The effect of MgO as a solid-solution additive in the sintering of Al2O3 was studied. The separate effects of the additive on densification and grain growth were assessed. Magnesia was found to increase the densification rate during sintering by a factor of 3 through a raising of the diffusion rate. The grain-size dependence of the densification rate indicated control primarily by grain-boundary diffusion. Magnesia also increased the grain growth rate during sintering by a factor of 2.5. The dependence of the grain growth rate on density and grain size suggested a mechanism of surface-diffusion-controlled pore drag. It was argued, therefore, that MgO enhanced grain growth by raising the surface diffusion coefficient. The effect of MgO on the densification rate/grain growth rate ratio was, therefore, found to be minimal and, consequently, MgO did not have a significant effect on the grain size/density trajectory during sintering. The role of MgO in the sintering of alumina was attributed mainly to its ability to lower the grain-boundary mobility.

Effect of MgO Solute on Microstructure Development in Al2O3

The grain-growth behavior of Al{sub 2}O{sub 3} compacts with small contents ({le}10 wt%) of various liquid-forming dopants was studied. Equiaxed and/or elongated grains were observed for the following dopants: MgO, CaO, SiO{sub 2}, or CaO + TiO{sub 2}. The platelike grains, defined as the abnormal grains larger than 100 {mu}m with an aspect ratio {ge}5 and with flat boundaries along the long axis, were observed when the boundaries were wet with the liquid phase and the codoping satisfied two conditions of size and valence. These dopings were Na{sub 2}O + SiO{sub 2}, CaO + SiO{sub 2}, SrO + SiO{sub 2}, or BaO + SiO{sub 2}. However, an addition of MgO to the Al{sub 2},O{sub 3} doped with CaO + SiO{sub 2} resulted in the change of grain shape from platelike to equiaxial. Equiaxed grains were also observed for the MgO + SiO{sub 2} doping, indicating that two conditions were necessary but not sufficient to develop the platelike grains. The fast growth rate of the platelike grains is explained by an increased interfacial reaction rate due to the codopants.

Origin and growth kinetics of platelike abnormal grains in liquid-phase-sintered alumina

In this investigation we have studied how the presence of a liquid phase affects the grain morphology and grain growth kinetics in Al2O3 at 1800°C using the growth of both matrix grains and large spherical single-crystal seeds growing into the matrix. The growth rates of the matrix grains were found to decrease in the following order: undoped Al2O3, AI2O3 with anorthite, AI2O3 with anorthite and MgO, and Al2O3 with MgO. Except for the samples doped with MgO alone, the matrix grains were faceted and appeared tabular in polished sections. In samples containing anorthite both with and without MgO, the single-crystal seeds exhibit basal facets with continuous liquid films and slow growth in the 〈0001〉 relative to all other crystallographic directions. When only MgO is added, the growth of the single-crystal seeds was not isotropic; however, no faceting was observed. We discuss how anisotropic growth rates caused by the anorthite additions can stimulate discontinuous grain growth in Al2O3.

Effect of a Liquid Phase on the Morphology of Grain Growth in Alumina

Dihedral angles, Ψs, from surface thermal grooves were measured using a metal reference line technique for polycrystalline MgO, undoped Al2O3, and MgO-doped Al2O3 The values of Ψs span the following ranges: 89° to 116° for MgO at 1520 K, 76° to 166° for undoped Al2O3 at 1870 K, and 90° to 139° for MgO-doped Al2O3 at 1870 K For all three systems, the median Ψs values are 105° to 113°, implying that the median γgb/γs is 11 to 12, in contrast to metals where γgb/γs ranges typically from 1/4 to 1/2 The widths in Ψs distributions were different for the three materials with the width increasing in the following order: MgO, MgO-doped Al2O3, undoped Al2O3 For all three materials, the grainboundary grooves and their corresponding Ψs were not symmetrical with respect to the surface normal The asymmetry for MgO was due to the pinning of the grain boundaries by the surface thermal grooves The range of inclination angles of the grain boundary to the surface was a function of Ψs, with the maximum inclination angles of ∼13°, in quantitative agreement with theory

Dihedral Angles in Magnesia and Alumina: Distributions from Surface Thermal Grooves

Evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder, through primary recrystallization and normal grain growth, to a size stabilized by second phases. To form the marbles, reagent grade CaCO3 was mixed with 0, 1 and 5 volume% mica and heat-treated under pressure with added water. Densification with negligible recrystallization occurred within one hour at 500° C and 500 MPa confining pressure. Primary recrystallization occurred at 500–550° C, causing increases of grain size of factors of 2–5. Resulting samples had uniform grain size, gently curved grain boundaries, and near-equilibrium triple junctions; they were used subsequently for normal grain growth studies. Normal grain growth occurred above 550° C; at 800° C, grain size (D) increased from 7 μm (D
0) to 65 μm in 24 hours. Growth rates fit the equation, D

 n
 -D

 0
 =Kt, where K is a constant and n≃2.6. Minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size, D
max, which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0.3 and 1. Once D
max is reached, normal growth continues only if second phases are mobile or coarsen, or if new driving forces are introduced that cause unpinning of boundaries. Normal grain growth in Solnhofen limestone was significantly slower than in pure synthetic marble, suggesting that migration is also inhibited by second phases in the limestone.

Grain growth in synthetic marbles with added mica and water

During an investigation to determine the effect of additives in inhibiting exaggerated grain growth, it was observed that several gave strikingly similar microstructures with pore clusters at grain centers surrounded by substantially pore-free annular regions. A few very large pores were found at the grain boundaries and occasionally in the small-pore-free annular regions. The development of this structure is explained as a sequence of exaggerated grain growth followed by normal grain growth. A discontinuous grain-boundary liquid phase facilitates the final grain-growth step and takes part in the coalescence of fine pores into coarse pores.

Giulio Rossi

Papers

Influence of Additives on the Microstructure of Sintered Al2O3

Influence of additives on the microstructure of sintered al2o3