Abstract
The alloying of Nb5Si3 modifies its properties. Actual compositions of (Nb,TM)5X3 silicides in developmental alloys, where X = Al + B + Ge + Si + Sn and TM is a transition and/or refractory metal, were used to calculate the composition weighted differences in electronegativity (Δχ) and an average valence electron concentration (VEC) and the solubility range of X to study the alloying and properties of the silicide. The calculations gave 4.11 < VEC < 4.45, 0.103 < Δχ < 0.415 and 33.6 < X < 41.6 at.%. In the silicide in Nb-24Ti-18Si-5Al-5Cr alloys with single addition of 5 at.% B, Ge, Hf, Mo, Sn and Ta, the solubility range of X decreased compared with the unalloyed Nb5Si3 or exceeded 40.5 at.% when B was with Hf or Mo or Sn and the Δχ decreased with increasing X. The Ge concentration increased with increasing Ti and the Hf concentration increased and decreased with increasing Ti or Nb respectively. The B and Sn concentrations respectively decreased and increased with increasing Ti and also depended on other additions in the silicide. The concentration of Sn was related to VEC and the concentrations of B and Ge were related to Δχ. The alloying of Nb5Si3 was demonstrated in Δχ versus VEC maps. Effects of alloying on the coefficient of thermal expansion (CTE) anisotropy, Young’s modulus, hardness and creep data were discussed. Compared with the hardness of binary Nb5Si3 (1360 HV), the hardness increased in silicides with Ge and dropped below 1360 HV when Al, B and Sn were present without Ge. The Al effect on hardness depended on other elements substituting Si. Sn reduced the hardness. Ti or Hf reduced the hardness more than Cr in Nb5Si3 without Ge. The (Nb,Hf)5(Si,Al)3 had the lowest hardness. VEC differentiated the effects of additions on the hardness of Nb5Si3 alloyed with Ge. Deterioration of the creep of alloyed Nb5Si3 was accompanied by decrease of VEC and increase or decrease of Δχ depending on alloying addition(s).
Highlights
Performance and environmental targets for future aero-engines could be met with changes in the propulsive and thermal efficiency of the engines and new materials that have capabilities beyond those of Ni-based superalloys
Available experimental data for tetragonal Nb5 Si3 silicides in developmental Nb-silicide based alloys was used to seek out relationships between the silicide parameters ∆χ and valence electron concentration (VEC), the hardness of tetragonal Nb5 Si3 and changes of the creep of Nb5 Si3 with alloying
It is necessary to know the actual compositions of the Nb5 Si3 silicides in studied alloys [4,5,6,30,41,42,43,44,45,46,47,48] in order to calculate the silicide parameters VEC and ∆χ
Summary
Performance and environmental targets for future aero-engines could be met with changes in the propulsive and thermal efficiency of the engines and new materials that have capabilities beyond those of Ni-based superalloys. The most important phases in the microstructure of Nb-silicide based alloys are the bcc Nb solid solution(s), Nbss and the Nb5 Si3 silicide The latter can be present as the tetragonal high temperature βNb5 Si3 , or the tetragonal low temperature αNb5 Si3 [2], or as the hexagonal γNb5 Si3 silicide. The effects of alloying on the solubility range of X in (Nb,TM) X3 where X = Al + B + Ge + Si + Sn and TM is a transition and/or refractory metal are discussed first, followed by relationships between solutes and their concentrations in Nb5 Si3 and the silicide parameters VEC and ∆χ and how alloying influences the hardness of tetragonal Nb5 Si3.
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