Abstract
The microstructures and properties of the alloys JZ3 (Nb-12.4Ti-17.7Si-6Ta-2.7W-3.7Sn-4.8Ge-1Hf-4.7Al-5.2Cr) and JZ3+(Nb-12.4Ti-19.7Si-5.7Ta-2.3W-5.7Sn-4.9Ge-0.8Hf-4.6Al-5.2Cr) were studied. The densities of both alloys were lower than the densities of Ni-based superalloys and many of the refractory metal complex concentrated alloys (RCCAs) studied to date. Both alloys had Si macrosegregation and the same phases in their as cast and heat treated microstructures, namely βNb5Si3, αNb5Si3, A15-Nb3X (X = Al, Ge, Si, Sn), C14-Cr2Nb and solid solution. W-rich solid solutions were stable in both alloys. At 800 °C only the alloy JZ3 did not show pest oxidation, and at 1200 °C a thin and well adhering scale formed only on JZ3+. The alloy JZ3+ followed parabolic oxidation with rate constant one order of magnitude higher than the single crystal Ni-superalloy CMSX-4 for the first 14 h of oxidation. The oxidation of both alloys was superior to that of RCCAs. Both alloys were predicted to have better creep at the creep goal condition compared with the superalloy CMSX-4. Calculated Si macrosegregation, solid solution volume fractions, chemical compositions of solid solution and Nb5Si3, weight changes in isothermal oxidation at 800 and 1200 °C using the alloy design methodology NICE agreed well with the experimental results.
Highlights
Refractory metal intermetallic composites (RMICs, list of abbreviations is given at the end of the paper) are candidate metallic materials to replace Ni-based superalloys in high pressure turbines to enable the latter to operate at temperatures significantly higher than those that are currently possible using Ni-based superalloys, and make it feasible for future aero-engines to meet strict environmental and performance targets [1]
Other metallic materials currently considered for similar applications are RHEAs, i.e., refractory metal (RM) high entropy alloys (HEA) and RM complex concentrated alloys (RCCAs) [2]
Ge and Sn addition [2], contain Ta and W with Ta/W = 2.4, which is higher than the ratio in RCCAs studied to date (Ta/W ≤ 1 [2]) and to have low ductile to brittle transition temperature (DBTT), owing to the strong negative effect of W on DBTT compared with Ta [5,30], contain Ti and Hf with Ti/Hf = 12, which is higher than the ratio in RCCAs studied to date (Ti/Hf ≤ 3 [2]) and the ratio suggested in [16] and to have low DBTT [30], have Al/Cr = 1 and Sn/Ge = 1 based on the results reported in [13] and have Ge and Sn concentrations each of 5 at.%
Summary
Refractory metal intermetallic composites (RMICs, list of abbreviations is given at the end of the paper) are candidate metallic materials to replace Ni-based superalloys in high pressure turbines to enable the latter to operate at temperatures significantly higher than those that are currently possible using Ni-based superalloys, and make it feasible for future aero-engines to meet strict environmental and performance targets [1]. Nb-silicide-based alloys are the RMICs that attract much attention for blade applications owing to their low densities and capacity to offer a balance of properties [3,4]. Nb-silicide-based alloys are developed to meet the specific property goals (see section) that were chosen for the new metallic materials. 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, one further challenge for alloy developers is to design/select alloys that (f) offer a balance of mechanical properties with acceptable oxidation resistance
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