On Nb Silicide Based Alloys: Alloy Design and Selection

Abstract

The development of Nb-silicide based alloys is frustrated by the lack of composition-process-microstructure-property data for the new alloys, and by the shortage of and/or disagreement between thermodynamic data for key binary and ternary systems that are essential for designing (selecting) alloys to meet property goals. Recent publications have discussed the importance of the parameters δ (related to atomic size), Δχ (related to electronegativity) and valence electron concentration (VEC) (number of valence electrons per atom filled into the valence band) for the alloying behavior of Nb-silicide based alloys (J Alloys Compd 748 (2018) 569), their solid solutions (J Alloys Compd 708 (2017) 961), the tetragonal Nb5Si3 (Materials 11 (2018) 69), and hexagonal C14-NbCr2 and cubic A15-Nb3X phases (Materials 11 (2018) 395) and eutectics with Nbss and Nb5Si3 (Materials 11 (2018) 592). The parameter values were calculated using actual compositions for alloys, their phases and eutectics. This paper is about the relationships that exist between the alloy parameters δ, Δχ and VEC, and creep rate and isothermal oxidation (weight gain) and the concentrations of solute elements in the alloys. Different approaches to alloy design (selection) that use property goals and these relationships for Nb-silicide based alloys are discussed and examples of selected alloy compositions and their predicted properties are given. The alloy design methodology, which has been called NICE (Niobium Intermetallic Composite Elaboration), enables one to design (select) new alloys and to predict their creep and oxidation properties and the macrosegregation of Si in cast alloys.

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

“…Examples of RCCAs or RMPEAs are the alloys NbTiV 2 Zr [9], Al 0.5 CrNbTi 2 V 0.5 [10], Al 2 Nb 3 TaTi 3 Zr [8], AlCrMoSi 0.05 TaTi [11] and Al 9.2 Cr 5.7 Hf 0.5 Mo 1.3 Nb 47 Ti 25 V 9.5 W 0.8 [12]. Some Nb-silicide-based alloys also satisfy the standard definition of HEA [13].The microstructures of Nb-silicide-based alloys consist of alloyed bcc Nb solid solution(s) (Nb ss ) and intermetallics [13,14] and references within. The latter include tetragonal M 5 Si 3 and M 3 Si silicides, hexagonal M 5 Si 3 silicides (M = TM,RM), C14 Laves and A15 phases [13].…”

“…Some Nb-silicide-based alloys also satisfy the standard definition of HEA [13].The microstructures of Nb-silicide-based alloys consist of alloyed bcc Nb solid solution(s) (Nb ss ) and intermetallics [13,14] and references within. The latter include tetragonal M 5 Si 3 and M 3 Si silicides, hexagonal M 5 Si 3 silicides (M = TM,RM), C14 Laves and A15 phases [13]. The former can be Ti-rich Nb ss , Si-free Nb ss and "normal" Nb ss [13] (see next section).…”

“…The former can be Ti-rich Nb ss , Si-free Nb ss and "normal" Nb ss [13] (see next section). Refractory metals confer solid solution strengthening to Nb solid solution [15][16][17][18] and are essential additions for achieving high temperature strength and creep property goals in Nb-silicide-based alloys [4,13,18]. Some Si-free Nb ss in the latter also satisfy the accepted definition of HEA [14].…”

“…Nb-silicide alloy development research has studied the effects of Mo and/or W on the microstructure and high temperature strength [4,[18][19][20]. Only a small number of papers has considered the effects of Ta addition [13] and to our knowledge there is no data for Nb-silicide-based alloys with Ta and W additions. Phases in RHEA/RCCA/RMPEA are one (or sometimes two) bcc (RM) ss , and Laves and/or M 5 Si 3 silicide(s) [8].…”

“…In this paper we refer to these materials as RCCAs.Many elements contribute to improving the oxidation resistance of Nb-silicide-based alloys. These include Al, B, Ce, Cr, Ge, Fe, Hf, Mo, Si, Sn, and Ti [3,5,13,[21][22][23] and references within. Pest oxidation at intermediate temperatures (600-900 • C) is suppressed by the synergy of Al, Cr, Si, and Ti with B, Ge, or Sn [3,13].…”