On the Nb5Si3 Silicide in Metallic Ultra-High Temperature Materials

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 & UDelta;& chi; 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 & UDelta;& chi; 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 & UDelta;& chi; 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.

Automated summary

This paper discusses the importance of choosing and typology of the alloyed refractory metal silicide Nb5Si3 for achieving a balance of properties and meeting property goals in metallic ultra-high temperature materials. Different types of alloyed Nb5Si3, including conventional, complex concentrated, and high entropy silicides, in metallic UHTMs with Nb and Si addition were studied to support the design and development of metallic UHTMs.