Modelling thermodynamic and kinetic reactions between ceramic oxides and ti-alloy melt

Moses, Joseph (2025). Modelling thermodynamic and kinetic reactions between ceramic oxides and ti-alloy melt. University of Birmingham. Ph.D.

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Abstract

Casting titanium alloys presents significant challenges due to the high reactivity of
titanium in its molten state. This reactivity renders ceramic mould oxides highly
susceptible to chemical reactions, leading to premature mould failure and contamination
of the liquid metal by ceramic inclusions. Developing robust oxide systems
is therefore crucial to mitigating mould degradation and ensuring the homogeneity,
precision and geometric accuracy of the final cast. While numerous experimental
studies have examined the behaviour of oxides in contact with liquid titanium alloys,
there is limited understanding of how to select and design new oxide systems. Furthermore,
modelling efforts in this field remain scarce, often constrained by the lack
of reliable thermodynamic data, insufficient insight into metal-mould interactions
and an over reliance on simplified binary system models. This study introduces a
combined thermodynamic and kinetic framework to analyse metal-mould reactions.
CALPHAD-based thermodynamic data was used to evaluate the interactions between
titanium and mould oxides, providing inputs for a kinetic model based on
the Frozen Gradient Approximation (FGA). The FGA was extended to incorporate
multi-component and multi-phase interactions, enabling a more comprehensive analysis
of oxide behaviour. The results demonstrate that all mould oxides dissolve to
some extent and none studied remain completely stable in contact with the TiAl alloy.
However, several strategies were identified to minimize reaction rates and
improve oxide stability. Among these, multi-phase oxide systems, such as Al2O3-
Mullite and YAG-YAP demonstrated enhanced stability when combined, compared
to their performance as individual oxides. This is attributed to the potential of multiphase
oxides with shared ions to serve as more durable mould materials. Notably,
Al2O3-Mullite proved effective in suppressing alpha-case formation, further enhancing
oxide stability in titanium casting applications. In existing literature, Y2O3 is
widely regarded as the most stable oxide for titanium casting, with CaO typically
ranked as the second most stable. However, the sequential oxide selection method
employed in this study reveals additional oxides and oxide combinations with stability
levels that fall between those of yttria and calcia. These include, systems
such as YAM, CaYAlO4 and YAM-YAP. These demonstrate significant potential as
alternative mould materials, offering intermediate stability.