Alloys for application at ultra-high temperatures: Nb-silicide in situ composites

On the Microstructure and Properties of Nb-12Ti-18Si-6Ta-5Al-5Cr-2.5W-1Hf (at.%) Silicide-Based Alloys with Ge and Sn Additions

“…Nb-silicide-based alloys with transition metal (TM)/refractory metal (RM) additions meet (are close to) the creep property goal [ 4 , 5 ]. The latter expects the creep strength to be greater than 170 MPa at a creep rate of 2 × 10 −8 s −1 at 1200 °C [ 3 , 15 ] (the creep goal assumes alloy density of 7 g/cm 3 , which can be met by Nb-silicide-based alloys [ 5 ]).…”

“…Nb-silicide-based alloys with transition metal (TM)/refractory metal (RM) additions meet (are close to) the creep property goal [ 4 , 5 ]. The latter expects the creep strength to be greater than 170 MPa at a creep rate of 2 × 10 −8 s −1 at 1200 °C [ 3 , 15 ] (the creep goal assumes alloy density of 7 g/cm 3 , which can be met by Nb-silicide-based alloys [ 5 ]). Creep resistant alloys occupy a particular area in the Δχ versus δ or VEC versus δ maps, see area B in the Figure 1 in [ 19 ] and Figure 19 in [ 5 ] (for the parameters Δχ, δ and VEC see list of abbreviations and Appendix A ).…”

“…The latter expects the creep strength to be greater than 170 MPa at a creep rate of 2 × 10 −8 s −1 at 1200 °C [ 3 , 15 ] (the creep goal assumes alloy density of 7 g/cm 3 , which can be met by Nb-silicide-based alloys [ 5 ]). Creep resistant alloys occupy a particular area in the Δχ versus δ or VEC versus δ maps, see area B in the Figure 1 in [ 19 ] and Figure 19 in [ 5 ] (for the parameters Δχ, δ and VEC see list of abbreviations and Appendix A ). For example, polycrystalline Nb-silicide-based alloys with nominal compositions (at.%) Nb-18Si-xRM-5Hf and Nb-zTi-18Si-xRM-yHf (RM = Mo, Ta, W, x = 2, 3, 5, y = 1, 5 and z = 8, 11) lie in the area B in the Figure 1 in [ 19 ], and at 1200 °C and 200 MPa have creep rates (compressive creep) in the range 1.1 × 10 −6 to 1.1 × 10 −8 s −1 , compared with creep rates in the range 1 × 10 −6 to 4.4 × 10 −8 s −1 that are predicted by NICE for T = 1200 °C and σ = 170 MPa.…”

“…Major challenges for alloy developers have been how to design alloys that (a) do not suffer from pest oxidation at intermediate temperatures, (b) have tolerable oxidation resistance at intermediate and high temperatures, (c) form scales that do not spall off and meet (d) the toughness goal and (e) the creep goal. Research in progress [ 4 , 5 ] has indicated that it is unlikely for an alloy to meet simultaneously the oxidation and mechanical property goals, thus one further challenge for alloy developers is to design/select alloys that (f) offer a balance of mechanical properties with acceptable oxidation resistance.…”

“…The alloy design methodology N iobium I ntermetallic C omposite E laboration (NICE) [ 4 ], which has been developed in our research group, addresses (a) to (f) and was used in the research described in this paper (NICE does not consider fracture toughness, but it can predict (calculate) the vol.% Nb ss , which is important for the toughness of Nb-silicide-based alloys). This research was motivated by four key findings, namely (i) the superior creep of Nb-silicide-based alloys with RM additions compared with Ni-based superalloys [ 5 , 6 , 7 ], (ii) the elimination of pest oxidation when Sn or Ge are in synergy with Al and Cr additions [ 4 , 7 , 8 , 9 , 10 , 11 , 12 ], (iii) the suppression of pest oxidation at intermediate temperature with the simultaneous elimination of scale spallation at a high temperature when Al, Cr, Ge and Sn were all simultaneously present in a Nb-silicide-based alloy without RM additions [ 13 ] and (iv) the improved adhesion of the scale that formed at 1200 °C on the alloy Nb-12Ti-18Si-6Ta-2.5W-1Hf-5Sn-5Ge (alloy JZ2 in [ 14 ]).…”