Creep of Polycrystalline MgO‐FeO‐Fe₂O₃ Solid Solutions

Abstract

Creep of Polycrystalline MgO-FeO-Fe 203 Solid Solutions42 1 strength retained for a given A T . In part, these deviations can be attributed to differences between the crack geometry in the refractory samples and that in the mechanical model used to derive Eq.(1). This derivation also assumes equal crack size and neglects crack interaction effects. Also, the proper values for ylroF. 0 , and E , are those corresponding to the quenching temperature, rather than to room temperature, as in the present calculations. A very likely explanation can also be based on an increase in crack density with increasing AT as the result of an expected nonuniform initial crack-size distribution. This increase will automatically give rise to experimental curves which are steeper than the theoretical curves for constant N . Crack densities which increased with increasing A T were observed by Roberts and Wronalj and Butsch et d . I 6 in the quenching of high-strength technical ceramics. Nevertheless, the crack-density values for the theoretical curves in Fig. 7 can be shown indirectly to be approximately correct for the corresponding experimental data on the following basis. Since the typical strength-vs-AT curve (Fig. 4) exhibits the gradual loss in strength typical for stable crack propagation, the crack length in the refractory specimens prior to quenching should correspond approximately to the minimum in the pIotsofAT(.rrcu~E,,/Zy,,,,.)"~ vsl. As indicated by Eq. (3). this minimum is a function of crack density. The experimental curves in Fig. 7 suggest that the minimum occurs for crack lengths near 0.1 cm. The crack lengths in the unquenched specimens, as calculated from K,, and MOR for all compositions, range from 0.078 to 0.254 cm, with corresponding crack densities of -10 to 0.5 cm-*, respectively, in good agreement with the values obtained by comparing the experimental and theoretical curves in Fig. 7. Because ofthe coarse microstructure, however, the crack densities could not be determined experimentally. It appears then that Eq. ( I ) may be used with a degree of reliability, especially when more information on crack length and crack density becomes available.In terms of development of materials with increased thermalshock resistance, the results of the present program suggest that, in addition to decreasing the coefficient of thermal expansion and Young's modulus, efforts devoted to increasing fracture energies, in particular yltor., should be successful. In addition, increasing crack densities through microstructural control also is expected to lead to improved spalling behavior. The latter approach has already been taken in the development of highly thermal-shock-resistant ZrO, partially stabilized with CaO,I7 foundry molds made by the Shaw process, and In summary, the present research has demonstrated the importance of fracture energies in establishing the spalling resistance of high-A120, refractories.

High temperature creep of ultrafine-grained Fe-doped MgO polycrystals

“…It is generally recognized that activation energy for boundary diffusion must be equal to or slightl~r less than that for lattice diffusion. The activation energy of 96 kcal mole -a found here for magnesium boundary diffusivity is lower than the average value of 120kcal mole -1 found in other creep studies by Trember et al [9] for the magnesium sublattice.…”

“…Consistently, all the energies above creep activation energy are higher than that reported for magnesium tracer diffusion in MgO [16][17][18]. It is generally suggested that high activation energy in an oxide which is doped with a transition metal probably results from a contribution of temperature dependence from both the Fe3+/Fe 2 § ratio and the solubility of the dopant [9,19]. However, it can be noted that Tagai" et al [3,4] found also a high activation energy of 104kcal mole -1 for both a 0.55% Fe cation-doped and a much purer MgO, and Tremper et al [9] reported a high activation energy of ~-117 kcal mole-1 in a [ar_ge range of dopant levels.…”

“…Following the analysis of Cannon et al in A1203 [15], controlling diffusivities are inferred from our data by assuming Equation 1 is applicable with either lattice or Figure 5 Comparison of apparent lattice diffusivities from this work and from [3,4,6,9], with tracer dfffusivities from [16][17][18]20] and with loop shrinkage measurements from [21]. This work: 0.5% Fe cation-doped MgO; A 0.1urn; o0.2#m; u 1/~m;in air.…”

“…Now lattice diffusion is supported by a number of creep data* for sizes in the range 10 to 30#m, where a grain size dependence closer to 2 than to 3 is found (Passemore etal. [2] : -2.5 from 5 to 20/~m; Terwilliger et al [6] :--2 from 15 to 30/1m; Tremper et al [9] :--1.95 from 10 to 30/am). Therefore, from the expression of -1 D complex given in Equation 2 where D1 ~ has been made negligible, the only possibility consistent with the above statement is d d…”

“…The mechanical properties of fully dense, finegrained magnesia polycrystals have been of interest for a long time [1][2][3][4][5][6][7][8][9]. In many creep experiments published on MgO polycrystals, viscous deformation has been reported for materials at small grain size (< 30/am) [1-4, 6, 8, 9].…”