Influence of eutectoid structures on the mechanical properties and oxidation behavior of Hf-27%Ta and Hf-42%Ta alloys

Hf–Ta alloys have recently received much attention due to their favorable oxidation resistance and mechanical properties at temperatures beyond 1200 °C. However, at lower temperatures the oxidation resistance becomes poor and the alloys experience brittle to ductile transition. Both effects are likely due to their two-phase structure formed during eutectoid transformation. In the present work, we report mechanical properties and oxidation behavior of Hf-27%Ta and Hf-42%Ta (compositions are in atomic percent), which experience eutectoid transformation at ∼1083 °C and contain two phases, Hf-rich HCP and Ta-rich BCC, below this temperature. Alloys of the compositions corresponding to these two eutectoid phases, Hf-90%Ta (BCC) and Hf-3%Ta (HCP), were also produced and studied for comparison. The results show that in the temperature range of 25–1000 °C (i.e., below the eutectoid transformation) the compression strengths of Hf-90%Ta and Hf-3%Ta are considerably smaller than the strength of Hf-27%Ta or Hf-42%Ta, while above the eutectoid transformation, where the Hf-27%Ta and Hf-42%Ta alloys have single-phase BCC structures, the Hf-90%Ta and Hf-3%Ta become stronger than either Hf-27%Ta or Hf-42%Ta. It was concluded that the interfaces in the eutectoid structure of Hf-27%Ta produce considerable strengthening, which can exceed the strengths of the phases which comprise the microstructure. At 800 °C and 1000 °C, Hf-90%Ta oxidized rapidly in comparison to Hf-3%Ta due to the extensive formation of Ta ₂O ₅ . It is suggested that the observed poor oxidation behavior of Hf-27%Ta below the temperature of the eutectoid transformation is likely controlled by the oxidation kinetics of the Hf-90%Ta BCC phase in the eutectoid structure of this alloy.