The effect of short fiber addition on the thermomechanical properties of calcium zirconate crucibles for titanium alloys

Abstract

This study investigated the addition of short fibers in low amounts of 0.1 or 0.5 wt% to CaZrO3 refractory compositions with the aim of improving the thermal shock resistance of crucibles for melting and casting highly reactive titanium alloys. To maintain the high corrosion resistance that is inherent to CaZrO3, short fibers of the same (CaZrO3) or similar (yttria-stabilized ZrO2) chemical composition were added. Physical, mechanical and thermomechanical properties as well as the microstructure of uniaxially pressed short fiber composites were evaluated. According to the main hypothesis, low amounts (0.1 wt%) of short fibers generated a thermal shock resistant microstructure with localized higher sintering shrinkage, which contributed to the Hasselman thermal shock parameter R′′′′ by reducing the strength after sintering and thus promoting resistance to crack propagation. Thermal shock experiments confirmed the lower relative strength loss than the fiber-free reference after up to 5 thermal shocks, whereas higher fiber amounts (0.5 wt%) impaired the thermal shock behavior due to cluster formation. The absolute values of the remaining strength (0.1 wt% short fibers) were similar to the reference, because the higher packing density by short fiber addition counteracted the benefits of the localized shrinkage effect to a certain extent, while the reference exhibited higher porosity, which correlates with higher crack density and work of fracture. The hypothesis was thus partially fulfilled. Crucibles were shaped by isostatic pressing. Casting trials with titanium alloys demonstrated the high corrosion resistance and good thermal shock behavior of the novel CaZrO3-based short fiber composite crucibles.