On the Nb5Si3 Silicide in Metallic Ultra-High Temperature Materials

Abstract

Refractory metal (RM) M5Si3 silicides are desirable intermetallics in metallic ultra-high temperature materials (UHTMs), owing to their creep properties and high Si content that benefits oxidation resistance. Of particular interest is the alloyed Nb5Si3 that forms in metallic UHTMs with Nb and Si addition. The choice of alloying elements and type of Nb5Si3 that is critical for achieving a balance of properties or meeting a property goal in a metallic UHTM is considered in this paper. Specifically, the different types of alloyed “normal” Nb5Si3 and Ti-rich Nb5Si3, namely “conventional”, “complex concentrated” (CC) or “high entropy” (HE) silicide, in metallic UHTMs with Nb and Si addition were studied. Advanced metallic UHTMs with additions of RMs, transition metals (TMs), Ge, Sn or Ge + Sn and with/without Al and with different Ti, Al, Cr, Si or Sn concentrations were investigated, considering that the motivation of this work was to support the design and development of metallic-UHTMs. The study of the alloyed silicides was based on the Nb/(Ti + Hf) ratio, which is key regarding creep, the parameters VEC and Δχ and relationships between them. The effect of alloying additions on the stability of “conventional”, CC or HE silicide was discussed. The creep and hardness of alloyed Nb5Si3 was considered. Relationships that link “conventional”, CC or HE bcc solid solution and Nb5Si3 in the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration) were presented. For a given temperature and stress, the steady state creep rate of the alloyed silicide, in which TMs substituted Nb, and Al and B substituted Si, depended on its parameters VEC and Δχ and its Nb/(Ti + Hf) ratio, and increased with decreasing parameter and ratio value, compared with the unalloyed Nb5Si3. Types of alloyed Nb5Si3 with VEC and Δχ values closest to those of the unalloyed Nb5Si3 were identified in maps of alloyed Nb5Si3. Good agreement was shown between the calculated hardness and chemical composition of Nb5Si3 and experimental results.

A Perspective of the Design and Development of Metallic Ultra-High Temperature Materials: Refractory Metal Intermetallic Composites, Refractory Complex Concentrated Alloys and Refractory High Entropy Alloys

“…The same phases can co-exist in other alloys (see below). For example, in the case of RM(Nb)ICs, RM(Nb)IC/RCCAs or RM(Nb)IC/RHEAs (see Abbreviations), bcc solid solution(s) can be "conventional", CC or HE according to their chemical composition, or "normal", Tirich, Si-free according to solute partitioning (see the Appendix B) [7][8][9][19][20][21]55], Nb 5 Si 3 silicide(s) can be "conventional", CC or HE according to their chemical composition, or Ti rich or Ti and Hf rich according to solute partitioning (see the Appendix B) [7,8,21,56], other compounds (e.g., A15 phases [54]) can be "conventional", CC or HE according to their chemical composition [19] and eutectics that contain bcc solid solution and Nb 5 Si 3 [57] can be "conventional", CC or HE according to their chemical composition [7,19,57], for example see the Figures 1-4 and 6 in [58] and the Figures 1-6 in [20]. Such intricateness (see Appendix A) of phases materialises from the correlations/relationships between elements, phases, alloys and their properties.…”

“…In the literature, how is synergy revealed by the aforementioned properties and parameters? The synergy is demonstrated by (a) The relationships/correlations between (i) alloy parameters (e.g., Figure 15b in [19], Figure 16 in [48], Figure 14 in [46], Figure 15a in [44], Figure 15 in [45], Figure 19 in [8], Figure 2 in [21], Figures 3 and 4 in [59]), (ii) phase parameters (e.g., Figures 1-4 and 6b in [58], Figure 10a-c in [20], Figure 3 in [9], Figure 8 in [48], Figure 5a,b in [60]), (iii) alloy parameters and alloy properties (e.g., Figure 13 in [8], Figure 6 in [9]), (iv) phase parameters and phase properties (e.g., Figure 5 in [58], Figure 5 in [9]), (v) alloy and phase parameters (e.g., Figure 17 in [7], Figure 6 in [8], Figure 17a in [21]), (vi) alloy parameters and processing (macrosegregation), e.g., Figure 8 in [46], (b) the "co-habitation" in parameter maps (vii) of phases (e.g., Figure 3a in [9], Figure <...…”

On the Microstructure and Properties of Complex Concentrated bcc Solid Solution and Tetragonal D8m M5Si3 Silicide Phases in a Refractory Complex Concentrated Alloy

“…In the literature, we can find data about the Vickers hardness of solid solution and/or intermetallic phases in metallic UHTMs, e.g., [11,19,32,40,60,61,[67][68][69][70][71][72]. The hardness of a phase that is measured using nanoindentation can be different from its Vickers hardness, meaning a correction factor must be applied.…”

“…The data for the alloy NT 1.2 presents some challenges for alloy designers using "alloy design landscapes" [9] to design metallic UHTMs. A challenge is to calculate the chemical composition of an A2 solid solution with the solutes that are essential to get a balance of properties for a single phase metallic UHTM, namely the transition metals Cr, Hf, Mo, Nb, Ti, W and the simple metal and metalloid elements Al, Ge, Si, Sn [11,19,20,40,59]. It should be possible to calculate properties of a multicomponent A2 solid solution with specific solute concentrations, say a (Nb,Ti,Al,Si,Cr,Ge,Hf,Mo,Sn,W) ss .…”

“…The constituent phases in the aforementioned multiphase metallic UHTMs can be "conventional" or high entropy (HE) or complex concentrated/compositionally complex (CC) [20,40]. In other words, in these materials, HE and/or CC phases can co-exist with "conventional" phases and vice versa.…”