0.035 to 2.26 cation % of nine other oxides and three binary mixtures thereof and of three seawater products (about 96, 98, and 99.5y0 MgO) was evaluated in transverse bending at 1200° to 1500°C, with strain rates of about 10−2%/hr, and average grain sizes of 5 to 50p. The results obtained were compared with those for pure magnesia. Most additives accentuated the plastic (diffusion-controlled) nature of the creep process presumably by pinning dislocations and/or slowing grain growth. In most cases the rate-determining diffusing species seemed to be the cation, Mg, but in two cases it was suspected that oxygen boundary diffusion was controlling. Porosities above ˜10% appear to increase the temperature dependence of creep, probably by introducing boundary sliding. The agreement of the creep data with those of other diffusion-controlled processes (electrical conductivity, sintering, and grain growth) is demonstrated.

Tuvia Zisner

Papers

High‐Temperature Creep of Polycrystalline Magnesia: I, Effect of Simultaneous Grain Growth

High‐Temperature Creep of Polycrystalline Magnesia: II, Effects of Additives *

Preferred Orientation in Hot-Pressed Magnesia

Comment on Note “Functional Relation Between Creep Rate and Porosity for Polycrystalline Ceramics”

Reactive Hot-Pressing of Magnesium Hydroxide