Rules for designing Nb silicide based alloys: The case for the solid solution phase

Nb silicide based alloys (or Nb in situ silicide composites) have the potential to replace Ni based superalloys in high temperature aero-engine applications. The two most important phases in the microstructure of these new alloys are the bcc Nb solid solution (Nbss) and the tetragonal Nb5Si3 silicide. Different types of solid solution can form in the new alloys. The Nbss is the key phase for achieving the mechanical property and oxidation goals. Alloying with transition metals, including refractory metals, and with simple metals and metalloids is essential for improving the mechanical behaviour and oxidation of the Nb silicide based alloys. Two questions (among many others) that are critical for the design of the new ultra-high temperature alloys are: (a) what are the rules that govern the formation of the bcc Nbss in Nb silicide based alloys and (b) can these rules discriminate between alloying additions in the bcc Nbss that are essential for improving the oxidation behaviour and creep resistance of the new alloys? The Nbss formed in different families of Nb silicide based alloys has been studied using thermo-physical parameters and properties that include enthalpies and entropies of mixing, atomic size and electronegativity and electronic structures. The presentation (i) will discuss the different types of Nbss that can form in the new alloys, and (ii) will use maps based on thermo-physical parameters and properties to discuss the rules that favour the stability of different forms of the Nbss

Eutectics and peritectics in the solidification processing of Nb silicide based alloys

There is strong industrial preference for cast Nb silicide alloys, in particular for directionally solidified (DS) alloys, owing to familiarity of industry with production and use of cast Ni superalloys for more than 30 years. Solidification processing of the new alloys can “control” their microstructures via the reactions L ® (Nb) + Nb3Si, L + bNb5Si3 ® Nb3Si, Nb3Si ® (Nb) + aNb5Si3 and L ® (Nb) + bNb5Si3, of which the former can be suppressed in favour of the latter that is metastable in the Nb-Si binary. In the open literature the solidification processing of Nb silicide based alloys has attracted very little attention compared with their microstructures, mechanical properties and oxidation, owing to the difficulty of scaling up from small arc melted buttons and the limited availability of facilities for melting and casting ultra-high temperature alloys (most of the developmental alloys have Tliquidus \u3e 2273 K) and for their secondary processing. The presentation will discuss the above reactions in model Nb silicide based alloys with particular emphasis on the formation and stability of the Nb3Si and bNb5Si3. New data about the dependence of the formation of the Nb3Si on solidification conditions will be presented for the first time and discussed. The experimental data will be from cast and heat treated alloys that were prepared using arc and plasma melting and optical floating zone (OFZ) melting

On the Alloying and Properties of Tetragonal Nb₅Si₃ in Nb-Silicide Based Alloys

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)

Powder route processing of Nb-silicide based alloys

For reasons of performance and fuel efficiency, aerospace engines of the future are expected to be able to operate at temperatures that are ever closer to and ultimately in excess of the melting temperatures of nickel-based superalloys. Consequently, alloys based on the refractory metal niobium are being investigated as alternatives to nickel-based superalloys. Alloys based on niobium with the addition of silicon show promising high temperature creep and room temperature toughness properties. They are also less dense than nickel-based superalloys. This suggests that they could, with development, meet the needs of future ultra-high temperature aerospace engine applications. To date, most of the published research has concentrated on cast and heat treated alloys. Powder route processing offers the potential for the manufacture of near net shape components but has attracted less attention in the open literature. This is probably due to the sluggish kinetics in Nb-Si based alloys, the sensitivity of Nb to contamination by interstitials and the availability of powders. This presentation will concentrate on powder route processing of model Nb-silicide alloys using elemental and pre-alloyed powders and plasma spark sintering or additive layer manufacturing. The microstructures produced by each processing method will be compared to ascertain the influence of processing route on microstructure. Emphasis will be given to phase selection and phase transformations and the contamination of the microstructure by interstitials during processing

Phase equilibria in the Nb-Si-Ge phase diagram

Niobium silicide-based in-situ composites have the potential to supersede nickel-based superalloys due to their excellent high temperature mechanical properties and low density. A thermodynamic database is being developed using the CALPHAD method to aid in alloy development. The addition of small amounts of germanium into these systems is of particular interest as it can significantly improve oxidation resistance. For example, germanium is reported to benefit high temperature oxidation resistance of coatings used on refractory silicide alloys by the formation of a glassy GeO2.SiO2 phase which fills cracks and is impermeable to further oxygen penetration. The effect of germanium on the phases formed in bulk niobium silicide-based in-situ composites is not particularly well understood, and limited data exists in the literature. To understand the effect of germanium on alloys, a thermodynamic description of the ternary Nb-Si-Ge phase diagram has been developed using the Calphad method. To support thermodynamic modelling samples were produced along the Nb5Ge3-Nb5Si3 pseudo binary and assessed using XRD. Experimental results show that germanium stabilises the high temperature Nb5Si3 (W5Si3 prototype) to low temperatures. The thermodynamic assessment will be presented and compared to experimental data from the current work and the literature

Phase equilibria in the Nb-rich region of Al-Nb-Sn at 900 and 1200 °C

The Al-Nb-Sn phase diagram was studied experimentally in the Nb-rich region to provide important phase equilibria information for alloy design of Nb-silicide based materials for aero engine applications. Three alloys were produced: Nb-17Al-17Sn, Nb-33Al-13Sn and Nb-16Al-20Sn (at.%). As-cast and heat-treated alloys (900 and 1200 °C) were analysed using XRD (X-ray diffraction) and SEM/EDS (scanning electron microscopy/ electron dispersive x-ray spectroscopy). Tin showed a high solubility in Nb2Al, reaching up to 21 at.% in the Sn-rich areas, substituting for Al atoms. Tin and Al also substituted for each other in the A15 phases (Nb3Al and Nb3Sn). Tin showed limited solubility in NbAl3, not exceeding 3.6 at.% as it substituted Al atoms. The solubility of Al in NbSn2 varied from 4.8 to 6.8 at.%. A ternary phase, Nb5Sn2Al with the tI32 W5Si3 crystal structure, was found to be stable. This phase was observed in the 900 °C heat-treated samples, but not in the 1200 °C heated samples

The role of Sn in the oxidation of Nb silicide based alloys

Aero-engine materials used in critical components must have acceptable oxidation behaviour at the temperatures of service. Niobium silicide based alloys have the potential to replace Ni based superalloys in future aero-engines owing to their lower densities, higher melting points and balance of properties. Niobium silicide based alloys must have inherent oxidation resistance to survive in case of coating failure . Great advances have been made towards improving the oxidation behaviour of developmental Nb silicide based alloys. Tin was reported [1, 2] to improve oxidation and subsequent research [3, 4] confirmed that Sn as an alloying addition contributes towards suppressing pest oxidation and is an essential alloying addition for suppressing the spallation of scale at high temperatures [5]. Evidence for the enrichment of the microstructure below the alloy/scale interface with Sn was provided for the first time by the group [3, 4]. “How Sn manages to deliver” better oxidation behaviour in Nb silicide based alloys was not understood. In the last “beyond the Ni superalloys” ECI conference some preliminary results were presented in a poster addressing this point. This presentation will be based on the results of recently completed systematic experimental and modelling research of model alloys and ternary systems to show how Sn affects microstructure and oxidation behaviour. Particular emphasis will be given to the links between phase stability, volume fraction and distributions of key intermetallic phases in the microstructure of Sn containing Nb silicide based alloys and their oxidation at 800 and 1200 oC. The role of Sn for the microstructure at the alloy/scale interface will be discussed

On the microstructure and properties of Nb-12Ti-18Si-6Ta-5Al-5Cr-2.5W-1Hf (at.%) silicide-based alloys with Ge and Sn additions

The microstructures and properties of the alloys JZ3 (Nb-12.4Ti-17.7Si-6Ta-2.7W-3.7Sn-4.8Ge-1Hf-4.7Al-5.2Cr) and JZ3+(Nb-12.4Ti-19.7Si-5.7Ta-2.3W-5.7Sn-4.9Ge-0.8Hf-4.6Al-5.2Cr) were studied. The densities of both alloys were lower than the densities of Ni-based superalloys and many of the refractory metal complex concentrated alloys (RCCAs) studied to date. Both alloys had Si macrosegregation and the same phases in their as cast and heat treated microstructures, namely βNb5Si3, αNb5Si3, A15-Nb3X (X = Al, Ge, Si, Sn), C14-Cr2Nb and solid solution. W-rich solid solutions were stable in both alloys. At 800 °C only the alloy JZ3 did not show pest oxidation, and at 1200 °C a thin and well adhering scale formed only on JZ3+. The alloy JZ3+ followed parabolic oxidation with rate constant one order of magnitude higher than the single crystal Ni-superalloy CMSX-4 for the first 14 h of oxidation. The oxidation of both alloys was superior to that of RCCAs. Both alloys were predicted to have better creep at the creep goal condition compared with the superalloy CMSX-4. Calculated Si macrosegregation, solid solution volume fractions, chemical compositions of solid solution and Nb5Si3, weight changes in isothermal oxidation at 800 and 1200 °C using the alloy design methodology NICE agreed well with the experimental results

Ab initio study of ternary W5Si3 type TM5Sn2X compounds (TM = Nb, Ti and X = Al, Si)

The adhesion of the scale formed on Nb-silicide based alloys at 1473 K improves when Al and Sn are in synergy with Si and Ti. This improvement is observed when there is segregation of Sn in the microstructure below the alloy/scale interface and a layer rich in intermetallics that include TM5Sn2X compounds is formed at the interface. Data for the ternary compounds is scarce. In this paper elastic and thermodynamic properties of the Nb5Sn2Al, Ti5Sn2Si, Ti5Sn2Al and Nb5Sn2Si compounds were studied using the first-principles, pseudopotential plane-wave method based on density functional theory. The enthalpy of formation of the ternary intermetallics was calculated using the quasi-harmonic approximation. The calculations suggest that the Nb5Sn2Si is the stiffest; that the Nb5Sn2Al and Ti5Sn2Si are the most and less ductile phases respectively; and that Nb significantly increases the bulk, shear and elastic moduli of the ternary compound compared with Ti

Microstructures and isothermal oxidation of the alumina scale forming Nb1.45Si2.7Ti2.25Al3.25Hf0.35 and Nb1.35Si2.3Ti2.3Al3.7Hf0.35 alloys

Coating system(s) will be required for Nb-silicide based alloys. Alumina forming alloys that are chemically compatible with the Nb-silicide based alloy substrate could be components of such systems. The intermetallic alloys Nb1.45Si2.7Ti2.25Al3.25Hf0.35 (MG5) and Nb1.35Si2.3Ti2.3Al3.7Hf0.35 (MG6) were studied in the cast, heat treated and isothermally oxidised conditions at 800 and 1200 °C to find out if they are αAl2O3 scale formers. A (Al/Si)alloy versus Nb/(Ti + Hf)alloy map, which can be considered to be a map for Multi-Principle Element or Complex Concentrated Nb-Ti-Si-Al-Hf alloys, and a [Nb/(Ti + Hf)]Nb5Si3 versus [Nb/(Ti + Hf)]alloy map were constructed making use of the alloy design methodology NICE and data from a previously studied alloy, and were used to select the alloys MG5 and MG6 that were expected (i) not to pest, (ii) to form αAl2O3 scale at 1200 °C, (iii) to have no solid solution, (iv) to form only hexagonal Nb5Si3 and (v) to have microstructures consisting of hexagonal Nb5Si3, Ti5Si3, Ti5Si4, TiSi silicides, and tri-aluminides and Al rich TiAl. Both alloys met the requirements (i) to (v). The alumina scale was able to self-heal at 1200 °C. Liquation in the alloy MG6 at 1200 °C was linked with the formation of a eutectic like structure and the TiAl aluminide in the cast alloy. Key to the oxidation of the alloys was the formation (i) of “composite” silicide grains in which the γNb5Si3 core was surrounded by the Ti5Si4 and TiSi silicides, and (ii) of tri-aluminides with high Al/Si ratio, particularly at 1200 °C and very low Nb/Ti ratio forming in-between the “composite” silicide grains. Both alloys met the “standard definition” of high entropy alloys (HEAs). Compared with HEAs with bcc solid solution and intermetallics, the VEC values of both the alloys were outside the range of reported values. The parameters VEC, Δχ and δ of Nb-Ti-Si-Al-Hf coating alloys and non-pesting Nb-silicide based alloys were compared and trends were established. Selection of coating alloys with possible “layered” structures was discussed and alloy compositions were proposed