Abstract
The paper presents the results of an experimental study of large (≈0.6 kg) arc melted buttons of four Ti free Nb-silicide based alloys with Sn addition with nominal compositions (at.%) Nb-18Si-5Hf-5Sn (EZ1), Nb-18Si-5Al-5Sn (EZ7), Nb-18Si-5Cr-5Hf-5Sn (EZ3) and Nb-18Si-5Al-5Hf-5Sn (EZ4). The alloys were studied in the as-cast and heat treated conditions. In all the alloys there was macrosegregation of Si (MACSi). Among the single element additions Hf had the weakest and Sn the strongest effect on MACSi. The simultaneous presence of Cr and Hf in the alloy EZ3 had the strongest effect on MACSi. In all the alloys the βNb5Si3 was the primary phase and was present after the heat treatment(s), the Nb3Si silicide was suppressed and the A15-Nb3Sn intermetallic was stable. The Nbss was not stable in the alloys EZ7 and EZ4 and the C14-NbCr2 Laves phase was stable in the alloy EZ3. Very Hf-rich Nb5Si3 was stable in the alloy EZ4 after prolonged heat treatments. Eutectics were observed in all the alloys. These were binary eutectics in the alloys EZ1 and EZ7, where respectively they consisted of the Nbss and βNb5Si3, and βNb5Si3 and A15-Nb3Sn phases. Most likely ternary eutectics consisting of the Nbss, C14-NbCr2 and βNb5Si3, and Nbss, βNb5Si3 and A15-Nb3Sn phases were observed, respectively in the alloys EZ3 and EZ4. The addition of Al increased the vol% of the Nb5Si3 and A15-Nb3Sn phases, particularly after the heat treatment(s). The lattice parameter of Nb respectively increased and decreased with the addition of Hf, and Al or Cr and the latter element had the stronger negative effect. Pest oxidation was not suppressed in the alloys of this study.
Highlights
Niobium silicide based alloys currently are developed owing to their potential to replace Ni-based superalloys in future aero-engines to enable the latter to meet new environmental and performance targets
We studied large (≈0.6 kg) arc melted buttons of the Nb-18Si-5Hf-5Sn (EZ1), Nb-18Si-5Al-5Sn (EZ7), Nb-18Si-5Cr-5Hf-5Sn (EZ3) and Nb-18Si-5Al-5Hf-5Sn (EZ4) alloys in the as-cast and heat treated conditions
We found that there was macrosegregation of Si (MACSi) in all the alloys
Summary
Niobium silicide based alloys (or Nb-silicide in situ composites) currently are developed owing to their potential to replace Ni-based superalloys in future aero-engines to enable the latter to meet new environmental and performance targets. These new ultra-high temperature alloys must meet property goals for toughness, creep resistance and oxidation. The aforementioned properties depend on the chemical composition, distribution (size and spatial) and volume fraction of the phases that are present in the microstructures of the alloys, where the most desirable ones are the bcc Nb solid solution (Nbss ) for toughness, and the tetragonal Nb5 Si3 silicide for creep resistance and oxidation. A high volume fraction of the Nbss has negative effect on the creep resistance and oxidation of the alloys. The volume fraction of Nb5 Si3 is crucial for the toughness and creep of the alloys
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