Discontinuous Dissolution of Iron Aluminate Spinel in the Al₂O₃–Fe₂O₃ System

Abstract

When sintered 85A1,0,-15Fe20, (in wt %) specimens consisting of corundum grains and spinel particles were annealed at temperature where only a corundum phase was stable, phase transformation of spinel into metastable FeAIO, and subsequently complete dissolution of the metastable phase occurred together with the migration of grain boundaries at the surface of the specimens. Since the grain boundary migration was induced by grain boundary diffusion of Fe,O, from the transforming and dissolving particles, the boundary migration by temperature decrease corresponds to a discontinuous dissolution of the spinel particles and a chemically induced grain boundary migration by temperature change. Inside the specimens, however, the transformation-dissolution and the grain boundary migration were suppressed because of unavailable accommodation of the volume expansion due to the transformation.

Copper‐alumina nanocomposites derived from CuAlO2: Phase transformation and microstructural coarsening

“…Specifically, the only report of discontinuous coarsening in a ceramic eutectic is that of Janecek and Pletka for NiO‐CaO. Prior studies of discontinuous coarsening related phenomena in ceramic systems have primarily been concerned with the coarsening (or dissolution) of precipitates or secondary phases via diffusion‐induced grain‐boundary migration (DIGM) . In the case of a metal‐ceramic system, Handwerker et al and Backhaus‐Ricoult and Peyrot‐Chabrol reported DIGM for the system alumina‐chromia‐chromium; it was found that chromium particles precipitated during a partial reduction process underwent discontinuous coarsening via DIGM.…”

Microstructure and mechanical properties of Al2O3 matrix nanocomposites produced by solid state precipitation

“…In contrast, the XRD pattern corresponding to the as-sintered A10F (pattern 3) contains only peaks corresponding to ␣-Al 2 O 3 revealing that the Fe 2 O 3 (∼10 wt.%) has completely dissolved in the Al 2 O 3 matrix during holding at the sintering temperature of 1450 • C, as is suggested by the Al 2 O 3 -Fe 2 O 3 phase diagram. 16 The magnified view of the {1 0 4} peak of ␣-Al 2 O 3 (Fig. 1b) shows a notable shift in the peak position towards the lower angle (2θ)/higher dspacing side after solution treatment of A10F (compare patterns 1 and 3).…”

“…The selected sintering temperature of 1450 • C corresponds to the temperature on the Al 2 O 3 -Fe 2 O 3 phase diagram where ∼15 wt.% Fe 2 O 3 is soluble in Al 2 O 3 . 16 Hence it is expected that all of the Fe 2 O 3 in the A10F specimens will be dissolved in the Al 2 O 3 during sintering to yield an alumina-based solid solution containing Fe 3+ cations. In this perspective, the sintering step is designed to fulfil the dual purpose of attaining densification and developing Al 2 O 3 -10 wt.% Fe 2 O 3 (A10F) solid solutions.…”

“…Fe 2 O 3 (haematite) has the same crystal structure as ␣-Al 2 O 3 (corundum) and possesses substantial solubility in Al 2 O 3 at moderately high temperatures in air (∼15 wt.% at ∼1410 • C). 16 Furthermore, under reducing conditions Fe 3+ (of dissolved Fe 2 O 3 ) can be reduced to Fe 2+ , which being aliovalent to Al 3+ possesses very limited solubility in Al 2 O 3 . Hence, it is expected that aging of Al 2 O 3 -Fe 2 O 3 solid solutions under reducing conditions should lead to the formation of Fe 2+ -containing precipitate particles.…”

Effect of yttria doping on the microstructure and mechanical properties of Al2O3–FeAl2O4 nanocomposites developed via solid state precipitation

“…19,20 Fe 2 O 3 (haematite) has the same crystal structure as ␣-Al 2 O 3 (corundum) and possesses substantial solubility in Al 2 O 3 at moderately high temperatures in air (∼15 wt.% at ∼1410 • C). 21 This allowed the development of dense polycrystalline solid solutions of Al 2 O 3 -10 wt.% Fe 2 O 3 during pressureless sintering in air. Aging of the fully dense solid solutions in a reducing atmosphere led to the precipitation of FeAl 2 O 4 particles in the Al 2 O 3 matrix.…”