Abstract
The microstructures and hardness of the as cast and heat treated (1400 °C/100 h) alloys Nb-24Ti-18Si-5Ge-5Cr-5Al (ZF6) and Nb-24Ti-18Si-5Ge-5Cr-5Al-5Hf (ZF9) were studied. Both alloys were compared with refractory metal bcc solid solution + intermetallic High Entropy Alloys (HEAs). There was macrosegregation of Si, Ti, Cr and Al in both alloys. The roles of Ge and Hf on macrosegregation are discussed. In both alloys the primary phase was the βNb5Si3. In the as cast alloy ZF6 the Nbss, βNb5Si3 and C14-NbCr2 Laves phase and Nbss + βNb5Si3 eutectic were formed. The microstructure of the as cast alloy ZF9 consisted of Nbss, βNb5Si3, γNb5Si3 and C14-NbCr2 Laves phase. The heat-treated microstructures of the alloys ZF6 and ZF9 consisted of Nbss, βNb5Si3 and αNb5Si3 and Nbss, βNb5Si3, αNb5Si3 and γNb5Si3, respectively. The surfaces of both alloys were contaminated by oxygen where TiO2 and HfO2 formed respectively in the alloys ZF6 and ZF9. Alloying with Hf increased the lattice parameter of Nbss and decreased the hardness of ZF9 and Nb5Si3. The roles of alloying additions on the hardness of the Nbss and Nb5Si3 and relationships between alloy hardness and alloy parameters VEC (valence electron concentration), δ (related to atomic size) and Δχ (related to electronegativity) were discussed.
Highlights
The search for new alloys capable of operating in the demanding conditions in gas turbine engines at temperatures higher than those experienced by the currently used Ni-based superalloys has concentrated on refractory metal silicide-based alloys, in particular Nb-Si based alloys and Mo-Si based alloys [1,2]
The former are known as Nb-silicide based alloys or Nb in situ composites and have microstructures consisting of Nbss and intermetallic(s), such as Nb5 Si3, Nb3 Si, NbCr2 and others, depending on alloy chemistry and processing [2,3]
Considering the reported beneficial effect of Ge on the oxidation of Nb-silicide based alloys [5] and the restrictions imposed on the Al, Cr and Hf contents of Nb-24Ti-18Si-zxAl-yCr-zHf based alloys owing to mechanical property targets, we embarked on the work presented in this paper in order to understand how the synergy of Ge with the alloying elements Al, Cr, Ti and Hf at the concentrations of the “base”
Summary
The search for new alloys capable of operating in the demanding conditions in gas turbine engines at temperatures higher than those experienced by the currently used Ni-based superalloys has concentrated on refractory metal silicide-based alloys, in particular Nb-Si based alloys and Mo-Si based alloys [1,2]. Considering the reported beneficial effect of Ge on the oxidation of Nb-silicide based alloys [5] and the restrictions imposed on the Al, Cr and Hf contents of Nb-24Ti-18Si-zxAl-yCr-zHf based alloys owing to mechanical property targets, we embarked on the work presented in this paper in order to understand how the synergy of Ge with the alloying elements Al, Cr, Ti and Hf at the concentrations of the “base”. Alloy Nb-24Ti-18Si-5Al-5Cr affects the stability of βNb5 Si3 and NbCr2 , and the vol.% of Nbss and the shape and size of Nb5 Si3 This knowledge is important for optimising room and elevated temperature mechanical properties and oxidation resistance [3]. The nominal compositions of the latter alloys are given in the Appendix A
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