Abstract
Diffusion in the Ti-Al-V system is studied and a CALculation of Phase Diagrams (CALPHAD) diffusion mobility description is developed. Diffusion couple experiments are performed to obtain diffusion paths in the hcp phase at 923 K, 1023 K and 1123 K. The diffusion coefficient of V in the hcp-Ti phase is found to decrease with increasing Al alloying. A forward-simulation analysis is used to evaluate the impurity diffusion coefficient for Al and V diffusion in the hcp Ti-V and the Ti-Al systems which are used as input in the mobility modeling. The composition dependency for the diffusion in the hcp phase in the ternary system is accounted for and a CALPHAD diffusion mobility description is obtained by directly optimizing the mobility parameters as a function of the experimental composition profiles from the diffusion couples. Both experimental data and previous diffusion mobility descriptions in the literature for the bcc Ti-Al-V phase are adopted. A complete description of diffusion in both the hcp and bcc phases for the Ti-Al-V system is presented with the aim to be used for design of Ti alloys and processes.
Highlights
A large portion of the Ti grades are comprised of a microstructure that contains the a-phase
A forward-simulation analysis is used to evaluate the impurity diffusion coefficient for Al and V diffusion in the hcp Ti-V and the Ti-Al systems which are used as input in the mobility modeling
The composition dependency for the diffusion in the hcp phase in the ternary system is accounted for and a CALculation of Phase Diagrams (CALPHAD) diffusion mobility description is obtained by directly optimizing the mobility parameters as a function of the
Summary
A large portion of the Ti grades are comprised of a microstructure that contains the a-phase (hcp, hexagonal close-packed). Despite the commonly occurring a and near-a alloys, surprisingly limited research has concerned diffusion of typical alloying elements in the a-phase. The lack of data for diffusion in a-Ti systems is associated with challenges doing low-temperature diffusion experiments (a is stable below about 1160 K for pure Ti) and low solubilities of many alloying elements in the a-Ti matrix. The maximal solubility of V in a-Ti is 0.0385 mol fraction[2,3] and is located at 830 K which makes design of diffusion experiments challenging. Another major challenge with diffusion in the a-Ti systems is the physical characteristics of the phase itself.
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