Abstract
We studied the microstructures and isothermal oxidation of the Nb-silicide-based alloys Nb-11.5Ti-18Si-5Mo-2W-4.9Sn-4.6Ge-4.5Cr-4.7Al-1Hf (JZ4) and Nb-21Ti-18Si-6.7Mo-1.2W-4.4Sn-4.2Ge-4Cr-3.7Al-0.8Hf (JZ5), calculated their average creep rate for the creep goal conditions of T = 1200 °C and σ = 170 MPa, and compared properties of the two alloys with those of other refractory metal (RM) complex concentrated alloys (RCCAs). Both alloys had a density less than 7.3 g/cm3 and lower than the density of multiphase bcc solid solution + M5Si3 silicide RCCAs. There was macrosegregation of Si in both alloys, which had the same phases in their as-cast microstructures, namely βNb5Si3, αNb5Si3, A15-Nb3X (X = Al, Ge, Si, Sn), TM5Sn2X (X = Al, Ge, Si), C14-Cr2Nb, but no solid solution. After heat treatment at 1500 °C for 100 h, a low volume fraction of a W-rich (Nb, W)ss solid solution was observed in both alloys together with βNb5Si3, αNb5Si3 and A15-Nb3X but not the TM5Sn2X, whereas the Laves phase was observed only in JZ4. At 800 °C, both alloys did not pest, and there was no spallation of their scales at 1200 °C. At both temperatures, both alloys followed parabolic oxidation kinetics and their weight changes were lower than those of Ti-rich Nb-silicide-based alloys. The oxidation of both alloys was superior to that of other RCCAs studied to date. For each alloy the Si macrosegregation, volume fraction of solid solution, chemical composition of solid solution and Nb5Si3, and weight changes in isothermal oxidation at 800 and 1200 °C that were calculated using the alloy design methodology NICE agreed well with the experimental results.
Highlights
Ultra-high temperature materials (UHTMs) are currently being developed as potential replacements of Ni-based superalloys to meet performance and environmental targets in future aero engines.Metallic UHTMs are refractory metal (RM) intermetallic composites (RMICs), RM high entropy alloys (RHEAs) and RM complex concentrated alloys (RCCAs) [1,2]
Nb-silicide-based alloys with RM additions compared with Ni-based superalloys [1,20,48,49], (ii) the superior solid solution strengthening of Nb by Mo compared with Ta [1,28,31], (iii) the elimination of scale spallation and/or pest oxidation when Sn and Ge were in synergy with Al, Cr and transition metal (TM), or Al, Cr, TM and Ta,W additions [45,46,47] and (iv) the attainment of oxidation rates in the Ti-poor
The isothermal oxidation of each alloy at 800 ◦ C and 1200 ◦ C was studied for 100 h using thermal gravimetric (TG) analysis (NETZSCH STA 449 F3 thermal analyser with the rate of 3 ◦ C per minute in both heating and cooling) and 3 × 3 × 3 mm3 specimens from the as-cast alloys that were ground to 1200 grit
Summary
Ultra-high temperature materials (UHTMs) are currently being developed as potential replacements of Ni-based superalloys to meet performance and environmental targets in future aero engines. Metallic UHTMs must meet property goals for creep, toughness and oxidation resistance [3,4,6]. Nb-silicide-based alloys can meet the creep and toughness goals and have closed substantially the gap with the oxidation goal. Nb-silicide-based alloys with RM additions compared with Ni-based superalloys [1,20,48,49], (ii) the superior solid solution strengthening of Nb by Mo compared with Ta [1,28,31], (iii) the elimination of scale spallation and/or pest oxidation when Sn and Ge were in synergy with Al, Cr and TM, or Al, Cr, TM and Ta,W additions [45,46,47] and (iv) the attainment of oxidation rates in the Ti-poor. A recommendation for future research is proposed at the end of the paper
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