TiAl-based intermetallic compounds have great potential applications as a kind of high temperature materials. The atomic diffusion is one of key factors to influence their microstructural stability and mechanical properties at high temperature. To achieve higher bearing temperature, the diffusion behaviors of alloying elements in both α2-Ti3Al and γ-TiAl should be involved to design new TiAl-based alloys. Since diffusion couple method is a simple way to investigate the diffusion behavior of metallic materials, it was adopted in this study to investigate the diffusion behaviors of alloying elements in both α2-Ti3Al and γ-TiAl phases, to relate their diffusion mechanisms with site occupancy behaviors and to character microstructural stability of two-phase TiAl alloys with full lamellar microstructure, in hope of providing fundamental information to favor alloying design.
Superalloy clamps are designed to prepare the binary and ternary diffusion couples of α2-Ti3Al and γ-TiAl by diffusion welding technique. They were welded at 1173K for 2h in vacuum and then cooled in furnace. After diffusion treatments at high temperature, concentration distributions of host and alloying elements were measured across the diffusion interface by electron probe microanalysis. As to Ti-25Al/Ti-34Al and Ti-50Al/Ti-52Al binary diffusion couples, the inter-diffusion coefficients of Ti and Al were found to follow the Arrhenius relation with pre-exponential factors and activation energies 6.4×10-2m2s-1 and 358kJ/mol for α2-Ti3Al phase as well as 3.1×10-7m2s-1 and 232kJ/mol for γ-TiAl phase. The results showed that the inter-diffusion rates of host elements in the γ-TiAl phase are easier than in the α2-Ti3Al phase.
To simplify the analysis of inter-diffusion coefficient, Al concentrations in pairs of ternary diffusion couple were kept as constants of 50Al and 34Al for the γ-TiAl phase and the α2-Ti3Al phase, respectively. To evaluate the relation of site occupancy behavior of alloying with its diffusion mechanism, V, Cr and Nb were selected as ternary alloying elements with contents of 2 atomic percent. Based on the hypothesis of diluted solution, tracer diffusion coefficients were calculated in the α2-Ti3Al phase and the γ-TiAl phase. The results showed that the activation energies of these elements increase in turn for V, Cr and Nb in both phases. Diffusion mechanisms are ascertained by the site occupancies of alloying elements in α2-Ti3Al and γ-TiAl.
Microstructural stability of the (α2 +γ) two-phase TiAl-based alloys with full lamellar microstructure was investigated in this thesis, in hope of relating microstructure evolution at high temperature with diffusion behavior of ternary alloying. Examining the experimental measurements of lamellar spacing, volume fractions and distributions of alloying elements in both the α2 and the γ phases, the following tendencies were found: 1) Cr and V contribute on grain refinement; 2) V tends to refine lamellar spacing whereas Nb has contrary effect; 3) Cr suppresses the formation of the α2 phase under the condition of keeping Al content as constant. Based on the established model by ledge growth mechanism, the above experimental findings were explained and the influence of alloying diffusion on microstructure evolution was discussed.
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