On the Nb silicide based alloys: Part I – The bcc Nb solid solution

This paper is about the three types of bcc Nbss solid solution, namely normal Nbss, Nbss rich in Ti and Nbss with no Si that are observed in multi-component Nb silicide based alloys of Nb-Si-TM-RM-X (TM = Cr, Hf, Ti, V, RM = Mo, Ta, W, X = Al, B, Ge, Sn) systems. The entropy (ΔSmix) and enthalpy (ΔHmix) of mixing, atomic size difference (δ), electronegativity difference (Δχ), valence electron concentration (VEC) and the parameter Q = Tm ΔSmix/|ΔHmix| were calculated for fifty four solid solutions. The values of these parameters were −2 0.179 had no B, Ta and V and the solid solutions with no W had Δχ 5) and the Nbss with no Si (δ < 5). The formation of Ti rich Nbss and Nbss with no Si was attributed to the partitioning of Mo, Ti and W

On Nb silicide based alloys: alloy design and selection

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

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

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)

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

A study of the effect of 2 at.% Sn on the microstructure and isothermal oxidation at 800 and 1200 °C of Nb-24Ti-18Si-based alloys with Al and/or Cr additions

Alloying with Al, Cr, Sn, and Ti significantly improves the oxidation of Nb silicide-based alloys at intermediate and high temperatures. There is no agreement about what the concentration of Sn in the alloys should be. It has been suggested that with Sn ≤ 3 at.% the oxidation is improved and formation of the brittle A15-Nb3Sn compound is suppressed. Definite improvements in oxidation behaviour have been observed with 5 at.% Sn or even higher concentrations, up to 8 at.% Sn. The research reported in this paper is about three model alloys with low Sn concentration and nominal compositions Nb-24Ti-18Si-5Cr-2Sn (ZX3), Nb-24Ti-18Si-5Al-2Sn (ZX5), and Nb-24Ti-18Si-5Al-5Cr-2Sn (ZX7) that were studied to understand the effect of the 2 at.% Sn addition on as-cast and heat-treated microstructures and isothermal oxidation in air at 800 and 1200 °C for 100 h. There was macrosegregation of Si and Ti in the alloys ZX3 and ZX5 and only of Si in the alloy ZX7. The Nbss was stable in all alloys. Tin and Ti exhibited opposite partitioning behaviour in the Nbss. The βNb5Si3 was the primary phase in all three cast alloys and had partially transformed to αNb5Si3 in the alloy ZX3. Aluminium in synergy with Sn increased the sluggishness of the βNb5Si3 to αNb5Si3 transformation during solidification. After the heat treatment the transformation of βNb5Si3 to αNb5Si3 had been completed in all three alloys. Fine precipitates were observed inside some αNb5Si3 grains in the alloys ZX5 and ZX7. In the latter alloys the A15-Nb3X (X = Al, Si, and Sn) formed after the heat treatment, i.e., the synergy of Al and Sn promoted the stability of A15-Nb3X intermetallic in these Nb-silicide-based alloys even at this low Sn concentration. A Nbss + Nb5Si3 eutectic formed in all three alloys and there was evidence of anomalous eutectic in the parts of the alloys ZX3 and ZX7 that had solidified under high cooling rate and/or high melt undercooling. A very fine ternary Nbss + Nb5Si3 + NbCr2 eutectic was also observed in parts of the alloy ZX3 that had solidified under high cooling rate. At 800 °C none of the alloys suffered from catastrophic pest oxidation; ZX7 had a smaller oxidation rate constant. A thin Sn-rich layer formed continuously between the scale and Nbss in the alloys ZX3 and ZX5. At 1200 °C the scales formed on all three alloys spalled off, the alloys exhibited parabolic oxidation in the early stages followed by linear oxidation; the alloy ZX5 gave the smallest rate constant values. A thicker continuous Sn-rich zone formed between the scale and substrate in all three alloys. This Sn-rich zone was noticeably thicker near the corners of the specimen of the alloy ZX7 and continuous around the whole specimen. The Nb3Sn, Nb5Sn2Si, and NbSn2 compounds were observed in the Sn-rich zone. At both temperatures the scales formed on all three alloys consisted of Nb-rich and Nb and Si-rich oxides, and Ti-rich oxide also was formed in the scales of the alloys ZX3 and ZX7 at 1200 °C. The formation of a Sn-rich layer/zone did not prevent the contamination of the bulk of the specimens by oxygen, as both Nbss and Nb5Si3 were contaminated by oxygen, the former more severely than the latter

The impact of Ti and temperature on the stability of Nb5Si3 phases: a first-principles study

Nb-silicide based alloys could be used at T > 1423 K in future aero-engines. Titanium is an important additive to these new alloys where it improves oxidation, fracture toughness and reduces density. The microstructures of the new alloys consist of an Nb solid solution, and silicides and other intermetallics can be present. Three Nb5Si3 polymorphs are known, namely αNb5Si3 (tI32 Cr5B3-type, D8l), βNb5Si3 (tI32 W5Si3-type, D8m) and γNb5Si3 (hP16 Mn5Si3-type, D88). In these 5–3 silicides Nb atoms can be substituted by Ti atoms. The type of stable Nb5Si3 depends on temperature and concentration of Ti addition and is important for the stability and properties of the alloys. The effect of increasing concentration of Ti on the transition temperature between the polymorphs has not been studied. In this work first-principles calculations were used to predict the stability and physical properties of the various Nb5Si3 silicides alloyed with Ti. Temperature-dependent enthalpies of formation were computed, and the transition temperature between the low (α) and high (β) temperature polymorphs of Nb5Si3 was found to decrease significantly with increasing Ti content. The γNb5Si3 was found to be stable only at high Ti concentrations, above approximately 50 at. % Ti. Calculation of physical properties and the Cauchy pressures, Pugh’s index of ductility and Poisson ratio showed that as the Ti content increased, the bulk moduli of all silicides decreased, while the shear and elastic moduli and the Debye temperature increased for the αNb5Si3 and γNb5Si3 and decreased for βNb5Si3. With the addition of Ti the αNb5Si3 and γNb5Si3 became less ductile, whereas the βNb5Si3 became more ductile. When Ti was added in the αNb5Si3 and βNb5Si3 the linear thermal expansion coefficients of the silicides decreased, but the anisotropy of coefficient of thermal expansion did not change significantly

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

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

In this paper two Nb-silicide-based alloys with nominal compositions (at.%) Nb-12Ti-18Si-6Ta-2.5W-1Hf-2Sn-2Ge (JZ1) and Nb-12Ti-18Si-6Ta-2.5W-1Hf-5Sn-5Ge (JZ2) were studied. The alloys were designed using the alloy design methodology NICE to meet specific research objectives. The cast microstructures of both alloys were sensitive to solidification conditions. There was macro-segregation of Si in JZ1 and JZ2. In both alloys the βNb5Si3 was the primary phase and the Nbss was stable. The A15-Nb3X (X = Ge,Si,Sn) was stable only in JZ2. The Nbss+βNb5Si3 eutectic in both alloys was not stable as was the Nb3Si silicide that formed only in JZ1. At 800 °C both alloys followed linear oxidation kinetics and were vulnerable to pesting. At 1200 °C both alloys exhibited parabolic oxidation kinetics in the early stages and linear kinetics at longer times. The adhesion of the scale that formed on JZ2 at 1200 °C and consisted of Nb and Ti-rich oxides, silica and HfO2 was better than that of JZ1. The microstructure of JZ2 was contaminated by oxygen to a depth of about 200 μm. There was no Ge or Sn present in the scale. The substrate below the scale was richer in Ge and Sn where the NbGe2, Nb5(Si1-xGex)3, W-rich Nb5(Si1-xGex)3, and A15-Nb3X compounds (X = Ge,Si,Sn) were formed in JZ2. The better oxidation behavior of JZ2 compared with JZ1 correlated well with the decrease in VEC and increase in δ parameter values, in agreement with NICE. For both alloys the experimental data for Si macrosegregation, vol.% Nbss, chemical composition of Nbss and Nb5Si3, and weight gains at 800 and 1200 °C was compared with the calculations (predictions) of NICE. The agreement was very good. The calculated creep rates of both alloys at 1200 °C and 170 MPa were lower than that of the Ni-based superalloy CMSX-4 for the same conditions but higher than 10−7 s−1

A study of the effects of Hf and Sn on the microstructure, hardness and oxidation of Nb-18Si silicide based alloys without Ti addition

The paper presents the results of an experimental study of large (≈0.6 kg) arc melted buttons of four Ti free Nb-silicide based alloys with Sn addition with nominal compositions (at.%) Nb-18Si-5Hf-5Sn (EZ1), Nb-18Si-5Al-5Sn (EZ7), Nb-18Si-5Cr-5Hf-5Sn (EZ3) and Nb-18Si-5Al-5Hf-5Sn (EZ4). The alloys were studied in the as-cast and heat treated conditions. In all the alloys there was macrosegregation of Si (MACSi). Among the single element additions Hf had the weakest and Sn the strongest effect on MACSi. The simultaneous presence of Cr and Hf in the alloy EZ3 had the strongest effect on MACSi. In all the alloys the βNb5Si3 was the primary phase and was present after the heat treatment(s), the Nb3Si silicide was suppressed and the A15-Nb3Sn intermetallic was stable. The Nbss was not stable in the alloys EZ7 and EZ4 and the C14-NbCr2 Laves phase was stable in the alloy EZ3. Very Hf-rich Nb5Si3 was stable in the alloy EZ4 after prolonged heat treatments. Eutectics were observed in all the alloys. These were binary eutectics in the alloys EZ1 and EZ7, where respectively they consisted of the Nbss and βNb5Si3, and βNb5Si3 and A15-Nb3Sn phases. Most likely ternary eutectics consisting of the Nbss, C14-NbCr2 and βNb5Si3, and Nbss, βNb5Si3 and A15-Nb3Sn phases were observed, respectively in the alloys EZ3 and EZ4. The addition of Al increased the vol% of the Nb5Si3 and A15-Nb3Sn phases, particularly after the heat treatment(s). The lattice parameter of Nb respectively increased and decreased with the addition of Hf, and Al or Cr and the latter element had the stronger negative effect. Pest oxidation was not suppressed in the alloys of this study