Influence of alkaline oxide on the deformation of ceramic shell mould at high temperatures during investment casting

In this investigation, the effect of alkaline oxides such as Na₂O and K₂O on the deformation of ceramic shell mould was studied at high temperatures. Two groups of ceramic shell mould samples were prepared by impregnating them with a solution of NaOH and KOH of different concentrations. Systematic creep test was conducted under different compression loads at 2?MPa, 4?MPa and 6?MPa using a specially designed creep testing equipment between 1200 and 1350?°C. The obtained results were analyzed based on Norton-Bailey-Arrhenius (NBA) equation. The phase transformation and micromorphology evolution of different samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. From the results it was observed that the activation energy of ceramic shell mould used was about 198?kJ/mol below 1300?°C, whereas it increased to 325?kJ/mol near 1350?°C with a stress exponent of around 1.50 at all the temperatures. Adding Na₂O and K₂O decreased the activation energy at low temperature and increased it at high temperature. Besides, the stress exponent obviously decreased to nearly 1.00 below 1300?°C indicated the dominance of interface sliding mechanism, which then increased back to 1.30–1.60 at 1350?°C suggesting a combined creeping mechanism. Based on the results of XRD and SEM, it could be noted that during the creeping process the temperature played an important role in changing the interface structure.