Initial stages of the refractory degradation process by molten Al-5Mg alloy were investigated by analyzing the degradation structures caused by an interaction with molten metal during heat cycles. The refractory aggregate, found to be poly-crystalline, consisting of cristobalite (SiO₂) and mullite (Al₆Si₂O₁₃), was deteriorated ahead of the surrounding matrix. Metal permeated into the refractory, forming a thin metal layer along the crucible surface below the molten metal line at an early stage of degradation process, which would later serve as the bases for the metal infiltration path through the refractory above the molten metal line. A discontinuous film (thickness<3μm), most likely MgAl₂O₄, was formed at the “melt / refractory” interface. At the experimental temperature of 1150°C, Mg in the molten metal would build up as Mg gas at MgAl₂O₄ film/refractory interface and easily diffuse into the aggregate body and react with the aggregate components of SiO₂ and Al₆Si₂O₁₃ to form MgAl₂O₄ and Al-Mg-Si-O compounds. This was followed by the molten metal infiltration into the refractory while reducing the Al-Mg-Si-O compound to form MgAl₂O₄. Through the initial degradation process MgAl₂O₄ would finally form in a successive chain of reactions, resulting in deterioration of the crucible.

The present work is aimed at clarifying the internal degradation processes of an alumina-silica refractory through microstructural observations and analysis after exposure to a molten Al-5Mg alloy for an extended period. An alumina-silica crucible was pretreated by a molten Al-5Mg alloy to cause the initial stage of degradations at the inner crucible surface. Fresh alloy was melted in the pretreated crucible and held at 1150˚C for 96 hrs to cause further degradations into the crucible wall. Results indicate that Mg reacts with aggregate (Al₆Si₂O₁₃, SiO₂) and matrix materials to form MgAl₂O₄, Si, Al, and Al-Si-Ca intermetallic compounds that were crystallized around MgAl₂O₄ to form network-like structures. In the absence of Mg, Al reacts with the aggregate to produce Si and αAl₂O₃ and reacts with calcium-aluminum silicate in the matrix to form Si and CaO-Al₂O₃ compounds. These indicate that MgAl₂O₄ spinel forms preferentially before the corundum (αAl₂O₃) formation. Mg and Ca, segregated at the interface of " αAl₂O₃ / Al₆Si₂O₁₃" or "αAl₂O₃ / SiO₂", may promote corundum formation. The lower concentration of O and Si in the degraded areas in comparison with those in un-degraded areas suggests that gaseous SiO might have been generated during the degradation processes.

In this study, degradation by so-called “Obake” , an advanced stage of an external corundum growth on alumina-silica refractory surfaces, was investigated. The Obake was successfully recreated by melting Al-5%Mg in the alumina-silica crucible under thermal cycle conditions. The thermal test temperatures were chosen at 1000°C or 1150°C. It was confirmed that internal degradation occurred by formation of spinel (MgAl₂O₄) where the refractory was in touch with molten Al-5Mg and by corundum growth at completely separate location that is at or in the vicinity of the melt line of the refractory surface where the following reaction would take place.

(4/3) Al_(l)＋O_2(g) → (2/3) Al₂O_3(s)

As the degradation of refractory progress, the concentration of Si, Fe, Ti, and Ca in the melt increased. Cracks observed at the "MgAl₂O₄ / refractory" interface, exposed to thermal cycles, are most likely due to the difference in the linear expansion coefficient between MgAl₂O₄ and the refractory. αAl₂O₃, which is a constituent phase of Obake, exists as an aggregate of fine particles including Al metal in between. All αAl₂O₃ is found in the eutectic regions (Al-Si), suggesting that the eutectic reaction (577°C) may be accompanied by the formation of αAl₂O₃.

Pure magnesium (Mg) powders together with pure titanium (Ti) powders were mechanically alloyed (MAed) using a vibration ball mill. Stearic acid was used as process control agent (PCA) for the mechanical alloying (MA) process. The MAed powders were consolidated into bulk materials by the spark plasma sintering (SPS). Changes in hardness and solid-state reactions of the MAed powders and the SPS materials have been examined by hardness measurements and an X-ray diffraction (XRD), respectively. The Vickers hardness of the MAed powders increased to 58 HV after MA 8 h. No solid-state reactions were observed in MAed powders. The Vickers hardness of the SPS materials fabricated from MA 8 h Mg-10 mass%Ti powders reached to 76 HV. Formation of TiH₂ by solid-state reaction was observed for the SPS materials, but TiH₂ did not act as a strengthening phase. The Vickers hardness of the SPS materials improved by increasing sintering temperature, but not improved by increasing the amount of titanium. Mg-Ti system of hardness can be improved by MA-SPS process.